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[Dalton_Bi3+ doped (Lu,Gd)2WO6.pdf](https://mdr.nims.go.jp/filesets/a56d9b45-a275-4e00-a86d-73ef9866d851/download)

## Creator

Xuejiao Wang, Xiaowen Feng, Maxim S. Molokeev, Huiling Zheng, Qiushi Wang, Chunyan Xu, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[Modulation of Bi3+ luminescence from broadband green to broadband deep red in Lu2WO6 by Gd3+ doping and its applications in high color rendering index white LED and near-infrared LED](https://mdr.nims.go.jp/datasets/d6a48f05-f21e-4711-ad61-5ecc7806a4be)

## Fulltext

1  Broadband green to broadband deep red modulation of Bi3+ luminescence in Lu2WO6 by Gd3+ doping and application in high color rendering index white LED and Near-infrared LED  Xuejiao Wang,a,b* Xiaowen Feng,a Maxim S. Molokeev,c,d,e Huiling Zheng,a Qiushi Wang,a Chunyan Xu,f 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 cLaboratory of Crystal Physics, Kirensky Institute of Physics, FRC KSC SB RAS, Krasnoyarsk 660036, Russia dResearch and Development Department, Kemerovo State University, Kemerovo 650000, Russia eSiberian Federal University, Krasnoyarsk 660041, Russia fJilin Engineering Laboratory for Quantum Information Technology, Institute for Interdisciplinary Quantum Information Technology, Jilin Engineering Normal University, Changchun 130052, China    *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    mailto:wangxuejiao@bhu.edu.cnmailto:li.jiguang@nims.go.jp2  Abstract Phosphors that exhibit tunable broadband emissions are highly desired in multi-functional LEDs, including pc-WLEDs and pc-NIR LEDs. In this work, broadband emissions were obtained and modulated in the unexpectedly wide spectral range of 517-609 nm for (Lu0.99-xGdxBi0.01)2WO6 phosphors via tuning the Gd3+ content (x = 0-0.99). The effects of Gd3+ doping on phase constituent, particle morphology, crystal structure, and photoluminescence were systematically investigated. Broadband green emission was obtained with the Gd3+-free (Lu0.99Bi0.01)2WO6 phosphor (x = 0), whose emission intensity was enhanced by 50% with 5 at% of Gd3+ (x = 0.05). Phase transition happened when x＞0.50, and broadband red-NIR emission was obtained when x = 0.75-0.99. Three luminescence centers were proved to be responsible for the broadband emissions via crystal structure, spectral fitting and fluorescence decay analysis. A pc-WLED with high color rendering index (Ra = 91.3), stable emission color, and low color temperature (3951 K) was fabricated from the (Lu0.94Gd0.05Bi0.01)2WO6 broadband green phosphor, and an LED device that simultaneously emits high color rendering white light and NIR light was obtained with the (Gd0.99Bi0.01)2WO6 broadband red-NIR phosphor. Night version and noninvasive imaging were also demonstrated for the latter LED device.    Keywords: Broadband Bi3+ luminescence, Gd3+ doping, Rare earth tungstate   3  1. Introduction Phosphors-converted LEDs (pc-LEDs), either traditional white light pc-WLEDs or recently on the rise near-infrared (NIR) pc-LEDs, have various advantages and are finding wide applications in the fields of general lighting, night vision, non-destructive analysis, biomedical, and so forth.1-4 Phosphors play a decisive role in pc-LEDs and determine the quality of the devices. For the fabrication of pc-WLEDs, one typical route is to combine red-green-blue (RGB) emitting phosphors with a UV LED chip or to pump a blend of red and green phosphors by a blue LED chip.5,6 However, due to the lack of a cyan (470-510 nm) spectral component between the PL spectra of blue and green phosphors, the obtained pc-WLEDs have a low color rendering index (CRI, 70-80), which limits their wide applications in general lighting.7,8 To address the cyan gap, developing UV or blue light excitable cyan-emitting phosphors becomes a logical-led solution, and thus considerable attention was paid to the synthesis of cyan emitting phosphors to enhance the CRI of the pc-LED devices.7,9-13 Though the CRI of a pc-LED device incorporating a cyan phosphor can indeed be improved in this way, the use of multiple phosphors will lead to fabrication complexity and reabsorption. Therefore, phosphors that emit broadband green light covering the cyan gap have been developed and used for high CRI and low correlated color temperature (CCT) pc-WLED devices.14,15 However, the development of such phosphors is yet rather limited, and the current research is mainly focused on garnet hosts with Eu2+ and Ce3+ as activators.14,15 Another route to obtain pc-WLED is coating a blue LED chip with the yellow-emitting Y3Al5O12:Ce3+ phosphor (YAG:Ce3+) but has the shortcoming of low CRI (<75) and unwanted high CCT (~7750 K, cold white) due