# Fileset [JORE.pdf](https://mdr.nims.go.jp/filesets/6f2d2a03-6d9c-4003-9fce-edfe112e26a2/download) ## Creator Fan Li, Zhenqi Song, Zhiyuan Pan, Sihan Feng, Qi Zhu, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361) ## Rights [Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/) ## Other metadata [Crystallization of RE2(OH)2CO3SO4.nH2O as a new family of layered hydroxides (RE = Gd−Lu lanthanides and Y), derivation of RE2O2SO4, photoluminescence and optical thermometry](https://mdr.nims.go.jp/datasets/f3053af1-af7e-49bd-8210-b5f209138112) ## Fulltext 1 Crystallization of RE2(OH)2CO3SO4nH2O as a new family of layered hydroxides (RE = Gd−Lu lanthanides and Y), derivation of RE2O2SO4, photoluminescence and optical thermometry Fan Lia, Zhenqi Songa, Zhiyuan Pana, Sihan Fenga, Qi Zhua, Ji-Guang Lib,∗ a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning, 110819, China b Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, Japan *Corresponding author Dr. Ji-Guang Li National Institute for Materials Science Tel: +81-29-860-4394 E-mail: li.jiguang@nims.go.jp mailto:li.jiguang@nims.go.jp2 Abstract Layered rare-earth hydroxides (LREHs) draw wide research interest because of their peculiar crystal structure, rich interlayer chemistry and abundant functionality of the RE element, but are limited to the two categories of RE2(OH)5A·nH2O (A: typical of Cl- or NO3-) and RE2(OH)4SO4·nH2O. On the other hand, RE2O2SO4 attracted attention for large-capacity oxygen storage, low-temperature magnetism and luminescence, whose preparation mostly involves toxic SOx gases and/or complicated procedures. This study produced RE2(OH)2CO3SO4nH2O as a new family of LREHs (RE = Gd-Lu lanthanides and Y) via hydrothermal reaction, from which phase-pure RE2O2SO4 was derived via subsequent annealing at 800 °C in air without the involvement of SOx. The compounds were thoroughly characterized to reveal the intrinsic influence of lanthanide contraction (RE3+ radius) on crystal structure, thermal behavior (dehydroxylation/decarbonation/desulfurization), vibrational property and crystallite morphology. Analyzing the photoluminescence of Eu3+ and Sm3+ in the Gd2O2SO4 typical host found that the 617 nm (Eu3+, λex = 275 nm) and 608 nm (Sm3+, λex = 407 nm) main emissions can retain as high as ~79.6 and 85.5% of their room-temperature intensities at 423 K, with activation energies of ~0.19 and 0.21 eV for thermal quenching, respectively. Application also indicated that both the phosphors have the potential for optical temperature sensing via the fluorescence intensity ratio (FIR) technology, whose maximum relative sensitivity reached ~2.70% K−1 for Eu3+ and 1.73% K−1 for Sm3+ at 298 K. Keywords: Layered hydroxide; RE2O2SO4; Luminescence; Optical temperature sensing 3 1. Introduction Rare-earth oxysulfates (RE2O2SO4) constitute an important family of inorganic compounds since they may show unusual low temperature magnetic properties (RE = Gd, Tb, Dy, Ho, Er, Tm),1-3 serve as the catalyst for automotive emission-control with their large capacity of oxygen storage (RE = La, Pr, Nd, Sm),4,5 and exhibit interesting luminescent properties once properly activated (RE = La, Gd, Y).6-8 RE2O2SO4 was initially proposed to be orthorhombic in structure,9 but later investigation through neutron, electron and X-ray diffraction found that it actually belongs to the monoclinic system (Space group C2/c).10,11 The family of compounds all have a layered crystal structure formed by alternative stacking of the [RE2O2]2+ main layers and inter-layer [SO4]2− along the a-axis, where two opposite O atoms of each [SO4] tetrahedron are coordinated with the RE in two adjacent [RE2O2]2+ layers.10,11 Thermal decomposition of RE2(SO4)3nH2O is the most common method to prepare RE2O2SO4,4,12 but is accompanied by the release of toxic SOx gas and has difficulty in particle morphology control. To avoid these problems, the precursor conversion method has been adopted to synthesize RE2O2SO4 of smaller particle sizes. For example, Machida et al.5,13,14 prepared RE2O2SO4 (RE = Pr, Y) powders of ~100 nm by calcining a dodecyl sulfate precursor in air at 600 °C, and Zhong et al.15 obtained RE2O2SO4 powders of ~1 μm (RE = La, Pr-Lu) via thermal decomposition at 700 °C in air of a polymer precursor precipitated from the RE(NO3)3-NaOH-C4H6O4S2 (dimercaptosuccinic acid) reaction system. Both the methods, however, appear to be difficult in phase-purity control of the final product. Layered rare-earth hydroxides (LREHs) draw a wide range of research interest during recent years because of their peculiar crystal structure, rich interlayer chemistry and abundant functionality of the RE 4 element, but are limited to the two categories of RE2(OH)5A·nH2O (A: typical of Cl- or NO3-) and RE2(OH)4SO4·nH2O (SO42−-LREH).16-20 Especially, the appearance of SO42−-LREH (RE = La-Lu lanthanide or Y; n = 0 or 2) provided a unique opportunity for the green synthesis of RE2O2SO4, since the compound would directly dehydrate to RE2O2SO4 upon being properly heated (RE2(OH)4SO4·nH2O → RE2O2SO4 + (n + 2)H2O).18,19 The hydrated form of SO42−-LREH (RE2(OH)4SO4·2H2O) can be synthesized for the relatively larger RE3+ (RE = La-Dy) via fluxing a mixed solution of RE sulfate, sodium sulfate (Na2SO4, mineralizer) and hexamethylenetetramine ((CH2)6N4, precipitant) or via hydrothermal reaction of a mixed solution of RE nitrate and ammonium sulfate ((NH4)2SO4) under ~100 °C and pH ~7-9.17,19,20 The anhydrous form of SO42−-LREH (RE2(OH)4SO4) is obtainable for the relatively smaller RE3+ (RE = Eu-Lu and Y) via