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## Creator

Jiantong Wang, Bowen Wang, Yuhan Teng, Changshuai Gong, Xuyan Xue, Xuejiao Wang, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/)

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[A novel K3(Y0.88Yb0.10Er0.02)Si2O7 silicate phosphor for multi-mode thermometry of high sensitivity through up-conversion luminescence](https://mdr.nims.go.jp/datasets/426f3b6e-b248-4f38-84ee-3f69c8b9f682)

## Fulltext

1  A novel K3(Y0.88Yb0.10Er0.02)Si2O7 silicate phosphor for multi-mode thermometry of high sensitivity through up-conversion luminescence    Jiantong Wang,a Bowen Wang,a Yuhan Teng,a Changshuai Gong,a Xuyan Xue,a Xuejiao Wang,a* Ji-Guang Lib*    aCollege of Chemistry and Materials Engineering, Bohai University, Jinzhou, Liaoning 121007, China bResearch Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan   *Corresponding author Dr. Xuejiao Wang Bohai University Jianzhou, China Tel: +86-416-3400708 E-mail: wangxuejiao@bhu.edu.cn  Dr. Ji-Guang Li National Institute for Materials Science Ibaraki, Japan Tel: +81-29-860-4394 E-mail: li.jiguang@nims.go.jp    Revised Manuscript (Clean Version) Click here to view linked References 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 https://www.editorialmanager.com/ceri/viewRCResults.aspx?pdf=1&docID=170022&rev=1&fileID=2989938&msid=4f49667c-1ce5-4ff5-988e-b67f8927afd3https://www.editorialmanager.com/ceri/viewRCResults.aspx?pdf=1&docID=170022&rev=1&fileID=2989938&msid=4f49667c-1ce5-4ff5-988e-b67f8927afd32  Abstract Non-contract optical thermometry shows great advantages for temperature measurement in harsh environments, and it is highly desired to develop novel and high-performance systems. In this work, a novel K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor was synthesized via solid-state reaction, and its up-conversion luminescence and optical thermometric performance were systematically investigated. Under 980 nm excitation, the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor showed a strong green UC emission through a three photon process. The phosphor was found well capable of sensing temperature via the multi-modes of fluorescence intensity ratio (FIR) of thermally/non-thermally coupled energy levels and fluorescence lifetime (FL). The maximum absolute (SA) and relative (SR) sensitivities are SA = 156×10-4 K-1 (I531/I555, 298 K) and SR = 1.80% K-1 (I519/I555, 298 K) via the FIR mode, and are SA = 661×10-4 K-1 (548 K) and SR = 9.77% K-1 (298 K) via the FL mode, which are higher than those of most previous studies.  Keywords: up-conversion; silicate; optical thermometry; rare earth luminescence     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3  1. Introduction Silicate compounds are an important phosphor family, which have aroused great research interests [1-9]. Phosphors based on silicates have good thermal and chemical stability and are widely used in cathode-ray luminescence and photoluminescence. Silicates include normal silicate, partial silicate, pyrosilicate, and other different systems. By controlling the ratio of chemical reagents and experimental conditions, different silicate phosphors can be obtained, such as Ba2La8(SiO4)6O2: Eu2+ [2], MgGd4Si3O13: Ce3+, Mn2+ [3], Mg2Y2Al2Si2O12: Tb3+, Eu3+, Tm3+ [4], K3ScSi2O7: Eu2+ [5], etc. Silicates are an important choice for luminescence because of their significant absorption of ultraviolet (UV)/near-UV/blue light and low cost. At the same time, silicate-based phosphors have good physical properties, such as oxidation resistance, moisture resistance, and no interaction with the packaging resin [2,4,8,9]. The [SiO4]4- unit, as the basic building block of silicates, can form relatively complicated crystal structures, which usually contain various complex anionic groups by different joining methods, forming islands, rings, chains, or layered structures [5,6,9]. Among the silicates, A3RESi2O7 (A = Na, K; RE = Y, Gd, Lu), especially the K3RESi2O7, has aroused intense research interests and various experiments have been carried out for the system [5,6,10-19]. It is found that different emissions, such as near-infrared emission [5,6], white emission [10], and near-ultraviolet emission [11], can be realized by adjusting the RE site. However, the up-conversion (UC) luminescence of this K3RESi2O7 system has not been reported up to date to the best of our knowledge. One of the reasons is that the compound is typically synthesized by the traditional solid-phase method, which, although effective for phosphor selection and synthesis, would produce micron-sized  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4  large particles. It is difficult to obtain phosphors of nanometer size by this method, which greatly limits the application of UC phosphors of this system in the fields of biomarkers and anti-counterfeiting. Another reason is that the research interests of UC system are mainly on fluoride with low phonon energy, which would produce high UC efficiency [20-22]. However, fluoride is toxic, and its toxicity is related to its ability to dissociate fluorine ions. Recently, the UC phosphors activated by Yb3+-Er3+ pair are finding promising application as non-contact thermometers [7,20-23]. The intensity ratio of the 520 nm (2H11/2→4I15/2) and 550 nm (4S3/2→4I15/2) green luminescence Er3+ ion is very sensitive to temperature [24]. The temperature change will cause the obvious change in the ratio of these two branches, and the temperature can be determined by the branch ratio. Such application needs high thermal stability phosphors and doesn’t require the phosphor particles to be nanosized. In previous studies, common UC phosphors usually use halogen-containing compounds as substrate [20-22], and Yb3+/Er3+ co-doped NaYF4(NaYF4: Yb3+/Er3+) is the most common UC material with the highest luminous efficiency. However, halogen-containing compounds need irritating and frequently toxic chemicals in the process of their synthesis. Therefore, various researches have been conducted to explore novel systems using the solid-state method [7,25-27]. In view of the outstanding down-conversion photoluminescence and thermal stability of K3YSi2O7-based phosphors [10,13,14], we consider K3YSi2O7 to be a promising host for UC luminescence. Thus, the typical ion pair of Yb3+-Er3+ was doped into K3YSi2O7 in this work, and the resulted phosphor was shown to be capable of multi-mode temperature measurement with high sensitivity through UC luminescence.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5  2. Experimental section 2.1 Reagents and synthesis The rare earth sources for the synthesis were oxides (Y2O3, Yb2O3, Er2O3, 99.99% pure) bought from Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China. The other reagents of K2CO3 (99.9% pure) and SiO2 (99.99% pure) were purchased from Aladdin Industrial Corporation (Shanghai, China) and were used without further purification. K3(Y0.88Yb0.10Er0.02)Si2O7 phosphors were synthesized by the conventional high-temperature solid-state method, where the contents of Yb and Er were taken as the optimal values of other UC systems [28,29]. The raw materials were weighed according to stoichiometric ratio, ground in agate mortar with alcohol for 20 minutes, mixed evenly, and then transferred to a corundum crucible. Finally, the mixture was heated at 1350 ℃ in a muffle furnace for 6 h in air, with a heating rate of 5 ℃/min before 800 ℃ and 3 ℃/min for 800-1350 ℃. 