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## Creator

Zhenqi Song, Fan Li, Sihan Feng, Zhiyuan Pan, Qi Zhu, Xudong Sun, [Ji‐Guang Li](https://orcid.org/0000-0002-5625-7361)

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This is the peer reviewed version of the following article: Systematic synthesis of (Gd1-xLax)2O2SO4:Tb3+ and (Gd1-xLax)2O2S:Tb3+ nanophosphors for remarkably enhanced luminescence, which has been published in final form at https://doi.org/10.1111/jace.19384. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. This article may not be enhanced, enriched or otherwise transformed into a derivative work, without express permission from Wiley or by statutory rights under applicable legislation. Copyright notices must not be removed, obscured or modified. The article must be linked to Wiley’s version of record on Wiley Online Library and any embedding, framing or otherwise making available the article or pages thereof by third parties from platforms, services and websites other than Wiley Online Library must be prohibited.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Systematic synthesis of (Gd1-xLax)2O2SO4:Tb3+ and (Gd1-xLax)2O2S:Tb3+ nanophosphors for remarkably enhanced luminescence](https://mdr.nims.go.jp/datasets/7978ce73-92fc-4122-84b6-89353178f829)

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For Peer Review1Systematic synthesis of (Gd1-xLax)2O2SO4:Tb3+ and (Gd1-xLax)2O2S:Tb3+ nanophosphors for remarkably enhanced luminescenceZhenqi Songa, Fan Lia, Sihan Fenga, Zhiyuan Pana, Qi Zhua,*, Xudong Suna,b, Ji-Guang Lic,*aKey Laboratory for Anisotropy and Texture of Materials and School of Materials Science and Engineering, Shenyang, Liaoning, 110819, ChinabFoshan Graduate School of Northeastern University, Foshan, Guangdong, 528311, ChinacResearch Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, JapanCorresponding authorDr. Qi ZhuNortheastern UniversityShenyang, ChinaE-mail: zhuq@smm.neu.edu.cnTel: +86-24-8367-2700Dr. Ji-Guang LiNational Institute for Materials ScienceTsukuba, JapanE-Mail: li.jiguang@nims.go.jpTel: +81-29-860-4394Page 1 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960mailto:zhuq@smm.neu.edu.cnmailto:li.jiguang@nims.go.jpFor Peer Review2AbstractCoprecipitation with rare-earth nitrate, ammonium sulfate and ammonium hydroxide produced hydroxide-type amorphous precursors incorporating sulfate and carbonate anions, from which [(Gd1-xLax)0.99Tb0.01]2O2SO4 and [(Gd1-xLax)0.99Tb0.01]2O2S (x=0, 0.15, 0.3, 0.5, 0.65, 0.8, 1) were obtained as two series of nanophosphors by calcination at 950°C in air and hydrogen, respectively. The detailed characterization by XRD, SEM/TEM, BET and particle sizing confirmed that solid solutions were directly formed and that the products have small crystallite size, unimodal size distribution and high specific surface area, revealing the advantages of the synthesis method. Photoluminescence study revealed that La3+ admixture may significantly improve the 545 nm main emission of Tb3+ for both the phosphor series. Furthermore, the 545 nm main emission of [(Gd1-xLax)0.99Tb0.01]2O2SO4 was identified to have an excellent thermal stability, which retained over 90% of its room temperature intensity at 150°C (no quenching for Gd2O2SO4:Tb3+). The two series of phosphors were comparatively studied for their excitation and luminescence performances, as a function of temperature and La3+ content, and the results were rationalized by considering bandgap, crystal structure, UV absorption, and the character of chemical bonds.Keywords: (Gd, La)2O2SO4:Tb3+, (Gd, La)2O2S:Tb3+, Solid solution, Nanophosphor, PhotoluminescencePage 2 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review31. IntroductionGd2O2S has an excellent X-ray blocking capacity by its high theoretical density (7.34 g/cm2) and the large atomic number of Gd (Z = 64) and is chemically/structurally stable under high-energy radiation, so it is widely used for scintillation purposes.1,2 The compound is also known to be an excellent host for photoluminescence, because of its wide band gap (~4.6-4.8 eV), low phonon energy (about 520 cm-1), and good tolerance for activator doping.3-5 Tb3+ doped Gd2O2S (Gd2O2S:Tb3+) powder, for example, is widely used as a green phosphor in TV screens, cathode ray tubes and X-ray enhancement screens because of its high intrinsic conversion efficiency and high light yield.6,7 Besides, Gd2O2S:Tb3+ nano-colloid was reported to be a useful