hydrothermal reaction under a strictly controlled temperature and solution pH, for example 150 °C and pH = 10 for RE = Eu and Gd, 150 °C and pH = 8-9 for RE = Tb and Dy, 150 °C and pH = 7 for RE=Ho, Er and Y, 180 °C and pH = 7 for RE = Er, Tm and Y, and 200 °C and pH = 7 for RE=Lu,18 where the tending to be lower pH with decreasing radius of RE3+ was believed to be a reflection of lanthanide contraction. It is noteworthy that a certain amount of CO32−, arising from atmospheric CO2 or the hydrolysis of (CH2)6N4 ((CH2)6N4 + H2O → CH2O + NH3, CH2O + H2O → CO2 + H2)21 is readily incorporated into the RE2(OH)4SO4·nH2O formula, and the CO32− was proposed to replace hydroxyls (OH−) owing to its high coordinating capability.17,22,23 However, it remains unclear that to what extent can the OH− be replaced and how would CO32− substitution affect crystal structure. To answer these questions, we autoclaved an aqueous solution of Gd(NO)3, (NH4)2SO4 and urea (CO(NH2)2) at 140 °C for 12 h, and obtained a precipitate that was 5 analyzed by a number of techniques to be Gd2(OH)2CO3SO4⋅nH2O (n~1.5).24 Structure analysis by applying multiple algorithms (ITO, DICVOL and TREOR) and Pawley refinement further revealed that the compound was crystallized in the tetragonal system (space group: P-421m) and is layer structured along the [001] crystallographic direction.24 It was also found that calcining the compound in air at 800 °C may yield Gd2O2SO4 via dehydration, dehydroxylation and decarbonation.24 It still remains unclear, however, whether such a formula exists for the other RE elements and, if yes, how would lanthanide contraction (RE3+ size) affect the crystal structure and physicochemical properties of such a compound. We thus performed hydrothermal synthesis for the full range of RE elements (RE = La-Lu lanthanides and Y) via reacting RE(NO)3, (NH4)2SO4 and Na2CO3 in this work, where Na2CO3 was employed as the CO32− source instead of urea for an easier control of solution pH and CO32− concentration, and such an initiation successfully produced RE2(OH)2CO3SO4⋅nH2O (REOCSH) for RE = Gd-Lu and Y. The compounds were characterized in detail by a number of techniques to manifest the intrinsic influence of lanthanide contraction on crystal structure, thermal behavior, vibrational property and crystallite morphology. Calcining the REOCSH in air at 800 °C also produced the corresponding RE2O2SO4 compound in each case, and the dependence of lattice parameters on RE3+ size was revealed. Through analysis of temperature-dependent photoluminescence, Eu3+ and Sm3+ doped Gd2O2SO4 phosphors were demonstrated to possess satisfactory thermal stability and have the potential for optical thermometry via the fluorescence intensity ratio (FIR) technology. 2. Experimental Section 6 2.1. Reagents and hydrothermal synthesis 99.99% pure Ce(NO3)36H2O, Pr6O11, Tb4O7 and RE2O3 were purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China), and analytical grade (NH4)2SO4, Na2CO3, HNO3 and NH4OH were purchased from Sinopharm Co., Ltd (Shanghai, China). The rare-earth oxides listed above were separately dissolved with a proper amount of HNO3 solution. Ultra-pure water (resistivity  18 Mcm) was used throughout the experiments. In a typical synthesis of RE2(OH)2CO3SO4⋅nH2O (REOCSH), 1.5 mmol of (NH4)2SO4 and 1.5 mmol of Na2CO3 were dissolved in 40 mL of water, to which 3 mmol of RE(NO3)3 was added under magnetic stirring, followed by pH adjustment with HNO3 and NH4OH while keeping the total volume at 60 mL. After 30 min of continuous stirring for homogenization, the mixture was transferred into a Teflon lined stainless steel autoclave of 100 mL capacity, followed by 24 h of reaction in an electric oven preheated at 140 °C. After natural cooling to room temperature, the precipitate was collected via centrifugation, washed with water and ethanol successively, and then dried at 60 °C for 12 h. With RE = Gd for example, a series of experiments were carried out to investigate the influence of solution pH. The Eu3+ and Sm3+ doped samples of (Gd0.95Eu0.05)2(OH)2CO3SO4nH2O and (Gd1-xSmx)2(OH)2CO3SO4nH2O (x = 0.005-0.03) were also synthesized according to the above procedure, where the content of Eu3+ was taken as the optimal value of 5 at.% according to a previous report on La2O2SO4:Eu3+.25 Since the luminescence property of Gd2O2SO4:Sm3+ has not been reported before to the best of our knowledge, the Sm3+ content was varied to determine the optimal value. RE2O2SO4 was produced by calcining the corresponding REOCSH in stagnant air at 800 °C for 1 h, using a heating rate of 8 °C/min for the ramp stage of heating. 7 2.2. Characterization Phase identification was performed via X-ray diffractometry (XRD, SmartLab, Rigaku, Tokyo, Japan) under 40 kV/200 mA, using nickel-filtered Cu-Kα radiation (λ= 0.15406 nm) and a scanning speed of 4° 2 /min. Product morphology and structure were analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo) under an acceleration voltage of 15 kV, transmission electron microscopy (TEM, Model JEM-2000FX, JEOL) under 200 kV, and atomic force microscopy (AFM, Model Nanosurf easyScan-2, Liestal, Switzerland) in the tapping mode with a scanning resolution of 256 points/line and a scanning rate of 1 s/line. Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Fisher Scientific, Waltham, USA) was conducted using the standard KBr pellet method. The elemental contents of the typical hydrothermal products were determined for RE via inductively coupled plasma-optical emission spectrometry (ICP-OES; Model iCAP 7400, Thermo Fisher Scientific) and for C and S via the inert gas fusion-infrared absorption/thermal conductivity technique (Elementar varioEL cube, Langenselbold, Germany). Thermogravimetry/differential thermal analysis (TG/DTA, Model SETSYS Evolution-16, Setaram, France) was performed at a constant heating rate of 10 °C/min in flowing simulated air (50 mL/min). Photoluminescence and fluorescence decay were analyzed with a Model FP-8600 fluorospectrophotometer (JASCO, Tokyo), using a 150 W xenon lamp for excitation, a scan speed of 100 nm/min, a slit width of 5 nm, and an HPC-836 accessory (JASCO) for temperature control. 3. Results and discussion 3.1 Characterization of the hydrothermal products 8 The Gd2(OH)2CO3SO4nH2O (GdOCSH) obtained from the Gd(NO)3-(NH4)2SO4-urea hydrothermal system in our previous work24 is the only known RE2(OH)2CO3SO4nH2O (REOCSH) compound. The GdOCSH can be viewed as a derivative of Gd2(OH)4SO4nH2O (SO42−-LREH) since it can be yielded by replacing two out of four hydroxyls of the latter with CO32−. The GdOCSH compound was analyzed to belong to the tetragonal system (space group: P-421m) with a crystal structure layered along the c-axis,24 whose XRD pattern is shown in Fig. S1. Though the exact coordination of Gd yet needs to clarify, it was tentatively proposed by considering the layered structure of SO42−-LREH18,20 that the main layers (hydroxide layers) of GdOCSH would be composed of Gd3+, OH− and H2O while the SO42− and CO32− anions are sandwiched between two adjacent hydroxide layers for charge balance. As aforesaid, urea (CO(NH2)2) does not allow a facile control of solution pH, though pH value may profoundly affect chemical potential and cation hydrolysis.26-28 The influence of solution pH on hydrothermal product was thus examined in this work for the Gd(NO)3-(NH4)2SO4-Na2CO3 reaction system. It is seen from Fig. S2 that the pH = 6 and 7 products conform to GdOCSH,24 though the pH = 6 one was much better crystallized, while the pH = 8 and 10 samples are essentially amorphous. Calcining the pH =10 product at 800 °C in air produced a phase mixture of cubic Gd2O3 (JCPDS No. 12-0797) and monoclinic Gd2O2SO4 (JCPDS No. 24-9775),8,18 as shown in Fig. S3a, indicating that the precipitate has an off-stoichiometric amount of SO42− when compared with GdOCSH. In the current reaction system, Gd3+ would undergo hydration and hydrolysis to form [Gd(OH)a(CO3)b(SO4)c(H2O)d]3-a-2b-2c complex ion,26,27 in which SO42− is less coordinating than CO32−.27 For this, a higher solution pH would increase the OH−/Gd3+ and meanwhile 9 decrease the SO42−/Gd3+ molar ratio of the complex ion to yield a precipitate that has less SO42−. As the above results imply that an alkaline environment is not conducive to GdOCSH formation, the synthesis of REOCSH was thus attempted under pH = 6 for the other RE elements. XRD analysis indicated that the La and Ce products are a mixture of hydroxyl carbonate (RE(OH)CO3) and an unknown phase in each case, the Pr and Nd products are a mixture of RE(OH)CO3 and NaRE(SO4)22H2O, the Sm and Eu products are amorphous (Fig. S4), and REOCSH is only obtainable for RE = Gd-Lu and Y (Fig. 1a). Noteworthy is that the Sm and Eu products yielded a mixture of RE2O3 and RE2O2SO4 by calcination at 800 °C (Fig. S3b,c), indicating that they contain less SO42− than the REOCSH formula. For composition verification of the claimed REOCSH, chemical analysis was performed for the four representative samples of RE = Gd, Ho, Lu and Y. From the results tabulated in Table S1 and by considering molecular neutrality, it can be concluded that the samples would indeed have the general formula of RE2(OH)2CO3SO4nH2O, since in each case the RE:C:S atomic ratio is very close to 2:1:1. It should be noted that, through affecting RE3+ hydrolysis and the composition of [RE(OH)a(CO3)b(SO4)c(H2O)d]3-a-2b-2c complex ion, reaction temperature and solution pH jointly affect REOCSH formation, as found by previous studies on the hydrothermal crystallization of SO42−-LREH compounds.18,19 Our preliminary experiments also showed that hydrothermal temperatures below 140 °C and above 180 °C would produce low-crystallinity REOCSH/amorphous mass and a mixture of REOCSH and impurity phase, respectively, and 140 °C and pH = 6 are the optimal parameters to obtain REOCSH of satisfactory crystallinity for an as wide as possible range of RE elements. 10 Fig. 1. XRD patterns (a) and correlation of lattice constants (a = b, c) and cell volume (V) with the ionic radius of RE3+ (b) for RE2(OH)2CO3SO4nH2O. Analyzing the XRD patterns of the series of REOCSH (Fig. 1a) with the Jade 6 software revealed that the cell parameters (a = b, c) and cell volume (V) almost linearly decrease with decreasing RE3+ size (Fig. 1b, coordination number CN = 9), conforming to lanthanide contraction. Nevertheless, the a parameter decreases much faster than c does, as perceived from the slopes of the corresponding plots (~4.27 for a and 1.77 for c). Such a phenomenon is mainly attributed to the layered crystal structure of REOCSH, and can be explained by considering that (1) the RE3+ ions are accommodated in the hydroxide main layers (ab planes), which makes the a parameter (a = b) more susceptible to the size variation of RE3+, and (2) the rigid pillaring of interlayer SO42−/CO32− makes the crystal structure less deformable along the c-axis, as in the case of SO42−-LREH.18 The tendency of a and c variations conform to the observation that the diffraction peaks arising from ab planes, such as (110) and (020), were substantially shifted to larger angles with decreasing RE3+ size while the (001) diffraction (perpendicular to the c-axis) was not appreciably affected (Fig. 1a). The above results also imply that the tetragonal unit cell of REOCSH would be non-uniformly deformed along with decreasing radius of RE3+. 