2.2 Characterization  The crystalline phases of the samples were identified by an X-ray diffractometer (XRD; Model Ultima IV, Rigaku, Tokyo, Japan) with Cu-Kα (λ = 0.15406 nm) radiation via the step-scan mode, with a step size of 0.01° and an accumulation time of 2 s in 2θ = 5-120o range. Temperature-dependent UC spectra (RT-548 K) were measured using an FLS 1000 fluorospectrophotometer (Edinburgh Instruments Ltd., Herrsching am Ammersee, Britain), using a 980 nm continuous wavelength laser (2 W, Model MDL-III-980-2W-18050833) for excitation and a TAP-02 accessory for temperature control. The fluorescence decay kinetics of the main UC emissions were measured with the lifetime testing unit of the FLS 1000 equipment.   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6  3. Results and discussion 3.1 Synthesis and visible-near-infrared luminescence of K3(Y0.88Yb0.10Er0.02)Si2O7  Figure 1 (a) Rietveld refinement for the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor, where the observed and calculated patterns, the difference, and the positions of Bragg reflections are in black, red, gray and green, respectively, (b) crystal structure of K3YSi2O7, together with the coordination environment of the cation. Figure 1(a) shows the Rietveld refinement results of the obtained K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. The refinement yielded stable results and acceptable reliability factors, which indicate that the product is of high purity. The detailed refinement results are shown in Table 1 and Table 2. According to Loye et al., K3YSi2O7 may have two polymorphs, namely K3YSi2O7(1) and K3YSi2O7(2), which belong to P63/mmc and P63/mcm space groups, respectively [10]. In this work, we obtained K3YSi2O7(2) with the second space group. Figure 1(b) are the crystal structure observed from the c-axis and the coordination polyhedra of cations for K3YSi2O7(2). The compound K3YSi2O7(2) (ICSD 430538) has a hexagonal structure with lattice constants a = 9.8450 Å, c = 14.3236 Å. c/a = 1.4549. It consists of SiO4, K1O8, K2O9, K3O6, Y1O6, and Y2O6 units connected by shared oxygen atoms. K3YSi2O7(2) has two distinct yttrium sites, one distinct silicon site, three distinct potassium sites, and three distinct oxygen sites [13]. The two yttrium sites are  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7  independent of each other and are located in a hexagonal coordination environment. Y(1) is located in a regular hexagonal octahedron, and Y(2) is located in a hexagonal trigonal prism. Y1O6 is connected to SiO4 by oxygen atoms at shared vertices, and SiO4 forms the Si2O7 pyrosilicate structure by bridging oxygen atoms at shared vertices. The coordination numbers of K(1), K(2), and K(3) are 8, 9, and 6 respectively. The K(1) site is aligned with Si along the c-axis, the K(2) site is aligned with Y(1) along the c-axis, and the K(3) site is aligned with Y(2) along the c-axis. In addition, it can be seen that the lattice parameters and cell volume of the Yb3+-Er3+ co-doped sample is smaller than those of the pure host as also shown in Table S1. This conforms well to the fact that Er3+ (R = 0.890, CN = 6) and Yb3+ (R = 0.868, CN = 6) have smaller ionic radii than Y3+ (R = 0.900, CN = 6) [30], and indicates successful incorporation of the dopant ions to form solid solution.  Table 1 Experimental parameters of powder XRD and refined crystallographic data for K3(Y0.88Yb0.10Er0.02)Si2O7. Chemical Formula K3(Y0.88Yb0.10Er0.02)Si2O7 Diffractometer X’Pert Pro, PANalytical Radiation type Cu Kα, λ = 1.54060 Å 2θ interval (o) 5.00-120.00 Step size of 2θ (o) 0.01 Space group P63/mcm (193) Z 6 a (Å) 9.8419(2) b (Å) 9.8419(2) c (Å) 14.3032(4) V (Å3) 1199.85(6) Number of structure parameters 27 Number of profile parameters 35 b Rp (%) 8.73 c Rwp (%) 12.21 d S 2.22   iiCP yyyR /,ib ;  2122,i /  iiiCiwpc ywYywR ; exp/ RRS wpd    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8  Table 2 Wyckoff positions, atomic coordinates, and occupancies of K3(Y0.88Yb0.10Er0.02)Si2O7. Atom Site x y z Occupancy Y1 4 0.3333 0.6667 0 0.89 Yb1 4 0.3333 0.6667 0 0.10 Er1 4 0.3333 0.6667 0 0.01 Y2 2 0 0 0.25 0.91 Yb2 2 0 0 0.25 0.08 Er2 2 0 0 0.25 0.01 K1 12 0.32798 0 0.09211 1 K2 4 0.3333 0.6667 0.25 1 K3 2 0 0 0 1 Si1 12 0.65938 0 0.14315 1 O1 24 0.67437 0.15042 0.09402 1 O2 12 0.81453 0 0.14696 1 O3 6 0.59540 0 0.25 1  Figure 2 (a) Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor in the visible light range (450-750 nm) at different excitation powers of 980 nm laser. (b) The proposed up-conversion mechanism, where ET means energy transfer. The UC spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 excited at different powers (0.60-1.20 W) are shown in Figure 2. The emission spectra consist of green emissions in the range of 510-575 nm and red emissions in the 630-700 nm region. The emission peaks at 531 nm and 555 nm belong to the transitions from 2H11/2 and 4S3/2 excited states to 4I15/2 ground state of Er3+, respectively, while the red emission near 670 nm is generated by the transition from 4F9/2 to 4I15/2. The number of photons (n) required to excite the ground state electron to the emission state can be estimated by the Iem∝Pn equation [29]. The logarithmic curves of Iem and Pn at different powers are shown in  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9  Figure S1 for the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. The slope of the graph is the number of photons required (n). The slope of the curve (n value) in Figure S1 is about 3 for green emission and 2 for red emission, indicating that the observed UC luminescence is essentially caused by three (green) and two-photon (red) mechanism. The energy level diagram of Yb3+ and Er3+ in K3(Y0.88Yb0.10Er0.02)Si2O7 and the emission mechanism of UC are shown in Figure 2b. The three-photon process corresponding to green emission is as follows. Under the excitation of 980 nm laser, the electrons of Yb3+ are excited to the 2F5/2 level, from which energy transfer to the neighboring Er3+ would take place. The energy absorption of Er3+ ion excites electrons from the ground state 4I15/2 to the excited state 4I11/2 (ET1), followed by non-radiative relaxation (NR) to the 4I13/2 level. With the energy of the second excitation photon, the Er3+ electrons were raised to the 4F9/2 level (ET2). Subsequently, the Er3+ electrons were eventually excited to the 4G11/2 level by ET3 and then relax to 2H11/2 through non-radiative transition. Backing jumping of the 2H11/2 electrons to the ground state then produced the green light. The whole process can also be expressed as follows [31,32]: (1) 2F7/2 (Yb3+) + 980 nm photon → 2F5/2 (Yb3+) (2) 2F5/2 (Yb3+) + 4I15/2 (Er3+) → 2F7/2 (Yb3+) + 4I11/2 (Er3+) (ET1) (3) 4I11/2 (Er3+) → 4I13/2 (Er3+) (NR) (4) 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+) (ET2) (5) 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 4G11/2 (Er3+) (ET3) (6) 4G11/2 (Er3+) → 2H9/2 (Er3+) → 2H11/2 (NR)  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10  (7) 2H11/2 (Er3+) → 4I15/2 (Er3+) + photons (531 nm) For the emission peaks at 555 nm, the first six processes are the same as those of the 531 nm emission. The difference is that the electrons at the 2H11/2 energy level of Er3+ relax to the 4S3/2 by NR, and the occurrence of 4S3/2 (Er3+)→4I15/2 (Er3+) and transitions emit photons at 555 nm. For the red emission located at 662 nm, its two-photon process is that, after the first four transition steps, the electron at the 4F9/2 energy level transits to the ground state and emits a photon. It can be seen that for K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor, the increasing excitation power does not change the shape and position of the emission peak, but the intensity of the emission gradually increased.  