to the lack of red spectral components. For this, various red phosphors have been developed and 4  applied for white lighting. In the meantime, NIR phosphors are under keen development for NIR-pc LED applications.1-4 Red light is adjacent to NIR light in the electromagnetic spectrum, and broadband-emitting red phosphors may thus cover the NIR region for simultaneous white and NIR light applications. Therefore, broadband green and broadband red phosphors are both highly desired. Bismuth is an intriguing element in that it has various valance states (Bi0, Bi1+, Bi2+, Bi3+, Bi5+).5,16-18 Amongst, Bi3+ ([Xe] 4f145d106s2 configuration) is relatively stable and easy to get valence. The 6s electrons of Bi3+ are susceptive to the surrounding environment, for which the emission position and excitation tail of Bi3+-doped phosphors are strongly dependent on crystal field and lattice covalency.19-22 Hence, by adjusting the host composition and crystal structure, tunable Bi3+ luminescence can be achieved. In this work, Bi3+ doped Lu2WO6 tungstate was synthesized via solid state reaction, and the luminescence of Bi3+ was successfully tuned from broadband green to broadband red-NIR via Gd3+ doping. High color rendering LED devices for general lighting and NIR lighting were also obtained with such phosphors.  2. Experimental section 2.1. Reagents and synthesis. The series of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.99) phosphors were synthesized via solid-state reaction, which was started with Lu2O3 (99.99% pure), Gd2O3 (99.99% pure), Bi2O3 (99.99% pure) and H40N10O41W12·xH2O (AR grade). All the reagents were bought from Aladdin Industrial Corporation (Shanghai, China). The starting materials were weighed according to the above formula, and the amount of H40N10O41W12·xH2O was 6 wt% in excess in each case to compensate evaporation 5  loss. The mixture was thoroughly ground in an agate mortar for 30 min and subsequently calcined in a muffle furnace at 1300 oC for 4 h with a heating rate of 5 oC/min at the ramp stage. The products were cooled to room temperature at 5 oC/min and were then slightly grounded for characterizations. 2.2. Fabrication of LED devices. Two LED devices were fabricated using the broadband green and broadband red-NIR phosphors obtained in this work. The LED1 for full-spectrum pc-WLED was fabricated by thoroughly mixing commercial CaAlSiN3:Eu2+ red phosphor (main emission at 640-650 nm; Intematix Co., Ltd, California, USA), commercial BaMgAl10O17:Eu2+ blue phosphor (BAM, main emission at 440-460 nm; Yantai Shield, Yantai, China) and the synthesized (Lu0.94Gd0.05Bi0.01)2WO6 broadband green phosphor in silicone (Leaftop 9300, Shengzhen Tegu New Materials Co., Ltd, Shenzhen, China), followed by coating on a 1 W NUV-LED chip (~365 nm emission; San'an Optoelectronics Co., Ltd, Xiamen, China), where the weight ratio of the above three phosphors is 4:1:2 and the weight ratio of total phosphor powder to silicone is 1:1. The LED2 for simultaneous white and NIR lighting was fabricated by blending the BAM phosphor and the prepared (Gd0.99Bi0.01)2WO6 broadband red-NIR phosphor in silicone, followed by coating on another NUV-LED chip, where weight ratio of the two phosphors is 1:4 and the weight ratio of total phosphor to silicone is 1:1.  2.3. Characterization. Phase identification was performed by X-ray diffractometry (XRD; Model Ultima IV, Rigaku, Tokyo, Japan) operated at 40 kV/40 mA, using nickel filtered Cu-Kα 6  radiation (λ=0.15406 nm) and a scanning speed of 2 o/min. Rietveld refinement was carried out using the TOPAS software.23 The crystallographic data were deposited in Cambridge Crystallographic Data Centre (CSD#2213277-2213283), and can be downloaded from the website (www.ccdc.cam.ac.uk/data_request/cif). The morphology and elemental distribution of the samples were analyzed by field emission scanning electron microscopy (FE-SEM; Model Tescan MIRA LMS, Tesken, Czech Republic) equipped with an energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) of the valence state of Bi was conducted using a Thermo Scientific K-Alpha analyzer (Thermo Fisher Scientific, Waltham, USA), where the chamber pressure is less than 2.0× 10-7 Mbar, the spot size is 400 μm, the working voltage is 12 kV, and the filament current is 6 mA. The L3-edge X-ray absorption spectrum of Bi was measured at the XRD station of beamline 4B9A of Beijing Synchrotron Radiation Facility (BSRF) in the fluorescence mode. Light absorption and bandgap energy were studied via UV-vis spectroscopy (Model PE-750, PerkinElmer, Waltham, USA). Photoluminescence was measured using an FLS 1000 fluorospectrophotometer (Edinburgh Instruments Ltd., Edinburgh, UK) with a 450 W Xe lamp