fluorescent probe in biomarkers.8 The Gd2O2S:Tb3+ bulk ceramic fabricated via sintering, on the other hand, finds important application in scintillation bio-probing, high-resolution neutron radiography and X-ray computed tomography (X-CT) for its excellent attenuation property, scintillation efficiency, and imaging quality.9-11 Gd2O2S-based phosphors are conventionally synthesized via flux reaction at 1200-1300°C, which uses oxide and/or carbonate as the rare-earth (RE) source, elemental sulfur and/or sodium thiosulfate as the sulfur source and alkali carbonate and/or halide as a reflux.12,13 Though the resulted powder has a high crystallinity because of its micron-sized large particles, contamination by alien cations and anions is hardly evitable. Treating a precursor with a sulfur-bearing gas14,15 or combustion with thiourea as the fuel16 may produce finer powders, but phase/chemical purity control is a frequent problem. Furthermore, the above methods all involved sulfur-containing toxic reactant and/or Page 3 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review4exhaust gas.The luminescence performance and application of Gd2O2S:Tb3+ can be affected by a number of factors, such as particle morphology/granule size, chemical/phase purity, Tb3+ content, and lattice defects. For example, micron-sized particles are favored for phosphor application because of their better luminescence but are not satisfactory for the fabrication of scintillation ceramics because of their low specific surface area and poor sintering activity. It is also known that, by varying the concentration of Tb3+, the emission color of Gd2O2S:Tb3+ can be adjusted by the cross-relaxation process of Tb3+ ions.17 Often, codoping of other ions can significantly affect the performance of Gd2O2S:Tb3+. For example, Dy3+ can remarkably promote Tb3+ luminescence because there is a strong energy transfer from Dy3+ to Tb3+.18 Replacing some of the host Gd3+ ions with aliovalent Ta5+ or Sn4+ may create anion traps and eliminate the cation traps caused by sulfur vacancies and, therefore, enhances the main emission of Gd2O2S:Tb3+.19 Ca2+ and Ru4+ ions also have a strong influence on the defects in Gd2O2S, especially those caused by sulfur loss, even at a small concentration.19 It is noteworthy that sulfur vacancy (Vs) instead of oxygen vacancy (Vo) is the dominant lattice defect in Gd2O2S, particularly in the scintillation ceramics fabricated by high-temperature sintering, since the formation energy of Vs is smaller than that of Vo.20  This is in conformance with the soft-hard acid-base theory (HSAB),21 which predicts that soft Lewis base S2- is less affinitive than O2- toward hard Lewis acid Gd3+. As sulfur vacancies have a huge influence and may even completely quench luminescence,22 suppressing the formation of such defects is essential to a high luminescence Page 4 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review5performance of Gd2O2S-based phosphors/scintillation ceramics.It should be noted that the stability of RE-S bond is significantly dependent on the ionic size of RE3+ (RE = La-Lu lanthanide, Y). Previous synthesis of RE2O2S by calcining the hydrothermally crystallized RE2(OH)4SO4∙nH2O precursor compound in hydrogen (RE2(OH)4SO4∙nH2O + 4H2 →  RE2O2S + (n + 6)H2O) found that the strength of RE-S tends to decrease with decreasing ionic radius of RE3+ and Gd3+ happened to be the boundary for a pure product.23,24 A similar observation was reported by other researchers in RE2O2S synthesis via thermolysis of RE chelate compounds in organic solvent.25 In this regard, doping Gd2O2S with La3+ may strengthen the Gd-S bond, allowing for the fabrication of high-quality phosphors and bulk ceramics through suppressing the formation of sulfur vacancies. Furthermore, La2O2S:Tb3+ itself (theoretical density ~5.83 g/cm3) is an important green phosphor for X-ray excited luminescence (XEL) and cathodoluminescence (CL), and was revealed to be even more efficient than Gd2O2S:Tb3+ for medium X-ray tube voltages.6,26 Application of La2O2S:Tb3+ for temperature sensing under UV excitation was also reported.27 Though the mechanical mixtures of commercial La2O2S:Tb3+ and Gd2O2S:Tb3+ phosphor powders were previously investigated for luminescence,6 a study can hardly be found in the literature for either (Gd, La)2O2SO4:Tb3+ or (Gd, La)2O2S:Tb3+ solid solutions. Although the aforesaid RE2(OH)4SO4∙nH2O compound can serve as an excellent precursor to derive RE2O2SO4 via