Figure 3 (a) Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor in the near-infrared range (1000-1600 nm) at different excitation powers of 980 nm laser. (b) Fluorescence decay kinetics of the 1542 nm emission at different excitation powers (0.25-0.50 W), where the numbers following the power are the average lifetimes. The emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 in the near-infrared region at different powers are shown in Figure 3a. The strongest peak is at 1542 nm, which corresponds to the 4I13/2→4I15/2 transition of Er3+ [33]. It can be seen that the sample has a strong broadband emission in the near-infrared region. The Stark splitting of the NIR emission is evident because of the splitting of the 4f level of Er3+ ions due to the  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 11  action of the crystal field. Figure 3b shows the decay kinetics of the 1542 nm emission, where it was found that all the curves are well fitted using the double-order exponential equation [34,35]: 𝐼(𝑡) = 𝐴1𝑒𝑥𝑝(−𝑡/𝜏1) + 𝐴2𝑒𝑥𝑝(−𝑡/𝜏2)     (1) where A1 and A2 are constants, t is decay time, I(t) is luminescence intensity at time t, τ1, and τ2 are the fast and slow components of lifetime respectively. The average lifetime (τ) can be obtained by the following equation [34,35]: τ = (𝐴1𝜏12 + 𝐴2𝜏22)/(𝐴1𝜏1 + 𝐴2𝜏2)     (2) The derived τ and A values and the chi-square factor (χ2) of fittings are summarized in Table S2. The 1542 nm emissions at different powers show similar average lifetime of around 14 ms. 3.2 Multi-mode thermometric properties of K3(Y0.88Yb0.10Er0.02)Si2O7  Figure 4 (a) Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor excited by 980 nm laser (0.5 W) at different temperatures (RT-548 K). (b) Relative emission intensity for different transitions as a function of the measured temperature.  In practical applications, especially in the field of temperature measurement, the relationship between luminescence intensity and temperature is of great importance. Therefore, we tested the PL spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 in the temperature range of 298-548 K, as shown in Figure 4a. As can be seen from Figure 4a, the  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 12  luminescence intensity of the sample decreases gradually with increasing temperature and the specific CIE coordinates are given in Figure S2 and Table S3. As can be seen from Figure S2, the color coordinates of K3(Y0.88Yb0.10Er0.02)Si2O7 did not shift much with increasing temperatures. Figure 4b shows the relationship between normalized emission intensity and temperature. The results show that the green emission intensity of 519/527 nm (2H11/2→4I15/2) increases with increasing temperature, reaching the maximum value at 373 K and maintaining 97.2% and 98.7% of the maximum intensity at 423 K, respectively. At the highest measurement temperature of 548 K, 78.9% and 83.0% of the maximum emission intensity can still be retained, respectively. For 555 nm (4S3/2→4I15/2) emission, the intensity decreases obviously with increasing temperature. The intensity loss rates of 2H11/2→4I15/2 and 4S3/2→4I15/2 are different with increasing temperature. This indicates that the prepared phosphor can sense temperature through the fluorescence intensity ratio (FIR) of the thermally coupled 2H11/2 and 4S3/2 levels and has potential application in the field of optical thermometry. The 4F9/2→4I15/2 (662 nm) emission also decreased monotonically with increasing temperature. Since 2H11/2 and 4S3/2 are thermally coupled energy levels (ΔE = 736 cm-1), their population follows Boltzmann distribution. The FIR of the two different green emissions can be expressed as [36,37]: FIR =𝐼𝐻𝐼𝑆= 𝐴 ∙ exp (−∆𝐸𝑘𝐵𝑇) + 𝐶     (3) where IH and IS represent the emission intensity of the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions; ΔE is the energy level gap between corresponding energy levels; kB is Boltzmann constant; T is the absolute temperature; A and C are constants.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 13  Equation (3) is used to fit the FIR relation with temperature of the two thermal coupled energy levels. The fitting curve is shown in Figure 5a, and the points in the figure are experimental data. It can be seen from the figure that as the temperature rises from 298 K to 548 K, The I531/I555 ratios of K3(Y0.88Yb0.10Er0.02)Si2O7 increase gradually. According to the Boltzmann distribution law, when the temperature increases, electrons are more easily pumped to the higher 2H11/2 energy level to reach equilibrium. In addition, the probability of the non-radiative transition from 4S3/2 to 4I15/2 increases, so the emission at 555 nm is significantly weakened, thus increasing the ratios of I531/I555.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 14   Figure 5 Different FIRs and their corresponding SA and SR values as a function of the measurement temperature for the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor.   In addition, sensitivity S is an important index to evaluate the sensitivity of phosphors to temperature change, which can be expressed by absolute sensitivity SA and relative sensitivity SR. The calculation equations are as follows [37]: 𝑆𝐴 = |𝑑(𝐹𝐼𝑅)𝑑(𝑇)| = (𝐹𝐼𝑅 − 𝐶) ×𝛥𝐸𝑘𝐵𝑇2     (4) 𝑆𝑅 = |𝑑(𝐹𝐼𝑅)𝑑(𝑇)1𝐹𝐼𝑅| =𝐹𝐼𝑅−𝐶𝐹𝐼𝑅×𝛥𝐸𝑘𝐵𝑇2      (5) Equation (4) and Equation (5) were used to calculate SA and SR, respectively. As shown in Figure 5b, the maximum values of SA and SR corresponding to I531/I555 are 156×10-4 K-1(298 K) and 1.22% K-1(298 K), respectively; while the maximum values of SA and SR corresponding to I519/I555 are 58×10-4 K-1(548 K) and 1.80% K-1(298 K), respectively.  The energy difference (ΔE) of thermally coupled levels is generally between 200 ~ 2000 cm-1. Compared with the traditional technology based on the FIR of thermally  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 15  coupled energy levels, the temperature measurement based on non-thermally coupled energy levels (NTCL) is not limited by the energy difference, and the number of energy level pairs is rich, which is beneficial to improve the accuracy of temperature measurement. We calculated the temperature sensitivity of the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. The temperature-dependent FIR of non-thermally coupled energy levels can be fitted by a polynomial function [38]: 𝐹𝐼𝑅 =𝐼555𝐼662= 𝐴 + 𝐵𝑇 + 𝐶𝑇2 + 𝐷𝑇3     (6) where A, B, C, and D are constants. As shown in Figure 6a, the variation of the fluorescence intensity ratio FIR(I555/I662) of the non-thermally coupled energy levels with temperature can be well fitted by Equation (6), with R2 = 0.99. In this case, the sensitivity of temperature sensing can be calculated with the following equation [38]: 𝑆 =𝑑(𝐹𝐼𝑅)𝑑𝑇= 𝐵 + 2𝐶𝑇 + 3𝐷𝑇2     (7) The curve of sensitivity as a function of temperature is shown in Figure 6b, where the sensitivity reached a maximum value of 157×10-4 K-1 at 298 K.   Figure 6 (a) The FIR of the 555 nm and 662 nm emission peaks of K3(Y0.88Yb0.10Er0.02)Si2O7 as a function of temperature and (b) sensitivity as a function of temperature.