for excitation, slit width of 1 nm for excitation and 0.7 nm for emission, a scanning speed of 1 nm/s, and a TAP-02 accessory for temperature control. The quantum yield and fluorescence decay were recorded using the integrating sphere system and the lifetime testing unit of the FLS 1000 equipment, respectively. The optical properties of the fabricated LEDs were measured by a Model OHSP-350M LED Fast-Scan Spectrophotometer (350-1050 nm range, Hangzhou Hopoo http://www.ccdc.cam.ac.uk/data_request/cif7  Light&Color Technology Co. Ltd., Hangzhou, China). The density of states of two typical samples were calculated using the Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) potential, where the exchange–correlation potential was treated with the generalized gradient approximation using the Perdew–Burke–Ernzerhof (PBE) function.24  3. Results and discussion 3.1 Phase composition, crystallite morphology, and crystal structure   Fig. 1 The XRD patterns of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.99) (a), the observed (black) and calculated (red) XRD profiles and the difference (gray) between the two for the (Lu0.94Gd0.05Bi0.01)2WO6 sample (x = 0.05) (b), where the Bragg reflections are indicated with green tick marks, and the correlation of lattice constants (c) and axis angle/cell volume (d) with the x value. Fig. 1a shows the XRD patterns of the products, it can be perceived from the diffraction intensity that Gd3+ doping greatly enhanced the crystallinity of the sample. Such a phenomenon is closely related to a gradual change in crystallite morphology 8  and will be explained later. The x = 0-0.25 samples can be well indexed to the monoclinic Lu2WO6 (PDF card No. 23-1211; P2/c space group) due to the similar ionic radii of Lu3+ (0.977 Å for CN = 8) and Gd3+ (1.053 Å for CN = 8). A phase mixture was resulted at x = 0.50 and a complete phase transition to monoclinic Gd2WO6 (PDF card No. 23-1074; C2/c space group) happened at x = 0.75. Starting from the crystal structures of Lu2WO625 and Gd2WO626, Rietveld refinement using the TOPAS 4.2 software24 yielded stable results and satisfactory R-factors (Table S1) for the two series of compounds (Fig. S1), as shown in Fig. 1b with the x = 0.05 sample for example. The derived atomic coordinates and main bond lengths are summarized in Table S2 and Table S3, respectively. The derived lattice constants (a, b, c), cell volume and axis angle (β) are shown in Fig. 1c,d, where it is seen that, except for axis angle, the structural parameters all monotonously increase with increasing Gd3+ doping. This proves the formation of solid solution and suggests that the designated chemical compositions are close to the real ones.  Fig. 2 FE-SEM morphologies of the (Lu0.99-xGdxBi0.01)2WO6 samples with (a) x=0, (b) x=0.05, (c) x=0.10, (d) x=0.25, (e) x=0.50, and (f) x=0.99. Product morphology was investigated via FE-SEM, and the results are shown in Fig. 2. It can be seen that, in most cases, irregular aggregates consisting of microrod-like crystallites were produced. The microrods grew longer with increasing Gd3+ doping until x = 0.50. This may be due to the fact that the formation of the (Lu0.99-xGdxBi0.01)2WO6 final product is the reaction of RE2O3 (RE=Lu or Gd; Lewis 9  base) and H40N10O41W12 (Lewis acid) at high temperatures. The basicity of Gd2O3 is stronger than Lu2O3 and, therefore, the oxides (Lewis base) more readily react with H40N10O41W12 (Lewis acid) with increasing portion of Gd2O3 in the reactant mixture. The rodlike morphology mostly vanished for the x = 0.99 sample owing to the different crystal structures of Lu2WO6 and Gd2WO6. Elemental maps of two typical samples revealed good composition homogeneity, as shown in Fig. 3 and Fig. S2 for (Gd0.99Bi0.01)2WO6 and (Lu0.94Gd0.05Bi0.01)2WO6, respectively.   Fig. 3 SEM morphology (a), EDS analysis (b), and elemental mapping (c-e) of the (Gd0.99Bi0.01)2WO6 sample.  As valence state strongly affects the luminescence of bismuth, it is necessary to make an analysis before further discussion. Fig. 4a shows the XPS survey spectra for the two typical samples of x = 0 and 0.99, from which it is obvious that all the spectral features, except for the C 1s level, are attributed to the constituent elements of (Lu0.99-xGdxBi0.01)2WO6. Fig. 4b shows the high-resolution XPS spectra of Bi 4f orbital. It was observed that the peaks for Bi 4f5/2 and Bi 4f7/2 are located at around 164.31 and 159.01 eV with a doublet splitting of 5.30 eV for the x = 0 sample and at 163.65 and 158.43 eV with a doublet splitting of 5.22 eV for the x = 0.99 sample. This 10  is consistent with the Bi 4f spin-orbit and confirmed the existence of Bi3+ in the two samples.27,28 The different splitting energy is due to change of the crystal structure. The observed doublet peaks for Bi 4f7/2 and Bi 4f5/2 are asymmetric in both the lattices, which is due to the fact that Bi3+ ions are located at multiple crystallographic sites, since the binding energy for Bi in different polyhedrons slightly varies. Furthermore, both the two peaks shifted to lower energy for the x = 0.99 sample, from 164.39 and 159.08 eV to 163.65 and 158.43 eV, respectively. This may be caused by a higher covalency of the Bi-O bond in (Gd,Bi)2WO6 than in (Lu,Bi)2WO6, since a higher covalency will cause a stronger nephelauxetic effect and make the electron easier to flee from the core.   