5 10 15 20 25 30 35 40 45 502Theta (deg.)Gd001011111311110020002112012 221301031032 222113411350 cps610 cpsTbDyYIntensity (a.u.)HoErTmYb Lu(a)1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.1110.510.610.710.810.98.568.608.648.688.7294596598510051025 a (Å)Ionic size (Å)LuYbTmErHoYDyTbGd c (Å)Lu YbTmErHoYDyTb Gd(b) V (Å3 )Lattice constant aLattice constant cCell volume VLuYbTmErHoYDyTbGd11 FTIR analysis was performed for the series of REOCSH products to examine the contained chemical species, and the results are shown in Fig. 2. It is seen that, as found previously,24 the Gd product exhibited vibrations at ~3553 cm−1 for the stretching of OH− (ν1) and at ~3420/1645 cm−1 for the stretching (ν1)/bending (ν2) of H2O.29,30 CO32− absorptions were found at ~842 cm−1 (ν2, weak) and in the regions of ~1390-1590 cm−1 (ν3, strong) and 670-808 cm−1 (ν4, medium strong), while the characteristic vibrations of SO42− are well identifiable at ~1010 cm−1 (ν1, weak) and in the regions of ~1025-1270 cm−1 (ν3 strong) and 583-645 cm−1 (ν4, medium strong).24,29 No other type of anion is detectable. The other samples have spectral profiles very similar to that of GdOCSH, but their CO32− and SO42− vibrations were gradually shifted to larger wavenumbers with decreasing RE3+ size, indicating a stronger interaction of both the types of ligands with the RE3+ center. Such a phenomenon might be understood by considering that the positively charged hydroxide main layers will have a higher charge density with decreasing a/b lattice parameters (Fig. 1b), which attract the negatively charged CO32− and SO42− anions for a stronger bonding. Fig. 2. FTIR spectra of the REOCSH products. 3900 3600 3300 3000 1500 1200 900 600TmLuYbYErHoDyTb Transmittance (%)Wavenumber (cm-1)Gd355334201590-13901270-1025583-645842808-67010101645 12 FE-SEM and TEM analysis found that the REOCSH compounds were all crystallized as nanoplates with edge lengths of up to ~400 nm, as shown in Fig. 3a-c for the typical samples of RE = Gd, Y and Lu and in Fig. S5 for the rest. It was noticed that the crystallites tend to be larger and less elongated as the RE3+ size decreases, which indicates a faster and more balanced development of the lateral dimensions (length and width). AFM analysis revealed that the thickness of the nanoplates tends to decrease with decreasing RE3+ size, which is ~20.9, 14.2 and 8.3 nm for RE = Gd, Y and Lu, respectively (Fig. 3d-f). The tending to be larger lateral dimension at a smaller RE3+ implies a lower density of REOCSH nucleation, which might be understood by considering that the stronger hydrolysis of RE3+ makes CO32− and particularly SO42− more difficult to reside in the coordination sphere of RE for the aforesaid [RE(OH)a(CO3)b(SO4)c(H2O)d]3-a-2b-2c complex ion. For the same reason, the thickness development of REOCSH crystallites, which occurs via repetitive stacking of the hydroxide main layers and interlayer CO32−/SO42− along the [001] direction, was restricted to produce thinner nanoplates. Besides, the much smaller thickness than lateral dimension and the tending to be smaller thickness may explain why the 00l diffractions (such as 001) are significant broadened than the non-00l ones and why the (001) diffraction tends to be broader with decreasing RE3+ size (Fig. 1a), respectively. Selected area electron diffraction from an individual GdOCSH nanoplate yielded a set of well-arranged spots (SAED, the inset of Fig. 3a), among which those with the measured d-spacings of ~5.44/5.43 and 7.67 Å can be assigned to (010)/(100) and (110) planes, respectively. In addition, the (010)/(100) dihedral angle was found from the SAED pattern to be ~90, well conforming to the tetragonal structure of GdOCSH. 13 Fig. 3. FE-SEM/TEM (a-c) and AFM (d-f) analysis of the typical REOCSH products of RE = Gd, Y and Lu. In parts (d-f), the height profiles (lower row) were measured along the dashed black arrows in the AFM images (upper row) in each case. The thermal behaviors of REOCSH were investigated by TG/DTA in flowing simulated air, and the results are shown in Fig. 4a for the TmOCSH representative and in Fig. S6 for the rest. It is seen that the compounds similarly decompose via five stages corresponding to dehydration (stage Ⅰ), partial dehydroxylation (stage Ⅱ), complete dehydroxylation/partial decarbonation (stage Ⅲ), complete decarbonation (stage Ⅳ), and desulfurization (stage Ⅴ).24 Unlike the other stages, stage Ⅳ is companioned by an exotherm in each case, which corresponds to the crystallization of RE2O2SO4. The decomposition data are tabulated in Table 1 for the entire series of REOCSH, together with the number of molecular water (n value) calculated from the first stage of decomposition. The good correspondence of the 14 observed (Table 1) and theoretical (Table S2) weight losses may further confirm the proposed decomposition procedures. The dehydroxylation, decarbonation and desulfurization of RE compounds would shift to a higher, lower and lower temperature with decreasing RE3+ size, respectively,19,31,32 and the phenomena can be explained from the Pearson hard-soft-acid-base (HSAB) theory by considering lanthanide contraction. As seen from the peak temperature of the thermal event (Table 1), the stage III and stage V of this work follow the aforesaid tendency but stage IV showed an opposite trend. It