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 16   Figure 7 Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor excited with different powers 980 nm laser at (a) 398 K, (b) 473 K, and (c) 548 K. (d) The variation trend of I555/I662 with excitation power at different temperatures. Although quite a number of references used Equation (6) for the FIR fitting of non-thermally coupled energy levels, the excitation power was not involved in the equation. Such fitting cannot reflect the influence of the excitation power on FIRs. In order to ensure the accuracy of the results, the power dependent emission spectra at different temperatures (398 K, 473 K, 548 K) were measured and the I555/I662 FIR at different excitation powers was evaluated. As seen from the results shown in Figure 7, I555/I662 FIR is stable under varying excitation power at different temperatures and, therefore, the influence of excitation power during the temperature-sensing calculation of the non-thermally coupled energy level can be excluded. The influence of excitation power on the other two FIRs were also checked and were shown in Figure S4. It was found that excitation power has a small influence on the FIRs  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 17  values. Table 3 summarizes the SA and SR values of several typical phosphors with optical temperature sensing behavior. Through comparison, it was found that the maximum value of SA and SR obtained in this work are higher than these of most systems. Among them, the sensitivity values calculated based on non-thermally coupled energy levels are higher than those obtained by the thermally coupled energy levels. The results show that K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor has potential application value in optical temperature measurement. Table 3. A summary of SA and SR values, and temperature sensing ranges for some typical temperature sensing phosphors. Ion pair Mode Host Range (K) SA(K-1)×10-4 SR(% K-1) Ref Yb3+-Er3+ FIR KY3F10(Core-Only) 310-366 - 1.51 (310 K) [20] Yb3+-Er3+ FIR KY3F10(Core-Shell) 300-365 - 1.24 (316 K) [20] Yb3+/Er3+/Mn2+ FIR NaBiF4 298-473 55.9 (473 K) 1.22 (298 K) [21] Yb3+,Er3+ FIR NaScF4 298-573 25.6 (548 K) 0.31 [22] Yb3+,Er3+ FIR NaScF4(EDTA)  298-573 32.8 (548 K) 0.41 [22] Yb3+,Er3+ FIR Na2GdMg2(VO4)3 303-573 74.9 (479 K) 0.98 (303 K) [23] Er3+ FIR GdTaO4 293-723 41 (475 K) 1.12 (298 K) [24] Yb3+,Er3+ FIR Ba5Y8Zn4O21 293-563 0.39 (563 K) 136 (293 K) [25] Er3+ FIR LaGdO3 298-873 43 (554 K) 1.20 (298 K) [26] Bi3+/Tb3+ FIR LaNdO4 303-483 410 2.36 [27] Bi3+/Eu3+ FIR LaNdO4 303-483 440 1.89 [27] Bi3+/Dy3+ FIR LaNdO4 303-483 80 1.26 [27] Bi3+/Sm3+ FIR LaNdO4 303-483 310 1.36 [27] Yb3+/Er3+ FIR Y2O3 323-573 196 (543 K) - [39] Pr3+ FL La2MgTiO6 298-548 2.85 1.81 (473 K) [40] Sm2+ FL SrBa4O7 298-723 - 3.36 (500 K) [41] Eu2+ FL Li4ScCa(SiO4)2 303-573 1470 15.0 [42] Dy3+/Mn4+ FL Li2TiO3/Y2O3 273-373 220 (308 K) 6.67 (339 K) [43] Yb3+,Er3+ FIR(I531/I555) K3YSi2O7 298-548 156 (298 K) 1.22 (298 K) This work Yb3+,Er3+ FIR(I527/I555) K3YSi2O7 298-548 59 (548 K) 1.48 (298 K) This work Yb3+,Er3+ FIR(I519/I555) K3YSi2O7 298-548 58 (548 K) 1.80 (298 K) This work Yb3+,Er3+ FIR(I524/I555) K3YSi2O7 298-548 139  1.34 (298 K) This work Yb3+,Er3+ FIRNTCL(I555/I662) K3YSi2O7 298-548 157 (298 K)  - This work Yb3+,Er3+ FL(531 nm) K3YSi2O7 323-548 661 (548 K) 9.77 (298 K) This work   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18   Figure 8 (a) Fluorescence decay kinetics of 531 nm emission at different temperatures (RT-548 K), and (b) the relationship of fluorescence lifetimes for 531 nm emission with measurement temperature (RT-548 K). The intensity in (a) is in logarithmic form. Figure 8a shows the fluorescence decay curves for the 531 nm emission of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor under 980 nm laser excitation at different temperatures. As shown in the figure, all the curves are well fitted using the double-order exponential Equation (1). Then, the average lifetime (τ) can be obtained by the following Equation (2). The derived lifetime of the 531 nm emission is shown in Figure 8b, where it is seen that the lifetime monotonously decreased from 80.31 μs (323 K) to 67.71 μs (548 K). The 531 nm emission at other temperatures showed a similar decay behavior and the derived τ and A values and the chi-square factor (χ2) of fittings are summarized in Table S4. The χ2 values for all the fittings are around 1, indicating the high quality of the fitting. It is noteworthy that the fluorescence lifetime of K3(Y0.88Yb0.10Er0.02)Si2O7 decreases with increasing temperature, which can be attributed to increased non-radiative transitions in the matrix lattice. This trend is the same as that observed from the temperature-dependent emission intensity. As shown in Figure 8b, the lifetime of the 531 nm emission decreases exponentially with increasing temperature T, and can be expressed as τ531 = -17.18exp (-624.78/T) + 190.91.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19   Figure 9 The relative sensitivity SR and absolute sensitivity SA based on the fluorescence lifetime of 531 nm emission for K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. Aside from those utilizing the FIR technology, a temperature sensing scheme based on fluorescence lifetime (FL) can also be realized with the thermal quenching characteristics of K3(Y0.88Yb0.10Er0.02)Si2O7. Similar to the scheme based on FIR, the absolute sensitivity SA and relative sensitivity SR of FL based temperature sensing can be calculated by the following Equation [40,42-44]: 𝑆𝐴 = |𝑑𝜏𝑑𝑇|     (8) 𝑆𝑅 =1𝜏|𝑑𝜏𝑑𝑇|     (9) As shown in Figure 9, the phosphor has maximum values were 661×10-4 K-1(548 K) and 9.77% K-1(548 K) for SA and SR, respectively. It can be seen from the figure that SA and SR both increase with increasing temperature. By comparing the SR value based on the FL mode and the SR value based on the FIR mode, it is obvious that the former is greater than the latter. Nevertheless, as shown in Table 3, the SR values of both the FIR and FL modes obtained in this work are better than those of most previous studies.   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20  Conclusions A novel K3(Y0.88Yb0.10Er0.02)Si2O7 silicate phosphor was prepared by solid reaction. The UC luminescence properties, UC mechanism as well as the performance of optical temperature sensing via multi-modes were systematically investigated. The main conclusions are as follows: (1) The K3(Y0.88Yb0.10Er0.02)Si2O7 new phosphor exhibits strong green luminescence through 2H11/2→4I15/2 transition (531 nm) under 980 nm excitation, which occurs through a three photon processes.  (2) K3(Y0.88Yb0.10Er0.02)Si2O7 is well capable of temperature sensing via fluorescence intensity ratio (FIR) with thermally coupled/non-thermally coupled energy levels (TCL/NTCL) and fluorescence lifetime (FL). The maximum SA and SR values are SA = 156×10-4 K-1 (I531/I555, 298 K) and SR = 1.80% K-1 (I519/I555, 298 K) for the FIR mode, and are SA = 661×10-4 K-1 (548 K) and SR = 9.77% K-1 (298 K) for the FL mode.  Acknowledgements This work is supported by Natural Science Foundation of Liaoning Province (Grant No. 2020-MS-286). The authors would like to thank Siqi Liu from Shiyanjia Lab (www.shiyanjia.com) for the XRD measurement. References [1] X. Ji, J. Zhang, Y. Li, S. Liao, X. Zhang, Z. Yang, Z. Wang, Z. Qiu, W. Zhou, L. Yu, S. Lian, Improving quantum efficiency and thermal stability in blue-emitting Ba2-xSrxSiO4: Ce3+ phosphor via solid solution, Chem. 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Alloys Compd. 881 (2021) 160601. https://doi.org/10.1016/j.jallcom.2021.160601.