Fig. 4 XPS survey spectra (a) and high-resolution XPS spectra of Bi 4f orbital (b) for the x = 0 and 0.99 typical compositions. (c) is the Bi L3-edge XANES spectra for the typical samples of x = 0.25, 0.75 and 0.99, with that of the Bi2O3 reference included for comparison. Fig. 4c shows the results of X-ray absorption near edge structure (XANES) analysis of Bi L3-edge for the three samples of x = 0.25, 0.75 and 0.99, using Bi2O3 as a 11  reference material. XANES is sensitive to the valence of absorption atoms and partly to the geometrical configuration of the surrounding atoms. The position of edge energy depends on the oxidation state of the absorption atoms, permitting the detection of the valence state of Bi.29,30 It can be seen that the synthesized samples and Bi2O3 share the same position for the edge, which further proves that the valence state of Bi is 3+ in the studied samples.    Fig. 5 UV-vis absorption spectra of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.99) (a), the partial density of states (DOS) of Bi, W, O, Gd/Lu atoms for (Lu0.99Bi0.01)2WO6 (b) and (Gd0.99Bi0.01)2WO6 (c), where the Fermi level is indicated by the vertical dashed line. Part (d) is for the determination of bandgap energies. Fig. 5a shows the absorption spectra in the 200-800 nm region of typical samples, together with a Lu2WO6 powder synthesized in this work for comparison. It is seen that Lu2WO6 shows absorption in the 200-350 nm region, and 1 at% of Bi3+ doping of Lu2WO6 ((Lu0.99Bi0.01)2WO6, x = 0) obviously enhanced the absorption and produced an intense new band at 373 nm that is contributed from 1S0→3P1 transition of Bi3+ 12  orbitals.31 Slight doping of Gd3+ (x = 0.05) did not obviously change the spectra. The x = 0.50 sample shows a similar spectral profile but lower intensity for the 373 nm band, and the 200-350 nm broadband is slightly stronger. When Lu3+ is completely replaced by Gd3+ (x = 0.99), the sample showed much stronger absorption in the 200-350 nm region and the 373 nm band disappeared. The absence of the 373 nm band was speculated to be due to the change of covalency experienced by Bi3+ and variation of electronic configuration for Bi3+ in the x = 0.99 sample. It was reported that the position of optical absorption for the 1S0→3P1 transition of Bi3+ strongly depends on covalency effect/nephelauxetic strength in the crystal lattice.32,33 Thus, it is reasonable to infer that the 1S0→3P1 transition of Bi3+ would left-shift in the UV-vis spectrum and overlap with the broadband of tungstate anions (200-350 nm) when Lu3+ (electronegativity: 1.27 eV) is completely replaced by Gd3+ (electronegativity: 1.20 eV). In view that the variation of electronic configuration for Bi3+ may also influence UV absorption, the partial DOS of Bi, W, O, Gd/Lu atoms were calculated in Fig. 5b,c for the two typical compositions of x = 0 and 0.99. It can be seen that, for (Lu0.99Bi0.01)2WO6 (x = 0), the valence band is mainly contributed from Lu 4f and O 2p and the conduction band is mainly composed of W 5d and O 2p, though Bi 6s and Bi 6p slightly contributed to the valence and conduction bands, respectively. For (Gd0.99Bi0.01)2WO6 (x = 0.99), the valence band is composed of Gd 4f, O 2p and Bi 6s while the conduction band consists of Gd 4f, W 5d and O 2p states. The 6s states of Bi are near the Fermi surface in (Lu0.99Bi0.01)2WO6 and obviously moves deeper into the valence band in (Gd0.99Bi0.01)2WO6. Furthermore, the orbital hybridization of Bi and O is also more prominent in (Gd0.99Bi0.01)2WO6. These suggest that the absorption of Bi3+ may overlap with that of tungstate anions and shift to a higher energy, which corresponds with the observed changing of absorption spectra. Estimation of bandgap 13  energy (Eg) can be made from the absorption spectra. The relation between absorption coefficient (α) and incident photon energy (hν) can be written as α=Bd(hν-Eg)1/2 for indirectly allowed transition.34,35 The α in the equation represents absorption coefficient, Bd is absorption constant, and hν is the energy of