was speculated that the gradually higher occurrence temperature of stage IV (complete decarbonation) is owing to more sluggish removal of hydroxyls. Fig. 4. TG/DTA curves for TmOCSH (a) and a comparison of the TG curves for the full series of REOCSH compounds (b). Table 1 A summary of the thermal decomposition data for the series of REOCSH compounds RE Peak 3 (°C) Peak 4 (°C) Peak 5 (°C) n value Weight loss Ⅰ (%) Weight loss Ⅱ-Ⅳ (%) Weight loss Ⅴ (%) Total weight loss (%) Gd 457 614 1246 1.44 4.88 11.00 14.54 30.42 Tb 459 616 1211 1.43 4.81 10.83 14.60 30.24 Dy 467 630 1173 1.47 4.89 10.83 13.91 29.76 Y 487 645 1151 1.53 6.97 15.40 19.33 41.70 Ho 478 651 1143 1.53 5.03 10.45 13.93 29.41 Er 500 660 1124 1.31 4.31 10.48 13.61 28.40 Tm 500 682 1094 1.46 4.74 11.22 14.28 30.24 Yb 504 707 1063 1.47 4.69 10.96 13.85 29.50 Lu 515 750 1057 1.57 4.97 10.81 13.79 29.57 150 300 450 600 750 900 1050 120065707580859095100Weight (%)Temperature (C)−4−2024Peak 5Peak 4Peak 3Peak 2Peak 1DTA (mV/mg)1094682500382175RE = Tm(a) Stage IStage IIStage IIIStage IVStage VExo150 300 450 600 750 900 1050 1200556065707580859095100Weight (%)Temperature (C) Gd Tb Dy Y Ho Er Tm Yb Lu(b)15 3.2 Derivation and characterization of RE2O2SO4 According to the results of TG (Fig. 4b), 800 °C was selected as a suitable temperature to derive RE2O2SO4 from REOCSH. XRD analysis (Fig. 5a) showed that phase-pure RE2O2SO4 (monoclinic, space group C2/c)8,18 was obtained in each case, but the diffraction peaks gradually shifted to higher angles with decreasing size of RE3+. Analyzing the XRD patterns with the Jade 6 software found that lattice constants (a, b and c) and cell volume (V) tend to linearly decrease as the radius of RE3+ decreases while axis angle β showed an opposite tendency (Fig. 5b, Table S3). FE-SEM observation indicated that the nanoplates of REOSCH tend to disintegrate by the mass loss during calcination and the primary crystallites of RE2O2SO4 are up to ~25 nm (Fig. S7). This size value is in line with the average crystallite sizes (~22 nm, Table S3) estimated by broadening analysis of the XRD peaks (Fig. 5a) with Scherrer formula. Fig. 5. XRD patterns of the derived RE2O2SO4 (a) and correlation of lattice parameters (a, b, c, β) and cell volume (V) with the ionic radius of RE3+ (b). The above results thus indicated that the incorporation of a pretty high amount of CO32- into SO42−-LREH to form RE2(OH)4-2x(CO3)xSO4∙nH2O (x ≤ 1.0) would not cause phase separation (formation of RE2O3) by calcination at 800 °C for RE = Gd-Lu lanthanide or Y. It can also be inferred that property control of RE2O2SO4 powder is viable through precursor engineering 10 15 20 25 30 35 40 45 50A(a)JCPDS (Gd2O2SO4): No.24-9775200110-202310-402-112-312022004-204020-602600510220 Intensity (a.u.)350 cps 600 cps1500 cps ErYHoDyTbGdTmYbLu1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.1113.013.213.413.64.084.114.144.174.207.927.988.048.10107.10107.24107.38107.52403416429442 a (Å)Ionic size (Å)Lu YbTmEr HoYDyTbGd(b)Lattice constant a b (Å)LuYbTm ErHoYDyTb GdLattice constant b c (Å)Lu YbTm ErHoYDy TbGd b () LuYbTmErHoYDyTbGdLattice constant cAxis angle b V (Å3 )Lu YbTmErHoYDy TbGdCell volume V16 and that precursor synthesis can be performed under much less stringent conditions (such as lower temperatures in open air), since as long as the precursor has the above formula, no matter it is crystalline or amorphous, RE2O2SO4 can be derived from it by proper annealing. 3.3 The photoluminescence properties of Gd2O2SO4:0.05Eu3+ and Gd2O2SO4:xSm3+ The Eu3+ and Sm3+-doped samples of (Gd0.95Eu0.05)2(OH)2CO3SO4nH2O (GdOCSH:0.05Eu) and (Gd1-xSmx)2(OH)2CO3SO4nH2O (x = 0.005-0.03; GdOCSH:xSm) were synthesized under the same hydrothermal conditions of GdOCSH. Phase analysis indicated that the products conform to GdOCSH and do not contain any impurity (Fig. S8a). Elemental mapping of the GdOCSH:0.05Eu and GdOCSH:0.03Sm typical samples revealed that the constituent elements are uniformly distributed across the crystallites (Fig. 6), implying the formation of solid solution. XRD analysis showed that phase-pure Gd2O2SO4:0.05Eu and Gd2O2SO4:xSm were produced by calcining their respective precursors in air at 800 °C (Fig. S8b). The formation of GdOCSH:Ln3+ and Gd2O2SO4:Ln3+ (Ln = Eu, Sm) solid solutions were further evidenced by lattice expansion (Table S4), since the effective ionic radii of Eu3+ (1.12 Å, CN = 9) and Sm3+ (1.132 Å, CN = 9) are both larger than that of Gd3+ (1.107 Å, CN = 9).33 Fig. 6. STEM morphologies (a, f) and the results of elemental mapping for GdOCSH:0.05Eu (a-e) and GdOCSH:0.03Sm (f-j). 17 Fig. 7a shows the excitation (PLE) and emission (PL) spectra of Gd2O2SO4:0.05Eu. It is seen that the PLE spectrum, taken by monitoring the 617 nm red emission, contains a broad and strong band centered at ~275 nm for O2−→Eu3+ charge transfer and 8S7/2 → 6IJ transition of Gd3+ and a few much weaker peaks in the longer wavelength region for intra-4f6 transitions of Eu3+.34,35 Under 275 nm excitation, the phosphor exhibited the typical 5D0 → 7FJ (J = 0-4) emissions of Eu3+, with the 5D0 → 7F2 one (~617 nm) being the most prominent. The phosphor has a quantum yield (QY) of ~32.3% and Commission Internationale de L’Eclairage (CIE) chromaticity coordinates (Fig. S9) of around (0.656, 0.344). It was also found that the 617 nm main emission decayed in a single exponential manner and has a fluorescence lifetime of ~2.43 ± 0.01 ms (Fig. 7c). Fig. 7. PLE and PL spectra (a, b) and fluorescence decay curves (c, d) of Gd2O2SO4:0.05Eu (a, c) and Gd2O2SO4:xSm (b, d) phosphors. 