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 1  A novel K3(Y0.88Yb0.10Er0.02)Si2O7 silicate phosphor for multi-mode thermometry of high sensitivity through up-conversion luminescence    Jiantong Wang,a Bowen Wang,a Yuhan Teng,a Changshuai Gong,a Xuyan Xue,a Xuejiao Wang,a* Ji-Guang Lib*    aCollege of Chemistry and Materials Engineering, Bohai University, Jinzhou, Liaoning 121007, China bResearch Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan   *Corresponding author Dr. Xuejiao Wang Bohai University Jianzhou, China Tel: +86-416-3400708 E-mail: wangxuejiao@bhu.edu.cn  Dr. Ji-Guang Li National Institute for Materials Science Ibaraki, Japan Tel: +81-29-860-4394 E-mail: li.jiguang@nims.go.jp    Revised Manuscript with Changes Marked 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2  Abstract Non-contract optical thermometry shows great advantages for temperature measurement in harsh environments, and it is highly desired to develop novel and high-performance systems. In this work, a novel K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor was synthesized via solid-state reaction, and its up-conversion luminescence and optical thermometric performance were systematically investigated. Under 980 nm excitation, the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor showed a strong green UC emission through a three photon process. The phosphor was found well capable of sensing temperature via the multi-modes of fluorescence intensity ratio (FIR) of thermally/non-thermally coupled energy levels and fluorescence lifetime (FL). The maximum absolute (SA) and relative (SR) sensitivities are SA = 156×10-4 K-1 (I531/I555, 298 K) and SR = 1.80% K-1 (I519/I555, 298 K) via the FIR mode, and are SA = 661×10-4 K-1 (548 K) and SR = 9.77% K-1 (298 K) via the FL mode, which are higher than those of most previous studies.  Keywords: up-conversion; silicate; optical thermometry; rare earth luminescence     1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3  1. Introduction Silicate compounds are an important phosphor family, which have aroused great research interests [1-9]. Phosphors based on silicates have good thermal and chemical stability and are widely used in cathode-ray luminescence and photoluminescence. Silicates include normal silicate, partial silicate, pyrosilicate, and other different systems. By controlling the ratio of chemical reagents and experimental conditions, different silicate phosphors can be obtained, such as Ba2La8(SiO4)6O2: Eu2+ [2], MgGd4Si3O13: Ce3+, Mn2+ [3], Mg2Y2Al2Si2O12: Tb3+, Eu3+, Tm3+ [4], K3ScSi2O7: Eu2+ [5], etc. Silicates are an important choice for luminescence because of their significant absorption of ultraviolet (UV)/near-UV/blue light and low cost. At the same time, silicate-based phosphors have good physical properties, such as oxidation resistance, moisture resistance, and no interaction with the packaging resin [2,4,8,9]. The [SiO4]4- unit, as the basic building block of silicates, can form relatively complicated crystal structures, which usually contain various complex anionic groups by different joining methods, forming islands, rings, chains, or layered structures [5,6,9]. Among the silicates, A3RESi2O7 (A = Na, K; RE = Y, Gd, Lu), especially the K3RESi2O7, has aroused intense research interests and various experiments have been carried out for the system [5,6,10-19]. It is found that different emissions, such as near-infrared emission [5,6], white emission [10], and near-ultraviolet emission [11], can be realized by adjusting the RE site. However, the up-conversion (UC) luminescence of this K3RESi2O7 system has not been reported up to date to the best of our knowledge. One of the reasons is that the compound is typically synthesized by the traditional solid-phase method, which, although effective for phosphor selection and synthesis, would produce micron-sized  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4  large particles. It is difficult to obtain phosphors of nanometer size by this method, which greatly limits the application of UC phosphors of this system in the fields of biomarkers and anti-counterfeiting. Another reason is that the research interests of UC system are mainly on fluoride with low phonon energy, which would produce high UC efficiency [20-22]. However, fluoride is toxic, and its toxicity is related to its ability to dissociate fluorine ions. Recently, the UC phosphors activated by Yb3+-Er3+ pair are finding promising application as non-contact thermometers [7,20-23]. The intensity ratio of the 520 nm (2H11/2→4I15/2) and 550 nm (4S3/2→4I15/2) green luminescence Er3+ ion is very sensitive to temperature [24]. The temperature change will cause the obvious change in the ratio of these two branches, and the temperature can be determined by the branch ratio. Such application needs high thermal stability phosphors and doesn’t require the phosphor particles to be nanosized. In previous studies, common UC phosphors usually use halogen-containing compounds as substrate [20-22], and Yb3+/Er3+ co-doped NaYF4(NaYF4: Yb3+/Er3+) is the most common UC material with the highest luminous efficiency. However, halogen-containing compounds need irritating and frequently toxic chemicals in the process of their synthesis. Therefore, various researches have been conducted to explore novel systems using the solid-state method [7,25-27]. In view of the outstanding down-conversion photoluminescence and thermal stability of K3YSi2O7-based phosphors [10,13,14], we consider K3YSi2O7 to be a promising host for UC luminescence. Thus, the typical ion pair of Yb3+-Er3+ was doped into K3YSi2O7 in this work, and the resulted phosphor was shown to be capable of multi-mode temperature measurement with high sensitivity through UC luminescence.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5  2. Experimental section 2.1 Reagents and synthesis The rare earth sources for the synthesis were oxides (Y2O3, Yb2O3, Er2O3, 99.99% pure) bought from Huizhou Ruier Rare Chemical Hi-Tech Co. Ltd., Huizhou, China. The other reagents of K2CO3 (99.9% pure) and SiO2 (99.99% pure) were purchased from Aladdin Industrial Corporation (Shanghai, China) and were used without further purification. K3(Y0.88Yb0.10Er0.02)Si2O7 phosphors were synthesized by the conventional high-temperature solid-state method, where the contents of Yb and Er were taken as the optimal values of other UC systems [28,29]. The raw materials were weighed according to stoichiometric ratio, ground in agate mortar with alcohol for 20 minutes, mixed evenly, and then transferred to a corundum crucible. Finally, the mixture was heated at 1350 ℃ in a muffle furnace for 6 h in air, with a heating rate of 5 ℃/min before 800 ℃ and 3 ℃/min for 800-1350 ℃. 2.2 Characterization  The crystalline phases of the samples were identified by an X-ray diffractometer (XRD; Model Ultima IV, Rigaku, Tokyo, Japan) with Cu-Kα (λ = 0.15406 nm) radiation via the step-scan mode, with a step size of 0.01° and an accumulation time of 2 s in 2θ = 5-120o range. Temperature-dependent UC spectra (RT-548 K) were measured using an FLS 1000 fluorospectrophotometer (Edinburgh Instruments Ltd., Herrsching am Ammersee, Britain), using a 980 nm continuous wavelength laser (2 W, Model MDL-III-980-2W-18050833) for excitation and a TAP-02 accessory for temperature control. The fluorescence decay kinetics of the main UC emissions were measured with the lifetime testing unit of the FLS 1000 equipment.   