incident photons. The (Ahν)2-hν plots, converted from the spectral data of Fig. 5a, are shown in Fig. 5d, where A presents absorbance and is proportional to α. Extrapolating the linear parts of the (Ahν)2-hν plots yielded Eg values of ~3.66, 2.91, 2.91 and 2.98 eV for Lu2WO6, (Lu0.99Bi0.01)2WO6, (Lu0.94Gd0.05Bi0.01)2WO6 and (Lu0.49Gd0.50Bi0.01)2WO6, respectively. The (Gd0.99Bi0.01)2WO6 sample, on the other hand, was found to have Eg value of 3.38 eV. 3.2 Photoluminescence properties      Fig. 6 The emission (a) and excitation (b) spectra of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.99), the average bond lengths (Å) of (Lu1-xGdx)On polyhedrons (c), and the results of Gaussian fitting of the emission spectrum of the x = 0.05 sample (d).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   14  The effect of Gd3+ doping on Bi3+ luminescence was investigated, and Fig. 6a shows the emission spectra for the series of (Lu0.99-xGdxBi0.01)2WO6 phosphors (x = 0-0.99; λex = 346 nm). It is seen that the x = 0 sample exhibits a broad and asymmetric luminescence band ranging from 400 to 720 nm (peaking at 510 nm) that arises from the 3P1 → 1S0 transition of Bi3+. Noteworthy is that the band has a large full width at half maximum (FWHM) of 0.61 eV and well covers the cyan gap. Therefore, the phosphor may have the potential for application in high color rendering white pc-LEDs. When x ≤0.50, Gd3+ doping did not obviously change the profile but influenced the intensity and position of Bi3+ luminescence. As seen from the relationship shown in Fig. S3, the 510 nm emission intensity increases until x = 0.05 and then decreases. The x = 0.10 and x = 0.25 samples have an almost equal intensity, which is still stronger than the Gd3+-free sample (x = 0) although weaker than the x = 0.05 one. The increased luminescence with Gd3+ doping (x = 0.02 and 0.05) may largely be due to the enhanced crystallinity of the sample as analyzed in Fig. 2, while the decreased luminescence with further doping of Gd3+ (x = 0.10 and 0.25) may be due to lattice expansion (lower lattice stiffness), which enhances phonon vibration and strengthens phonon-photon coupling to some degree.36-39 Besides, the intensified distortion of Bi-O polyhedron by Gd3+ doping is also harmful to the 3P1 → 1S0 emission of Bi3+, since the transition is parity allowed and can be suppressed by a larger extent of polyhedron distortion. The emission intensity of the x = 0.50 sample is even lower than the Gd3+-free one, and this may be due to the multi-phase nature of this sample (Fig. 1). Furthermore, the normalized PL spectra revealed that the emission band tends to blue shift with increasing x for the x = 0-0.50 samples, as seen in Fig. S4.  The type of polyhedron and bond length are important factors determining the 15  crystal field strength (Dq) of the sites occupied by Bi3+ according to crystal field theory, and Dq can be estimated by the following expression:40                 (1) where Z is the charge of the ligand anion, e is the charge of electron, r is the radius of the d wave function, and R is the average distance between the central cation and its ligand anion. As Dq is inversely proportional to R5, an increasing R would result in a smaller Dq. The average bond lengths of the three types of polyhedrons in the host were calculated according to the aforesaid results of Rietveld structure refinement, which are presented in Fig. 6c as a function of the Gd3+ content. It can therefore be concluded from the figure that the tending to be larger average bond length with increasing x is responsible for the observed blue shifting of emission band for the x = 0-0.25 samples. The asymmetric profile of the broad green emission can be decomposed into three Gaussian sub-bands as shown in Fig. 6d for the x = 0.05 sample, which also indicates the presence of three kinds of luminescence centers. The results of Gaussian fitting for the other compositions can be found in Fig. S5 and Table S4. Basically, unchanged positions of the three sub-peaks were observed until x = 0.10, and a blue shift was observed with further doping at x = 0.25 and especially at x = 0.50. For the x = 0-0.25 (Lu0.99-xGdxBi0.01)2WO6 samples, it was found from Fig. 6c that bond length decreases in the order (Lu,Gd)2–O ＞  (Lu,Gd)1–O ＞  (Lu,Gd)3–O and, therefore, the sub-peaks at ~500, 555 and 678 nm should arise from the Bi3+ ions residing in (Lu,Gd)2O8, (Lu,Gd)1O8 and (Lu,Gd)3O7 polyhedrons, respectively, in view of the gradually stronger crystal field of the three sites. With further doping of Gd3+ at x = 0.75 and x= 0.99, Bi3+ showed a completely 16  different broadband red emission ranging from 425 to 800 nm (centered at 610 nm). It is encouraging to find that the FWHM of this band is as high as 0.72 eV for the x = 0.75 sample and 0.70 eV for the x = 0.99 sample and the spectra cover the near-infrared (NIR) region. Such Bi3+ luminescence, though reported before,8,13 is yet rather rare. The observed broadband red-NIR emission should be due to more distortion of the polyhedrons accommodating Bi3+ ions by the change of crystal structure and an enhanced nephalauxetic effect by a higher covalency of the Bi-O bond, as explained below.    