200 250 300 350 400 450 500 550 600 650 700 750 800010002000300040005000Intensity (a.u.)Wavelength (nm)lem=617 nm617 nm (5D0→7F2)lex=275 nm702 nm (5D0→7F4)596 nm (5D0→7F1)580 nm (5D0→7F0)395 nm (7F0→5L6) 702 nm (5D0→7F3)363 nm (7F0→5D4) 379 nm (7F0→5L7) 415 nm (7F0→5D3) 275 nm PLE PL(a)200 250 300 350 400 450 500 550 600 650 700 750020406080100120140160214 nmIntensity (a.u.)Wavelength (nm) x=0.005 x=0.01 x=0.015 x=0.02 x=0.03lem=608 nm lex=407 nm608 nm (4G5/2→6H7/2)275 nm (Gd3+ 3S7/2→6I J)345 nm (6H5/2→4K7/2)377 nm (6H5/2→3F9/2)(b)407 nm (6H5/2→4K11/2)478 nm (6H5/2→4I 11/2 + 4I 13/2)567 nm (4G5/2→6H5/2)650 nm (4G5/2→6H9/2)708 nm (4G5/2→6H11/2)PLE PL14 16 18 20 22 24 26 28 30 32 34 36 3805001000150020002500300035004000Intensity (a.u.)Decay Time (ms) Gd2O2SO4:0.05Eu Fitting curveλem = 617 nm, λex = 275 nmτ = 2.43(0.01) ms, χ2 = 1.00(c)14 16 18 20 22 24 26 28 30 32 34 36 380102030405060Intensity (a.u.)Decay Time (ms) Gd2O2SO4:0.01Sm Fitting curveλem = 608 nm, λex = 407 nmτ = 2.40(0.01) ms, χ2 = 1.00(d)18 Fig. 7b shows the photoluminescence spectra of the series of Gd2O2SO4:xSm phosphors. It is seen that the PLE spectra, recorded by monitoring the 608 nm main emission, consist of three parts in each case, with the peaks centered at ~214 and 275 nm for O2− → Sm3+ charge transfer and 8S7/2 → 6IJ transition of Gd3+, respectively,36,37 and the ones in the 325-500 nm region for intra-4f5 transitions of Sm3+. Since 214 nm is too close to the vacuum ultraviolet region, PL spectra were taken under 407 nm excitation (6H5/2 → 4K11/2 transition, the second strongest). It is clear that the phosphors emit at 567, 608 (the strongest), 650 and 780 nm in each case, which can be assigned to transitions from the 4G5/2 excited level to 6H5/2, 6H7/2, 6H9/2 and 6H11/2 ground states of Sm3+, respectively. The intensity of emission improves with increasing Sm3+ concentration up to x = 0.01, followed by a gradual decrease owing to concentration quenching. The operating mechanism of concentration quenching can be analyzed with the following equation:38 log (𝐼𝑥) = (−𝑠3) log(𝑥) + 𝑎 (1) where I is the intensity of the 608 nm emission, x is the content of Sm3+, a is a constant, and s is an indicator of interaction type, with the values of 3, 6, 8, and 10 for exchange, electric dipole-dipole, electric dipole-quadrupole, and electric quadrupole-quadrupole interactions, respectively. Linear fitting of the log(I/x) versus log(x) plot yielded an s value of ~4.50 (Fig. S10), which is midway between 3 and 6. To differentiate exchange interaction from electric dipole-dipole interaction, we analyzed the separation distance (R) of Sm3+ with the equation R ≈ 2[3V/(4xN)]1/3,39 where V is the volume of the unit cell (~441.3 Å3) and N is the number of Gd3+ sites in the unit cell (N = 8), and found R values of ~2.19, 1.92, 1.74 and 1.52 nm for x = 0.01, 0.015, 0.02 and 0.03, respectively. As the R values are all obviously larger than the ~0.4 19 nm required for exchange interaction, it can be said that concentration quenching largely took place via electric dipole-dipole interaction for Gd2O2SO4:xSm. The Gd2O2SO4:0.01Sm optimal composition was analyzed from its PL spectrum to have CIE color coordinates (Fig. S9) of about (0.593, 0.407) and QY of ~14.7%. Fluorescence decay analysis via single-exponential fitting found that the 608 nm main emission of Gd2O2SO4:0.01Sm has a lifetime of ~2.40 ms (Fig. 7d). 3.4 Thermal stability and temperature sensing performance of Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm 450 475 500 525 550 575 600 625 650 675 700 7250200400600800100012001400522 528 534 540 546 552 558 564 570 576024681012141618Intensity (a.u.)Wavelength (nm) 533 nm (5D1→7F1)540 nm (5D1→7F2)Intensity (a.u.)Wavelength (nm) 298 K 323 K 348 K 373 K 398 K 423 K 448 K 473 K 498 K 523 Klex = 275 nm(a)617 nm (5D0→7F2)200 225 250 275 300 325 350 375 400 4250200400600800100012001400Intensity (a.u.)Wavelength (nm) 298 K 323 K 348 K 373 K 398 K 423 K 448 K 473 K 498 K 523 Klem = 617 nm(b) 275 nm 395 nm 500 550 600 650 700 75005001000150020002500Intensity (a.u.)Wavelength (nm) 298 K 323 K 348 K 373 K 398 K 423 K 448 K 473 K 498 K 523 Kλex = 407 nm(c)608 nm (4G5/2→6H7/2)200 225 250 275 300 325 350 375 400 4250100020003000400050006000Intensity (a.u.)Wavelength (nm) 298 K 323 K 348 K 373 K 398 K 423 K 448 K 473 K 498 K 523 Kλem = 608 nm(d) 214 nm 275 nm 407 nm 20 Fig. 8. Temperature-dependent emission (a, c) and excitation (b, d) spectra of the Gd2O2SO4:0.05Eu (a, b) and Gd2O2SO4:0.01Sm (c, d) phosphors. Parts (e) and (f) are for relative intensities of the 617 and 608 nm emissions of Eu3+ and Sm3+, respectively. The insets in (a) and (e) respectively show an enlarged view of the 520-576 nm region and relative intensity of the 533 nm emission of Eu3+. To examine thermal stability, temperature-dependent photoluminescence spectra (298-523 K) were taken in Fig. 8 for the Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm phosphors. It is seen that the 5D0 → 7FJ (J = 0–4) emissions of Gd2O2SO4:0.05Eu (Fig. 8a) and the 4G5/2 → 6HP/2 (P = 5, 7, 9, 11) emissions of Gd2O2SO4:0.01Sm (Fig. 8c) were gradually weakened with increasing temperature owing to thermal quenching, though appreciable change in peak shape/position was not found. Noteworthy is that the 5D1 → 7F1,2 luminescence of Eu3+ was steadily enhanced by a higher temperature, as revealed by an enlarged view of the 520-576 nm spectral region (the inset in Fig. 8a) and temperature-dependent relative intensity of the 533 nm emission (5D1 → 7F1 transition, the inset in Fig. 8e). This is due to thermal