annealing in air (RE2(OH)4SO4∙nH2O → RE2O2SO4 + (n + 2)H2O; RE = La-Lu lanthanide and Y) and to derive RE2O2S (RE = La-Gd lanthanide) via annealing in hydrogen,23,24,28 it cannot produce rounded nanoparticles because the compound easily crystallizes as submicron/micron sized flakes by its layered crystal structure, and the derived products tend to inherit a flaky particle morphology.23,28 With the simple reactants of RE(NO3)3, (NH4)2SO4 and NH4OH, this Page 5 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review6work produced hydroxide-like amorphous precursors containing both SO42- and CO32- anions via coprecipitation at ~4°C and pH=9 in open air, from which [(Gd1-xLax)0.99Tb0.01]2O2SO4 and [(Gd1-xLax)0.99Tb0.01]2O2S (x = 0, 0.15, 0.3, 0.5, 0.65, 0.8, 1.0) nanophosphors having a spherical particle morphology, good dispersion and narrow size distribution were successfully obtained via calcination at 950°C in air and hydrogen, respectively. The synthesis process avoided the use and emission of any sulfur-bearing hazardous substance and is more efficient than either the hydrothermal 23,28 or the thermolysis25 method. More importantly, the incorporation of La3+ was confirmed to be able to significantly enhance Tb3+ luminescence for both the types of phosphors, and the [(Gd1-xLax)0.99Tb0.01]2O2SO4 series were identified to possess an excellent thermal stability since they retained ~92-100% of their room temperature emission intensities at 150°C. In the following sections, we report the synthesis, characterization, and luminescence properties of the materials.2. Experimental procedure2.1 Raw materials and synthesisThe starting rare-earth materials for the experiments were Gd2O3, La2O3 and Tb4O7 (99.99% pure, Huizhou Ruier Rare Earth High-Tech Co., Ltd., Huizhou, China), which were separately dissolved with 1 mol/L nitric acid (analytical grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) to make nitrate solutions. Ultrapure water (resistivity 18 Mcm) was used throughout the experiments. For powder synthesis, proper portions of the nitrate solutions were mixed together according to the formula of [(Gd1-xLax)0.99Tb0.01]3+ (x=0, 0.15, 0.3, 0.5, 0.65, 0.8 and 1.0), followed by the addition of 15 mmol (NH4)2SO4 (analytical grade, Sinopharm Chemical Reagent; SO42-/[(Gd1-xLax)0.99Tb0.01]3+ = 1.5 molar ratio) and volume adjustment of the reaction system to 100 ml. The mixed solution was cooled to ~4°C in an ice-water bath under Page 6 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review7magnetic stirring, followed by dropwise addition of ammonium hydroxide solution (Sinopharm Chemical Reagent) until pH=9 to obtain precipitate.29 After homogenization under magnetic stirring at ~4°C for 1 h, the precipitate was collected via centrifugation at 4000 rpm for 3 min, washed three times with water and once with anhydrous ethanol, and then dried in an oven at 60°C for 24 h. Finally, the fluffy precipitate (precursor) cake was lightly crushed with a mortar and pestle into a fine powder for characterization and further processing. [(Gd1-xLax)0.99Tb0.01]2O2SO4 was produced by calcining the precursor powder in air at 950°C for 1 h and [(Gd1-xLax)0.99Tb0.01]2O2S was obtained by calcining [Gd1-xLax)0.99Tb0.01]2O2SO4 in hydrogen at the same temperature for 2 h, where a heating rate of 5°C /min was used for the rising phase of calcination.2.2. Characterization techniquesPhase identification of the product was performed with an X-ray diffractometer (XRD; Model SmartLab, Rigaku, Tokyo, Japan) in the 2θ range of 10-70o, using nickel-filtered Cu Kα line (λ= 0.15406 nm) as the radiation source, an acceleration voltage/current of 40 kV/40 mA and a scan rate of 10o 2θ/min. Fourier transform infrared spectroscopy (FTIR; Model Nicolet iS5, Thermo Fisher Scientific, Madison, USA) was conducted by the standard KBr pellet method. The morphology and fine structure of the particles were analyzed using field emission scanning electron microscopy (FESEM, Model JSM-7001F, JEOL, Tokyo) at an acceleration voltage of 15 kV and transmission electron microscopy (TEM; Model FEI Talos F200X G2, Thermo Fisher Scientific) at 200 kV. Photoluminescence studies were performed in the temperature range of 25-250°C with an FP-8600 fluorescence spectrophotometer (JASCO, Tokyo), which was equipped with a Model ISF 513 (JASCO) integrating sphere ( 60 mm in diameter), a 150 W xenon lamp for excitation and a Model HPC-836 accessory (JASCO) for temperature control, and