Fig. 7 The crystal structures of (Lu0.99-xGdxBi0.01)2WO6 for x = 0-0.25 (a) and x = 0.75-0.99 (b).  As shown in Fig. 7, the crystal structure and type of polyhedron change with increasing Gd3+ doping. The polyhedrons where Bi3+ ions are accommodated change from [(Lu1/Gd1)O8], [(Lu2/Gd2)O8] and [(Lu3/Gd3)O7] in the x = 0-0.25 samples to [(Lu1/Gd1)O8], [(Lu2/Gd2)O8] and [(Lu3/Gd3)O8] in the x = 0.75-0.99 ones. With the broadband green emitting sample (x = 0.05) and broadband red emitting sample (x = 0.99) for example, polyhedral distortion index (D) was calculated with the following equation to quantitatively characterize the distortion of different polyhedrons:41,42 D=                   (2) 17  Where li is the distance from the central atom to the ith coordinating oxygen atom and lav is the average bond length of (Lu/Gd/Bi)-O. As the D values for the x = 0.05 sample is lower than those of the x = 0.99 sample (Fig. 7), it can be said that the substantially higher polyhedral distortion of the x = 0.99 sample may have enhanced crystal splitting to lower the 3P1 level of Bi3+ for the red-NIR luminescence. On the other hand, the electronegativity of Gd (1.20 eV) is smaller than that of Lu (1.27 eV). Therefore, significant substitution of Lu with Gd may lead to a stronger polarizability and covalency of the Bi-O bond if we consider (Lu/Gd)-O-Bi moiety. The stronger crystal field arising from an more significant nephelauxetic effect may also red-shift the luminescence of Bi3+, as observed and proved by other researchers.43-45 It may also be inferred from the blue-shifted emission of the x ≦ 0.50 samples that stronger bond covalency should may have played a more significant role in the luminescence behavior of the x = 0.75 and 0.99 samples. The sudden change of broadband green into broadband red-NIR emission without transition at x = 0.75, where crystal structure changes, indicates the importance of host lattice in Bi3+ luminescence. Moreover, the Gd3+ has special electronic configuration (4f7) and may have effects on luminescence centers and cause the red-NIR emission. Gaussian fitting of the broad red-NIR band into three sub-bands did not yield convergent results, since the right-hand tail of the band extends to longer wavelengths and is out of the measured range of this work.  The PLE spectra of the x = 0-0.50 and x = 0.75-0.99 samples showed asymmetric bands centered at 346 and 333 nm, respectively (Fig. 6b). The excitation/emission wavelengths and the FWHM values are summarized in Table S5 for the series of (Lu0.99-xGdxBi0.01)2WO6 phosphors. The quantum yield (QY) of the series samples was 18  also investigated and the results are shown in Table S5 and Fig. S6. It is seen that the QY value follows the variation trend of emission intensity and reached its maximum at x =0.05 (QY = 18.88%). The broadband red emission sample (x=0.99) has a QY value of 4.09%.    Fig. 8 The emission spectra (a, b) and intensity-normalized emission spectra (c, d) obtained under varying excitation wavelength in the range of 300-365 nm, Gaussian deconvolution of the emission band obtained under 300 nm excitation (e), and intensity variation of the deconvoluted three sub-bands with excitation wavelength (f) for the (Lu0.94Gd0.05Bi0.01)2WO6 phosphor.  19  As structure analysis revealed the existence of three different types of emission centers, each center should have its own excitation peak. This can be the reason for the asymmetric excitation bands in Fig. 6b. To further elucidate the luminescence behavior of Bi3+, PL spectra were measured for the (Lu0.94Gd0.05Bi0.01)2WO6 phosphor by varying the excitation wavelength from 300 to 365 nm. As shown in Fig. 8a and Fig. 8b, the overall emission band did not show obvious profile change, but the intensity of emission varied and the strongest emission was obtained under 346 nm excitation, in accordance with the results of Fig. 6b. From the intensity-normalized PL spectra (Fig. 8c,d), it is seen that the center of the luminescence band tends to red-shift with increasing excitation wavelength until 346 nm. Gaussian deconvolution further revealed the variation trend of the three sub-bands for peak intensity and peak position, as shown in Fig. 