coupling of the 5D0 and 5D1 energy levels, which makes 5D0 electrons be readily promoted to the higher lying 5D1 level by thermal activation. The Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm phosphors exhibited good thermal stability and their 617 nm (5D0 → 7F2) and 608 nm (4G5/2 → 6H7/2) main emissions maintained ~79.6 and 85.5% of their room-temperature intensities at 423 K, respectively (Fig. 8e,f). The activation energy (Ea) of thermal quenching can be assayed with the Arrhenius equation:40 275 300 325 350 375 400 425 450 475 500 525 5500.60.70.80.91.0300 350 400 450 500 5501.01.52.02.53.03.54.0 533 nm emissionRelative Intensity (a.u.)Temperature (K)100%219.1%389.5% 617 nm emission of Eu3+Relative Intensity (a.u.)Temperature (K)100%79.6%65.1%(e)275 300 325 350 375 400 425 450 475 500 525 5500.60.70.80.91.0Relative Intensity (a.u.)Temperature (K) 608 nm emission of Sm3+ 100%85.5%64.6%(f)21 𝐼𝑇 =𝐼01 + 𝑐exp (−𝐸𝑎𝑘𝑇) (2) where I0 and IT are the emission intensities at room temperature and temperature T, respectively, c is a rate constant, and k is the Boltzmann constant (8.629  10−5 eV/K). From the ln(I0/IT − 1) versus 1/(kT) plots shown in Fig. 9, Ea values of ~0.19 and 0.21 eV were derived via linear fitting for the 617 and 608 nm emissions of Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm, respectively. From the temperature-dependent excitation spectra of Gd2O2SO4:0.05Eu (Fig. 8b) and Gd2O2SO4:0.01Sm (Fig. 8d), it was found that the intensity of each excitation peak also gradually decreases with increasing temperature by thermal quenching. Noteworthy is that temperature-induced red shift of charge transfer band (CTB) edge was clearly observed in both the cases, which is owing to increased population of higher vibrational sublevels of the ground state with increasing temperature.41,42 Fig. 9. ln(I0/IT − 1) vs. 1/(kT) plots for the 617 nm emission of Gd2O2SO4:0.05Eu (a) and the 608 nm emission of Gd2O2SO4:0.01Sm (b). Non-contact temperature sensing with the fluorescence intensity ratio (FIR) technology is arousing a wide range of interest in recent years.43 The FIR technology mostly utilizes the up-conversion luminescence of thermally coupled energy levels (TCLs), particularly the 22 24 26 28 30 32 34 36−3.5−3.0−2.5−2.0−1.5−1.0−0.5slope = −0.19±0.01Adj. R-square = 0.9491ln(I0/IT−1)1/(kT) Experimental data Fitting line(a)22 24 26 28 30 32 34 36−3.5−3.0−2.5−2.0−1.5−1.0−0.5 Experimental data Fitting lineln(I0/IT−1)1/(kT)(b)slope = −0.21±0.01Adj. R-square = 0.999122 4F7/2/4F3/2 of Nd3+, the 3F2,3/3H4 of Tm3+ and the 2H11/2/4S3/2 of Er3+,44 but the accuracy of temperature sensing is affected by the heating effect of the infrared excitation light. The problem can nevertheless be avoided by employing down-conversion luminescence. Especially, highly sensitive temperature sensing was recently achieved with the non-TCLs FIR technology, which utilized temperature-induced shift of CTB edge in vanadate systems doped by activators like Eu3+ and Sm3+.42,45 For Gd2O2SO4:0.05Eu, the FIR of the thermally coupled 5D1/5D0 levels of Eu3+ can be described by the following offset-corrected Boltzmann equation:43 𝐹𝐼𝑅(5D1/5D0) =𝐼1𝐼2= 𝐴exp (−∆𝐸f𝑘𝑇) + 𝐵 (3) where I1 and I2 represent the integral intensities of the 5D1 → 7F1 (533 nm) and 5D0 → 7F2 (617 nm) luminescence under 275 nm excitation, respectively, A is a fitting constant, ΔEf is the energy gap between 5D0 and 5D1, k is the Boltzmann constant, T is the absolute temperature, and B is an offset factor. With the intensity data presented in Fig. 8e, it was found that FIR(5D1/5D0) follows the equation FIR(5D1/5D0) = 1.73exp(−2396.02/T) + 0.003 (Fig. 10a), and the derived ΔEf value of 1667 cm-1 is in good agreement with that (1745 cm-1) assayed from the emission spectra (Fig. 8a). The results thus indicate that the 5D1 and 5D0 levels of Eu3+ can be utilized for temperature sensing. On the other hand, the 617 nm emission of Gd2O2SO4:0.05Eu under 325 nm excitation (CTB edge, Fig. 8b) and 395 nm excitation (5F0 → 5L6 transition, Fig. 7a) gradually gained and lost intensity with increasing temperature (Fig. S11) owing to red-shift of CTB edge and thermal quenching, respectively. These opposite tendencies provided an opportunity for thermal sensing, and analyzing the data shown in Fig. 10b found an equation of FIR(I325/I395) = 85.02exp(−2408.09/T) + 0.129, where I325 and I395 23 represent integral intensities of the 617 nm emission under 325 and 395 nm excitation, respectively. Opposite tendencies of temperature influence (Fig. 8f, Fig. S12) were also found for the 608 nm emission of Gd2O2SO4:0.01Sm under excitation at 244 nm (CTB edge, Fig. 8d) and 407 nm (6H5/2 → 4K11/2 transition, Fig. 7b). As shown in Fig. 10c, fluorescence intensity ratio FIR(I244/I407) follows the equation FIR(I244/I407) = 10.06exp(−1533.4/T) + 0.116. The performance of temperature measurement can be evaluated by absolute sensitivity (Sa) and relative sensitivity (Sr). It is well recognized that Sr is more practical than Sa,46 and is more widely used to evaluate the performance of temperature sensing. Sr can be correlated to FIR and ΔEf via the equation:47 𝑆r =1FIR𝑑FIR𝑑𝑇=∆𝐸f𝑘𝑇2 (4) For Gd2O2SO4:0.05Eu, both the Sr(5D1/5D0) and Sr(I325/I395) tend to decrease with increasing temperature, following the equations of Sr(5D1/5D0) = 2396.02/T2 (Fig. 10d) and Sr(I325/I395) = 1954.33/T2 (Fig. 10e), and have almost the same maximum value of ~2.70% K−1 at T = 298 K. The value is higher than those of most Eu3+-activated thermosensitive materials, such as 1.4% K−1 at 300 K for CaEu2(WO4)4,48 1.68% K−1 