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6  3. Results and discussion 3.1 Synthesis and visible-near-infrared luminescence of K3(Y0.88Yb0.10Er0.02)Si2O7  Figure 1 (a) Rietveld refinement for the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor, where the observed and calculated patterns, the difference, and the positions of Bragg reflections are in black, red, gray and green, respectively, (b) crystal structure of K3YSi2O7, together with the coordination environment of the cation. Figure 1(a) shows the Rietveld refinement results of the obtained K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. The refinement yielded stable results and acceptable reliability factors, which indicate that the product is of high purity. The detailed refinement results are shown in Table 1 and Table 2. According to Loye et al., K3YSi2O7 may have two polymorphs, namely K3YSi2O7(1) and K3YSi2O7(2), which belong to P63/mmc and P63/mcm space groups, respectively [10]. In this work, we obtained K3YSi2O7(2) with the second space group. Figure 1(b) are the crystal structure observed from the c-axis and the coordination polyhedra of cations for K3YSi2O7(2). The compound K3YSi2O7(2) (ICSD 430538) has a hexagonal structure with lattice constants a = 9.8450 Å, c = 14.3236 Å. c/a = 1.4549. It consists of SiO4, K1O8, K2O9, K3O6, Y1O6, and Y2O6 units connected by shared oxygen atoms. K3YSi2O7(2) has two distinct yttrium sites, one distinct silicon site, three distinct potassium sites, and three distinct oxygen sites [13]. The two yttrium sites are  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7  independent of each other and are located in a hexagonal coordination environment. Y(1) is located in a regular hexagonal octahedron, and Y(2) is located in a hexagonal trigonal prism. Y1O6 is connected to SiO4 by oxygen atoms at shared vertices, and SiO4 forms the Si2O7 pyrosilicate structure by bridging oxygen atoms at shared vertices. The coordination numbers of K(1), K(2), and K(3) are 8, 9, and 6 respectively. The K(1) site is aligned with Si along the c-axis, the K(2) site is aligned with Y(1) along the c-axis, and the K(3) site is aligned with Y(2) along the c-axis. In addition, it can be seen that the lattice parameters and cell volume of the Yb3+-Er3+ co-doped sample is smaller than those of the pure host as also shown in Table S1. This conforms well to the fact that Er3+ (R = 0.890, CN = 6) and Yb3+ (R = 0.868, CN = 6) have smaller ionic radii than Y3+ (R = 0.900, CN = 6) [30], and indicates successful incorporation of the dopant ions to form solid solution.  Table 1 Experimental parameters of powder XRD and refined crystallographic data for K3(Y0.88Yb0.10Er0.02)Si2O7. Chemical Formula K3(Y0.88Yb0.10Er0.02)Si2O7 Diffractometer X’Pert Pro, PANalytical Radiation type Cu Kα, λ = 1.54060 Å 2θ interval (o) 5.00-120.00 Step size of 2θ (o) 0.01 Space group P63/mcm (193) Z 6 a (Å) 9.8419(2) b (Å) 9.8419(2) c (Å) 14.3032(4) V (Å3) 1199.85(6) Number of structure parameters 27 Number of profile parameters 35 b Rp (%) 8.73 c Rwp (%) 12.21 d S 2.22   iiCP yyyR /,ib ;  2122,i /  iiiCiwpc ywYywR ; exp/ RRS wpd    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8  Table 2 Wyckoff positions, atomic coordinates, and occupancies of K3(Y0.88Yb0.10Er0.02)Si2O7. Atom Site x y z Occupancy Y1 4 0.3333 0.6667 0 0.89 Yb1 4 0.3333 0.6667 0 0.10 Er1 4 0.3333 0.6667 0 0.01 Y2 2 0 0 0.25 0.91 Yb2 2 0 0 0.25 0.08 Er2 2 0 0 0.25 0.01 K1 12 0.32798 0 0.09211 1 K2 4 0.3333 0.6667 0.25 1 K3 2 0 0 0 1 Si1 12 0.65938 0 0.14315 1 O1 24 0.67437 0.15042 0.09402 1 O2 12 0.81453 0 0.14696 1 O3 6 0.59540 0 0.25 1  Figure 2 (a) Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor in the visible light range (450-750 nm) at different excitation powers of 980 nm laser. (b) The proposed up-conversion mechanism, where ET means energy transfer. The UC spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 excited at different powers (0.60-1.20 W) are shown in Figure 2. The emission spectra consist of green emissions in the range of 510-575 nm and red emissions in the 630-700 nm region. The emission peaks at 531 nm and 555 nm belong to the transitions from 2H11/2 and 4S3/2 excited states to 4I15/2 ground state of Er3+, respectively, while the red emission near 670 nm is generated by the transition from 4F9/2 to 4I15/2. The number of photons (n) required to excite the ground state electron to the emission state can be estimated by the Iem∝Pn equation [29]. The logarithmic curves of Iem and Pn at different powers are shown in  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 9  Figure S1 for the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. The slope of the graph is the number of photons required (n). The slope of the curve (n value) in Figure S1 is about 3 for green emission and 2 for red emission, indicating that the observed UC luminescence is essentially caused by three (green) and two-photon (red) mechanism. The energy level diagram of Yb3+ and Er3+ in K3(Y0.88Yb0.10Er0.02)Si2O7 and the emission mechanism of UC are shown in Figure 2b. The three-photon process corresponding to green emission is as follows. Under the excitation of 980 nm laser, the electrons of Yb3+ are excited to the 2F5/2 level, from which energy transfer to the neighboring Er3+ would take place. The energy absorption of Er3+ ion excites electrons from the ground state 4I15/2 to the excited state 4I11/2 (ET1), followed by non-radiative relaxation (NR) to the 4I13/2 level. With the energy of the second excitation photon, the Er3+ electrons were raised to the 4F9/2 level (ET2). Subsequently, the Er3+ electrons were eventually excited to the 4G11/2 level by ET3 and then relax to 2H11/2 through non-radiative transition. Backing jumping of the 2H11/2 electrons to the ground state then produced the green light. The whole process can also be expressed as follows [31,32]: (1) 2F7/2 (Yb3+) + 980 nm photon → 2F5/2 (Yb3+) (2) 2F5/2 (Yb3+) + 4I15/2 (Er3+) → 2F7/2 (Yb3+) + 4I11/2 (Er3+) (ET1) (3) 4I11/2 (Er3+) → 4I13/2 (Er3+) (NR) (4) 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+) (ET2) (5) 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 4G11/2 (Er3+) (ET3) (6) 4G11/2 (Er3+) → 2H9/2 (Er3+) → 2H11/2 (NR)  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10  (7) 2H11/2 (Er3+) → 4I15/2 (Er3+) + photons (531 nm) For the emission peaks at 555 nm, the first six processes are the same as those of the 531 nm emission. The difference is that the electrons at the 2H11/2 energy level of Er3+ relax to the 4S3/2 by NR, and the occurrence of 4S3/2 (Er3+)→4I15/2 (Er3+) and transitions emit photons at 555 nm. For the red emission located at 662 nm, its two-photon process is that, after the first four transition steps, the electron at the 4F9/2 energy level transits to the ground state and emits a photon. It can be seen that for K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor, the increasing excitation power does not change the shape and position of the emission peak, but the intensity of the emission gradually increased.  