8e, Fig. S7 and Table S6. Clearly, the intensities of sub-peaks 1 and 2 gradually increase till λex = 346 nm and then decrease, showing an optimal excitation wavelength of 346 nm, while that of sub-peak 3 is basically unchanged till λex = 315 nm and then substantially decreased at λex = 325 nm, revealing that a short wavelength is more efficient for excitation. That is, site-selective excitation behaviors were observed for the multiple cation sites in the host.5,16,45 Besides, the three sub-peaks were obviously red-shifted with increasing excitation wavelength from 300 to 335 nm (Table S6). For even longer excitation wavelength, sub-peaks 1 and 2 show basically unchanged position till λex = 365 nm while sub-peak 3 keeps unchanged up to λex = 355 nm and then obviously red-shifted. These largely corresponds with the observed peak-center variation for the overall 20  band with increasing excitation wavelength (Fig. 8c and d). Temperature-dependent emission spectra were also measured for the (Lu0.94Gd0.05Bi0.01)2WO6 sample. As exhibited in Fig. S8, the PL spectrum and peak position did not change with increasing temperature. The intensity of emission, however, decreased by 77.5% from 298 to 373 K, indicating that the thermal stability of luminescence needs improvement. The activation energy (ΔE) of thermal quenching can be calculated by using the Arrhenius equation, and a ΔE value of ~0.323 eV was obtained.  Fig. 9 Fluorescence decay curves for the main emissions of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.99), where λex = 346 nm and λem = 510 nm for the x = 0-0.50 samples and λex = 333 nm and λem = 610 nm for the x = 0.75-0.99 samples. Fig. 9 shows the fluorescence decay curves for the main emissions of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.99) under the excitation of optimal wavelength. The curves can be fitted with the exponential polynomial I(t)=A1exp(-t/τ1)+A2exp(-t/τ2)+A3exp(-t/τ3) (Fig. S9), where I(t) and t are the fluorescence intensity and decay time, respectively, A1, A2 and A3 are constants, and τ1, τ2 and τ3 are the decay time of exponential components. This corresponds well 21  with the presence of three types of luminescence centers. The average lifetime can be obtained using the equation of τ    τ     τ     τ    τ    τ    τ . The fitting results, including the derived τ*, A1/A2/A3 values and the chi-square factor (χ2) of fitting, are summarized in Table S7. The χ2 values are around 1 for all the fittings, indicating the high quality of the fitting. It can be found that the average lifetime is in the ranges of 0.843-1.518 μs and 0.640-0.989 μs for the x =0-0.25 and x = 0.75-0.99 samples, respectively, and the x = 0.50 mixture sample has a lifetime of 0.665 μs.  3.3 Application of the broadband green phosphor in high color rendering white lighting  Fig. 10 The luminescence spectrum (a), color rendering index Ra and R1-R15 factors (b, c) and CIE chromaticity coordinates (f) of LED1 under 20 mA current driving. Parts (d) and (e) are for CIE and CCT under varying current of driving, and the inserts (a) show the appearances of the device with current on and off. To evaluate the application potential of the (Lu0.94Gd0.05Bi0.01)2WO6 broadband green phosphor, a WLED device (LED1) was constructed using the R-G-B phosphor excited by UV-LED chip method, where the red and blue phosphors are commercially available CaAlSiN3:Eu2+ and BAM, respectively, and the excitation source is a 365 nm LED chip. Fig. 10a and Fig. 10f show the luminescence spectrum and the CIE 22  chromaticity coordinates of LED1 under 20 mA driving. It is seen that the device emits a warm white light and the luminescence spectrum covers the entire visible range, with the cyan gap being well covered. The device shows a satisfactory color correlated temperature (CCT) of 3951 K and a high Ra value of 91.3. In addition, all the other R parameters (R1-R15) are also satisfactory (Fig. 10b, c). To study the stability of the device, optical properties were measured by varying the driving current in the range of 20-100 mA, and the obtained main parameters are summarized in Table S8. It was found that CIE chromaticity coordinates remain almost stable at around (0.38, 0.38), indicating a high color stability of the device (Fig. 10d). Furthermore, the CCT of the device is quite stable at ~3900 K, irrespective of the driving current in the range of this study (Fig. 10e).  