at 298 K for GdVO4:0.12Eu,49 1.8% K−1 at 333 K for YBO3:0.02Eu50 and 2.23% K−1 at 298 K for NaLaCaWO6:0.3Eu,51 but is lower than the 4.36% K−1 at 300 K for YVO4:0.1Eu.52 The Sr(I244/I407) of Gd2O2SO4:0.01Sm follows the equation Sr(I244/I407) = 1533.40/T2 and has its maximum value of ~1.73% K−1 at 298 K (Fig. 10f), which is lower than the 3.68% K−1 at 300 K for GdVO4:0.05Sm42 and the 1.80% K−1 at 300 K for GdNbTiO6:0.03Sm53 but is larger than the 1.60% K−1 at 300 K for La3NbO7:0.01Sm.54 It was also noticed that the maximum Sr values of both Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm are significantly larger than those of the recently reported 24 Li6CaLa2Nb2O12:Yb,Er (1.6% K−1 at 298 K),55 Sr3Y(PO4)3:Yb,Ho (1.1% K−1 at 298 K)56 and NaLuF4:Yb,Er (0.5% K−1 at 300 K)57 up-conversion phosphors, though inferior to those of La2MoO6:Yb,Er (3.3% K−1 at 298 K)58 and Bi2MoO6:0.02Er,0.02Tm,0.15Yb (5.90 % K−1 at 293 K).59 Fig. 10. FIR (a-c) and Sr (d-f) as a function of temperature for Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm. Temperature resolution (T) manifests the minimal detectable temperature change of a luminescent thermometer, which is defined as:60,61 𝛿𝑇 = 1𝑆r × 𝛿𝐹𝐼𝑅𝐹𝐼𝑅 (5) where FIR/FIR is the relative uncertainty of FIR and is dependent on the experimental setup. In our case, the FIR/FIR value was taken as 0.4% according to the standard deviation of the baseline reading fluctuation at 298 K. Therefore, the T(5D1/5D0) and T(I325/I395) of Gd2O2SO4:0.05Eu are ~0.15 and 0.18 K at 298 K, respectively, and the T(I244/I407) of Gd2O2SO4:0.01Sm is ~0.23 K at 298 K. Repeatability and reversibility are important for practical application of a luminescent 300 350 400 450 500 5500.0000.0050.0100.0150.0200.025 Gd2O2SO4:0.05Eu Fitting curveFIR(5D1/5D0)Temperature (K)Adj-Square = 0.9989(a)300 350 400 450 500 5500.00.20.40.60.81.0 Gd2O2SO4:0.05Eu Fitting curveFIR(I325/I395)(b)Temperature (K)Adj-Square = 0.9977300 350 400 450 500 5500.10.20.30.40.50.60.7 Gd2O2SO4:0.01Sm Fitting curveFIR(I244/I407)Temperature (K)Adj-Square = 0.9996(c)300 350 400 450 500 5500.51.01.52.02.53.0 Gd2O2SO4:0.05EuSr(5D1/5D0) (% K-1)Temperature (K)T = 298 K, Sr = 2.70%K-1(d)300 350 400 450 500 5500.250.500.751.001.251.501.752.002.252.50 Gd2O2SO4:0.05Eu Sr(I 325/I395) (% K-1)Temperature (K)T = 298 K, Sr = 2.21%K-1(e)300 350 400 450 500 5500.40.60.81.01.21.41.61.8 Gd2O2SO4:0.01SmSr(I 244/I407) (% K-1)Temperature (K)T = 298 K, Sr = 1.73%K-1(f)25 thermometer. Three cycles of heating-cooling tests were conducted in this work for the two types of phosphors, and the results are shown in Fig. 11. It is seen that the FIR(5D1/5D0) and FIR(I325/I395) of Gd2O2SO4:0.05Eu (Fig. 11a,b) and the FIR(I244/I407) of Gd2O2SO4:0.01Sm (Fig. 11c) are essentially fully reversible for each cycle and are quite stable at each temperature point for the three cycles. The average repeatability factor (Rc) was quantified with the following equation to be better than 98.7% for both the types of optical thermometers:60 Rc = 1 −max|∆c − ∆i|∆c (6) where i is the thermometric signal (FIR) at temperature i in each cycle and c is the mean of three i values. The results thus indicated that both the types of thermometers have good repeatability and reversibility for optical temperature sensing. Fig. 11. Reversibility and repeatability tests of FIR for Gd2O2SO4:0.05Eu (a, b) and Gd2O2SO4:0.01Sm (c). 4. Conclusion RE2(OH)2CO3SO4nH2O was obtained in this work as a new type of layered hydroxide for RE = Gd-Lu lanthanides and Y. Non-uniform contraction of the unit cell, stronger interaction of CO32−/SO42− with the hydroxide main layers, increasing temperature of dehydroxylation/decarbonation and decreasing temperature of desulfurization were observed at a smaller RE3+. Phase-pure RE2O2SO4 can be readily obtained from the hydroxide 298 398 5230.0020.0040.0060.0080.0100.0120.0140.0160.0180.0200.022523 398 298298 398 523523 398 298298 398 523523 398 298FIR(5D1/5D0)Heating(a) CoolingTemperature (K)Heating Cooling Heating Cooling298 398 5230.10.20.30.40.50.60.70.80.91.0523 398 298298 398 523523 398 298298 398 523523 398 298FIR(I325/I395)(b) Heating CoolingTemperature (K)Heating Cooling Heating Cooling298 398 5230.150.200.250.300.350.400.450.500.550.600.650.70523 398 298298 398 523523 398 298298 398 523523 398 298FIR(I244/I407)(c) Heating CoolingTemperature (K)Heating Cooling Heating Cooling26 compounds by calcination in air at 800 °C. The optimal Sm3+ content was determined to be ~1 at.% for the Gd2O2SO4 host, and concentration quenching of luminescence was suggested to occur via electric dipole-dipole interaction. The Gd2O2SO4:0.05Eu and Gd2O2SO4:0.01Sm phosphors were analyzed to have fluorescence lifetimes of ~2.43 and 2.40 ms and be able to retain ~79.6 and 85.5% of the room-temperature intensity at 423 K for their 617 nm (λex = 275 nm) and 608 nm (λex = 407 nm) main emissions, respectively. Application in optical thermometry via the fluorescence intensity ratio technology indicated that the Gd2O2SO4:0.05Eu phosphor has a maximum relative sensitivity (Sr) of ~2.70% K−1 at 298 K for the thermally coupled 5D0/5D1 energy levels and for excitation under 325 nm (CTB edge)/395 nm (5F0 → 5L6 transition), while the Gd2O2SO4:0.01Sm phosphor has a maximum Sr of ~1.73% K−1 at 298 K for excitation under 244 nm (CTB edge)/407 nm (6H5/2 → 4K11/2 transition). The temperature resolution reached ~0.15 and 0.23 K for the FIR(5D1/5D0) and FIR(I325/I395) of Gd2O2SO4:0.05Eu and ~0.23 K for the FIR(I244/I407) of Gd2O2SO4:0.01Sm at 298 K. 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