Figure 3 (a) Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor in the near-infrared range (1000-1600 nm) at different excitation powers of 980 nm laser. (b) Fluorescence decay kinetics of the 1542 nm emission at different excitation powers (0.25-0.50 W), where the numbers following the power are the average lifetimes. The emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 in the near-infrared region at different powers are shown in Figure 3a. The strongest peak is at 1542 nm, which corresponds to the 4I13/2→4I15/2 transition of Er3+ [33]. It can be seen that the sample has a strong broadband emission in the near-infrared region. The Stark splitting of the NIR emission is evident because of the splitting of the 4f level of Er3+ ions due to the  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 11  action of the crystal field. Figure 3b shows the decay kinetics of the 1542 nm emission, where it was found that all the curves are well fitted using the double-order exponential equation [34,35]: 𝐼(𝑡) = 𝐴1𝑒𝑥𝑝(−𝑡/𝜏1) + 𝐴2𝑒𝑥𝑝(−𝑡/𝜏2)     (1) where A1 and A2 are constants, t is decay time, I(t) is luminescence intensity at time t, τ1, and τ2 are the fast and slow components of lifetime respectively. The average lifetime (τ) can be obtained by the following equation [34,35]: τ = (𝐴1𝜏12 + 𝐴2𝜏22)/(𝐴1𝜏1 + 𝐴2𝜏2)     (2) The derived τ and A values and the chi-square factor (χ2) of fittings are summarized in Table S2. The 1542 nm emissions at different powers show similar average lifetime of around 14 ms. 3.2 Multi-mode thermometric properties of K3(Y0.88Yb0.10Er0.02)Si2O7  Figure 4 (a) Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor excited by 980 nm laser (0.5 W) at different temperatures (RT-548 K). (b) Relative emission intensity for different transitions as a function of the measured temperature.  In practical applications, especially in the field of temperature measurement, the relationship between luminescence intensity and temperature is of great importance. Therefore, we tested the PL spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 in the temperature range of 298-548 K, as shown in Figure 4a. As can be seen from Figure 4a, the  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 12  luminescence intensity of the sample decreases gradually with increasing temperature and the specific CIE coordinates are given in Figure S2 and Table S3. As can be seen from Figure S2, the color coordinates of K3(Y0.88Yb0.10Er0.02)Si2O7 did not shift much with increasing temperatures. Figure 4b shows the relationship between normalized emission intensity and temperature. The results show that the green emission intensity of 519/527 nm (2H11/2→4I15/2) increases with increasing temperature, reaching the maximum value at 373 K and maintaining 97.2% and 98.7% of the maximum intensity at 423 K, respectively. At the highest measurement temperature of 548 K, 78.9% and 83.0% of the maximum emission intensity can still be retained, respectively. For 555 nm (4S3/2→4I15/2) emission, the intensity decreases obviously with increasing temperature. The intensity loss rates of 2H11/2→4I15/2 and 4S3/2→4I15/2 are different with increasing temperature. This indicates that the prepared phosphor can sense temperature through the fluorescence intensity ratio (FIR) of the thermally coupled 2H11/2 and 4S3/2 levels and has potential application in the field of optical thermometry. The 4F9/2→4I15/2 (662 nm) emission also decreased monotonically with increasing temperature. Since 2H11/2 and 4S3/2 are thermally coupled energy levels (ΔE = 736 cm-1), their population follows Boltzmann distribution. The FIR of the two different green emissions can be expressed as [36,37]: FIR =𝐼𝐻𝐼𝑆= 𝐴 ∙ exp (−∆𝐸𝑘𝐵𝑇) + 𝐶     (3) where IH and IS represent the emission intensity of the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions; ΔE is the energy level gap between corresponding energy levels; kB is Boltzmann constant; T is the absolute temperature; A and C are constants.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 13  Equation (3) is used to fit the FIR relation with temperature of the two thermal coupled energy levels. The fitting curve is shown in Figure 5a, and the points in the figure are experimental data. It can be seen from the figure that as the temperature rises from 298 K to 548 K, The I531/I555 ratios of K3(Y0.88Yb0.10Er0.02)Si2O7 increase gradually. According to the Boltzmann distribution law, when the temperature increases, electrons are more easily pumped to the higher 2H11/2 energy level to reach equilibrium. In addition, the probability of the non-radiative transition from 4S3/2 to 4I15/2 increases, so the emission at 555 nm is significantly weakened, thus increasing the ratios of I531/I555.    1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 14   Figure 5 Different FIRs and their corresponding SA and SR values as a function of the measurement temperature for the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor.   In addition, sensitivity S is an important index to evaluate the sensitivity of phosphors to temperature change, which can be expressed by absolute sensitivity SA and relative sensitivity SR. The calculation equations are as follows [37]: 𝑆𝐴 = |𝑑(𝐹𝐼𝑅)𝑑(𝑇)| = (𝐹𝐼𝑅 − 𝐶) ×𝛥𝐸𝑘𝐵𝑇2     (4) 𝑆𝑅 = |𝑑(𝐹𝐼𝑅)𝑑(𝑇)1𝐹𝐼𝑅| =𝐹𝐼𝑅−𝐶𝐹𝐼𝑅×𝛥𝐸𝑘𝐵𝑇2      (5) Equation (4) and Equation (5) were used to calculate SA and SR, respectively. As shown in Figure 5b, the maximum values of SA and SR corresponding to I531/I555 are 156×10-4 K-1(298 K) and 1.22% K-1(298 K), respectively; while the maximum values of SA and SR corresponding to I519/I555 are 58×10-4 K-1(548 K) and 1.80% K-1(298 K), respectively.  The energy difference (ΔE) of thermally coupled levels is generally between 200 ~ 2000 cm-1. Compared with the traditional technology based on the FIR of thermally  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 15  coupled energy levels, the temperature measurement based on non-thermally coupled energy levels (NTCL) is not limited by the energy difference, and the number of energy level pairs is rich, which is beneficial to improve the accuracy of temperature measurement. We calculated the temperature sensitivity of the K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. The temperature-dependent FIR of non-thermally coupled energy levels can be fitted by a polynomial function [38]: 𝐹𝐼𝑅 =𝐼555𝐼662= 𝐴 + 𝐵𝑇 + 𝐶𝑇2 + 𝐷𝑇3     (6) where A, B, C, and D are constants. As shown in Figure 6a, the variation of the fluorescence intensity ratio FIR(I555/I662) of the non-thermally coupled energy levels with temperature can be well fitted by Equation (6), with R2 = 0.99. In this case, the sensitivity of temperature sensing can be calculated with the following equation [38]: 𝑆 =𝑑(𝐹𝐼𝑅)𝑑𝑇= 𝐵 + 2𝐶𝑇 + 3𝐷𝑇2     (7) The curve of sensitivity as a function of temperature is shown in Figure 6b, where the sensitivity reached a maximum value of 157×10-4 K-1 at 298 K.   Figure 6 (a) The FIR of the 555 nm and 662 nm emission peaks of K3(Y0.88Yb0.10Er0.02)Si2O7 as a function of temperature and (b) sensitivity as a function of temperature.