3.4 Application of the broadband red-NIR phosphor in simultaneous white and NIR lighting  Fig. 11 The luminescence spectra (a), color rendering index Ra and R1-R15 factors (b, c) and CIE chromaticity coordinates (f) of LED2 under 100 mA current driving. Parts (d) and (e) are for CIE and CCT under varying current of driving, and the inserts (a) show the appearances of the device with current on and off.  23   Fig. 12 Photographs of a finger (a, b) and a fruit (c, d) obtained under natural light (a, c) and the NIR light from LED2 (b, d). The luminescence spectrum of (Gd0.99Bi0.01)2WO6 has a large FWHM and covers a broad region from red to NIR, and thus the phosphor is promising for application in simultaneous white and NIR lighting. Fig. 11a shows the luminescence spectrum of LED2, which was fabricated using (Gd0.99Bi0.01)2WO6, BaMgAl10O17:Eu2+ green phosphor and a 365 nm LED chip. It is seen that the spectrum covers the entire visible range and the NIR region up to 900 nm. The device emits high-quality warm white light (inset of Fig. 11a), which has a relatively low CCT of 3904 K and a high Ra value of 93.7 (Fig. 11b). In addition, all the other R parameters (R1-R15) have satisfactory values (Fig. 11b, c). The optical properties of LED2 under different driving currents (20-100 mA) were measured, and the obtained main parameters are summarized in Table S9. It was found that the CIE chromaticity coordinates of the device slightly drifted from (0.39, 0.39) to (0.37, 0.34) with increasing driving current (Fig. 11d) and, accordingly, CCT slightly increased from 3721 K to 3904 K (Fig. 11e). As the luminescence spectrum of LED2 also covers the NIR region, application of the device in NIR imaging was also explored. Fig. 12 shows the photographs of a finger and a fruit taken under natural light and the NIR light from LED2. It can be seen from Fig. 12a,b that the finger can be noninvasively imaged and recognized, indicating potential application of the device in medical diagnosis. The photograph of the peach 24  can also be clearly captured by a NIR camera (Fig. 12 c and d), implying that the phosphor may also be applicable in night-vision technology. Conclusions Photoluminescence regulation of Bi3+ in Lu2WO6 was achieved via Gd3+ doping, and the effect of Gd3+ on phase composition, crystal structure, crystallite morphology and optical properties were systematically discussed. Application of the obtained typical phosphors in high color rendering index lighting, night vision and noninvasive imaging were also demonstrated. The main conclusions are as follows: (1) Gd3+ doping greatly enhanced the crystallinity of the phosphor. The crystal structure of (Lu0.99-xGdxBi0.01)2WO6 (x = 0-0.25) remained as that of monoclinic Lu2WO6 (P2/c space group) up to x = 0.25 and then changed into that of monoclinic Gd2WO6 (C2/c space group) at the high doping levels of x = 0.75-0.99.  (2) The photoluminescence of Bi3+ was tuned from broadband green to broadband red-NIR light with increasing Gd3+ doping. Three different kinds of Bi3+ centers were proved via crystal structure analysis, spectral analysis and fluorescence decay analysis to be responsible for the observed broadband emissions. White-LED device, with high color rendering index (Ra = 91.3), stable emission color, and low correlated color temperature (3951 K) can be fabricated with the broadband (Lu0.94Gd0.05Bi0.01)2WO6 green phosphor.  (3) LED device that simultaneously emits high color rendering white light and near-infrared light can be obtained with the broadband (Gd0.99Bi0.01)2WO6 red-NIR phosphor for night version and noninvasive imaging applications.   25  Author contributions Xuejiao Wang: conceptualization, funding acquisition, writing – review & editing. Xiaowen Feng: investigation, data curation, writing – original draft. Maxim S. Molokeev: data curation, resources. Huiling Zheng: data curation, resources. Qiushi Wang: investigation, data curation. Chunyan Xu: data curation, resources. Ji-Guang Li: conceptualization, supervision, funding acquisition. All authors contributed to the general discussion. Conflicts of interest There are no conflicts to declare. Acknowledgements This work is supported in part by the Project of Education Department of Liaoning Province (Grant No. LQ2019014) and Natural Science Foundation of Liaoning Province (Grant No. 2020-MS-286). The authors wish to thank the facility’s support of the 4B9A beamline of the Beijng Synchrotron Radiation Facility (BSRF) (Project No. 2021-BEPC-PT-005290). The authors would like to thank Siqi Liu from Shiyanjia Lab (www.shiyanjia.com) for the XPS analysis. References 1. Y. Xiao, W. Xiao, D. Wu, L. Guan, M. Luo and L. Sun, Adv. Funct. Mater., 2022, 32, 2109618. 2. D. Liu, G. Li, P. Dang, Q. Zhang, Y. Wei, H. Lian, M. M. Shang, C. Lin and J. Lin, Angew. Chem. Int. Ed., 2021, 60, 14644-14649. 3. F. He, E. Song, Y. Zhou, H. Ming, Z. Chen, J. Wu, P. Shao, X. Yang, Z. G. Xia and Q. Zhang, Adv. Funct. 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