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 16   Figure 7 Emission spectra of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor excited with different powers 980 nm laser at (a) 398 K, (b) 473 K, and (c) 548 K. (d) The variation trend of I555/I662 with excitation power at different temperatures. Although quite a number of references used Equation (6) for the FIR fitting of non-thermally coupled energy levels, the excitation power was not involved in the equation. Such fitting cannot reflect the influence of the excitation power on FIRs. In order to ensure the accuracy of the results, the power dependent emission spectra at different temperatures (398 K, 473 K, 548 K) were measured and the I555/I662 FIR at different excitation powers was evaluated. As seen from the results shown in Figure 7, I555/I662 FIR is stable under varying excitation power at different temperatures and, therefore, the influence of excitation power during the temperature-sensing calculation of the non-thermally coupled energy level can be excluded. The influence of excitation power on the other two FIRs were also checked and were shown in Figure S4. It was found that excitation power has a small influence on the FIRs  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 17  values. Table 3 summarizes the SA and SR values of several typical phosphors with optical temperature sensing behavior. Through comparison, it was found that the maximum value of SA and SR obtained in this work are higher than these of most systems. Among them, the sensitivity values calculated based on non-thermally coupled energy levels are higher than those obtained by the thermally coupled energy levels. The results show that K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor has potential application value in optical temperature measurement. Table 3. A summary of SA and SR values, and temperature sensing ranges for some typical temperature sensing phosphors. Ion pair Mode Host Range (K) SA(K-1)×10-4 SR(% K-1) Ref Yb3+-Er3+ FIR KY3F10(Core-Only) 310-366 - 1.51 (310 K) [20] Yb3+-Er3+ FIR KY3F10(Core-Shell) 300-365 - 1.24 (316 K) [20] Yb3+/Er3+/Mn2+ FIR NaBiF4 298-473 55.9 (473 K) 1.22 (298 K) [21] Yb3+,Er3+ FIR NaScF4 298-573 25.6 (548 K) 0.31 [22] Yb3+,Er3+ FIR NaScF4(EDTA)  298-573 32.8 (548 K) 0.41 [22] Yb3+,Er3+ FIR Na2GdMg2(VO4)3 303-573 74.9 (479 K) 0.98 (303 K) [23] Er3+ FIR GdTaO4 293-723 41 (475 K) 1.12 (298 K) [24] Yb3+,Er3+ FIR Ba5Y8Zn4O21 293-563 0.39 (563 K) 136 (293 K) [25] Er3+ FIR LaGdO3 298-873 43 (554 K) 1.20 (298 K) [26] Bi3+/Tb3+ FIR LaNdO4 303-483 410 2.36 [27] Bi3+/Eu3+ FIR LaNdO4 303-483 440 1.89 [27] Bi3+/Dy3+ FIR LaNdO4 303-483 80 1.26 [27] Bi3+/Sm3+ FIR LaNdO4 303-483 310 1.36 [27] Yb3+/Er3+ FIR Y2O3 323-573 196 (543 K) - [39] Pr3+ FL La2MgTiO6 298-548 2.85 1.81 (473 K) [40] Sm2+ FL SrBa4O7 298-723 - 3.36 (500 K) [41] Eu2+ FL Li4ScCa(SiO4)2 303-573 1470 15.0 [42] Dy3+/Mn4+ FL Li2TiO3/Y2O3 273-373 220 (308 K) 6.67 (339 K) [43] Yb3+,Er3+ FIR(I531/I555) K3YSi2O7 298-548 156 (298 K) 1.22 (298 K) This work Yb3+,Er3+ FIR(I527/I555) K3YSi2O7 298-548 59 (548 K) 1.48 (298 K) This work Yb3+,Er3+ FIR(I519/I555) K3YSi2O7 298-548 58 (548 K) 1.80 (298 K) This work Yb3+,Er3+ FIR(I524/I555) K3YSi2O7 298-548 139  1.34 (298 K) This work Yb3+,Er3+ FIRNTCL(I555/I662) K3YSi2O7 298-548 157 (298 K)  - This work Yb3+,Er3+ FL(531 nm) K3YSi2O7 323-548 661 (548 K) 9.77 (298 K) This work   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 18   Figure 8 (a) Fluorescence decay kinetics of 531 nm emission at different temperatures (RT-548 K), and (b) the relationship of fluorescence lifetimes for 531 nm emission with measurement temperature (RT-548 K). The intensity in (a) is in logarithmic form. Figure 8a shows the fluorescence decay curves for the 531 nm emission of K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor under 980 nm laser excitation at different temperatures. As shown in the figure, all the curves are well fitted using the double-order exponential Equation (1). Then, the average lifetime (τ) can be obtained by the following Equation (2). The derived lifetime of the 531 nm emission is shown in Figure 8b, where it is seen that the lifetime monotonously decreased from 80.31 μs (323 K) to 67.71 μs (548 K). The 531 nm emission at other temperatures showed a similar decay behavior and the derived τ and A values and the chi-square factor (χ2) of fittings are summarized in Table S4. The χ2 values for all the fittings are around 1, indicating the high quality of the fitting. It is noteworthy that the fluorescence lifetime of K3(Y0.88Yb0.10Er0.02)Si2O7 decreases with increasing temperature, which can be attributed to increased non-radiative transitions in the matrix lattice. This trend is the same as that observed from the temperature-dependent emission intensity. As shown in Figure 8b, the lifetime of the 531 nm emission decreases exponentially with increasing temperature T, and can be expressed as τ531 = -17.18exp (-624.78/T) + 190.91.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19   Figure 9 The relative sensitivity SR and absolute sensitivity SA based on the fluorescence lifetime of 531 nm emission for K3(Y0.88Yb0.10Er0.02)Si2O7 phosphor. Aside from those utilizing the FIR technology, a temperature sensing scheme based on fluorescence lifetime (FL) can also be realized with the thermal quenching characteristics of K3(Y0.88Yb0.10Er0.02)Si2O7. Similar to the scheme based on FIR, the absolute sensitivity SA and relative sensitivity SR of FL based temperature sensing can be calculated by the following Equation [40,42-44]: 𝑆𝐴 = |𝑑𝜏𝑑𝑇|     (8) 𝑆𝑅 =1𝜏|𝑑𝜏𝑑𝑇|     (9) As shown in Figure 9, the phosphor has maximum values were 661×10-4 K-1(548 K) and 9.77% K-1(548 K) for SA and SR, respectively. It can be seen from the figure that SA and SR both increase with increasing temperature. By comparing the SR value based on the FL mode and the SR value based on the FIR mode, it is obvious that the former is greater than the latter. Nevertheless, as shown in Table 3, the SR values of both the FIR and FL modes obtained in this work are better than those of most previous studies.   1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 20  Conclusions A novel K3(Y0.88Yb0.10Er0.02)Si2O7 silicate phosphor was prepared by solid reaction. The UC luminescence properties, UC mechanism as well as the performance of optical temperature sensing via multi-modes were systematically investigated. The main conclusions are as follows: (1) The K3(Y0.88Yb0.10Er0.02)Si2O7 new phosphor exhibits strong green luminescence through 2H11/2→4I15/2 transition (531 nm) under 980 nm excitation, which occurs through a three photon processes.  (2) K3(Y0.88Yb0.10Er0.02)Si2O7 is well capable of temperature sensing via fluorescence intensity ratio (FIR) with thermally coupled/non-thermally coupled energy levels (TCL/NTCL) and fluorescence lifetime (FL). The maximum SA and SR values are SA = 156×10-4 K-1 (I531/I555, 298 K) and SR = 1.80% K-1 (I519/I555, 298 K) for the FIR mode, and are SA = 661×10-4 K-1 (548 K) and SR = 9.77% K-1 (298 K) for the FL mode.  Acknowledgements This work is supported by Natural Science Foundation of Liaoning Province (Grant No. 2020-MS-286). The authors would like to thank Siqi Liu from Shiyanjia Lab (www.shiyanjia.com) for the XRD measurement. References [1] X. Ji, J. Zhang, Y. Li, S. Liao, X. Zhang, Z. Yang, Z. Wang, Z. Qiu, W. Zhou, L. Yu, S. Lian, Improving quantum efficiency and thermal stability in blue-emitting Ba2-xSrxSiO4: Ce3+ phosphor via solid solution, Chem. 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Alloys Compd. 881 (2021) 160601. https://doi.org/10.1016/j.jallcom.2021.160601.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65   e-componentClick here to access/downloade-componentSupporting information  (Clean Version).docxhttps://www.editorialmanager.com/ceri/download.aspx?id=2989947&guid=b621a998-045f-496e-847c-ae0cdc7e3a00&scheme=1  e-componentClick here to access/downloade-componentSupporting information with Changes Marked.docxhttps://www.editorialmanager.com/ceri/download.aspx?id=2989948&guid=db1cd7a9-1313-4a9d-a3e3-f2db0b7c79f5&scheme=1  cif fileClick here to access/downloade-componentK3YSi2O7 Yb Er_str.cifhttps://www.editorialmanager.com/ceri/download.aspx?id=2989937&guid=bf13dd62-9edf-4cbf-8001-16e061e39b1d&scheme=1Declaration of interests  ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.  ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:        Declaration of Interest Statement