all the measurements were conducted Page 7 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review8at a scan speed of 100 nm/min and a slit width of 5 nm. The spectral response of the spectrophotometer was corrected with a Rhodamine-B solution (5.5 g/L in ethylene glycol) and a standard light source unit (ECS-333, JASCO) for the ranges of 220–600 nm and 350–850 nm, respectively, and the quantum yield of luminescence was read out by a built-in analysis software of the equipment. The light absorption spectra of the samples were recorded using a UV-Vis-NIR spectrophotometer (Model UV-3600 Plus, Shimadzu, Kyoto, Japan). The specific surface area of the calcination product was measured by the Brunauer-Emmett-Teller (BET) method via nitrogen adsorption at 77 K, using a Model TriStar II 3020 instrument (Micromeritics, GA, USA). Particle sizing was conducted with a laser diffraction particle sizer (Model Zetasizer Nano ZS90, Malvern, UK), and the suspension for analysis was prepared by ultrasonic dispersion of 2.5 mg powder in 20 mL ethanol in the absence of surfactant.3. Result and discussionFIGURE 1 XRD patterns (A) and FTIR spectra (B) for the typical precursors of x = 0, 0.5 and 1.0.Figure 1A shows the XRD patterns of three typical precursors (x = 0, 0.5, 1.0) obtained by the coprecipitation method, from which it is seen that they all exhibited an amorphous character. To assess chemical composition, the precursors were subjected to FTIR analysis, and the results are exhibited in Figure 1B. It was found that the three samples have virtually the same spectral features, and all showed a broad and strong Page 8 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review9absorption in the ~3000-3750 cm-1 region for the vibration of hydroxyl groups (~3500-3750 cm-1) and the stretching of O-H bond in water molecules (~3000-3500 cm-1). The shallow peak at ~1644 cm-1 further proved the presence of molecular water and can be assigned to the H-O-H bending mode of H2O. SO42- absorptions are clearly identifiable at ~1100, 980, and 610 cm-1, which correspond to v3, v1, and v4 vibrations, respectively.30 The other three absorption peaks at ~1518, 1398 and 845 cm-1 were caused by CO32- 29,30 owing to solution absorption of ambient CO2. The results thus indicated that the precursors would be Ln2(OH)4-2m(CO3)m(SO4)∙nH2O (Ln = (Gd1-xLax)0.99Tb0.01), where the SO42-/[(Gd1-xLax)0.99Tb0.01]3+ molar ratio was assigned to be 1/2 because all the precursors directly transformed into [(Gd1-xLax)0.99Tb0.01]2O2SO4 by annealing in air at 950°C. The precursor of this work can be viewed as a derivative of the aforesaid RE2(OH)4SO4∙nH2O sulfate hydroxide since its composition can be resulted by replacing a part of the hydroxyls in RE2(OH)4SO4∙nH2O with CO32- anions. FE-SEM observation (Figure S1) showed that all the three samples, unlike the hydrothermally produced RE2(OH)4SO4∙nH2O, are loose agglomerates of nanosized spherical particles, which is beneficial to derivation of dispersed nanophosphors by calcination. Previous analysis via thermogravimetry in flowing air (heating rate 10°C /min) showed that RE2(OH)4SO4∙nH2O would yield RE2O2SO4 in the temperature ranges of ~405-1300°C for RE = La, ~440-1120°C for RE = Gd and ~450-1110°C for RE = Tb,23 and RE(OH)CO3 would decompose into RE2O3 via removal of hydroxyls and then carbonate anions up to ~800°C.31 We thus selected 950°C as a suitable annealing temperature to derive [(Gd1-xLax)0.99Tb0.01]2O2SO4 from the Ln2(OH)4-2m(CO3)m(SO4)∙nH2O precursors.Figure 2A shows the XRD patterns of the products after calcination in air at 950°C, and it was found that they can all be indexed with the standard diffractions of monoclinic Gd2O2SO4 (JCPDS No.24-9775). The XRD spectra steadily drifted toward Page 9 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review10lower diffraction angles with increasing x (La3+ content), which is due to more replacement of Gd3+ by larger La3+ (1.10 Å for La3+ and 1.00 Å for Gd3+ under 7-fold coordination). We analyzed the XRD patterns with the Jade 6 software, and found that lattice constants a, b and c and cell volume V all almost linearly increase with increasing x value. This clearly indicated that (Gd, La)2O2SO4:Tb3+ solid solutions have been formed. Broadening analysis of the XRD peaks with Scherrer formula revealed that the x = 0, 0.15, 0.3, 0.5, 0.65, 0.8 and 1.0 products have average crystallite sizes of ~31.3, 30.8, 28.7, 27.4, 20.7, 22.6, and 76.5 nm, respectively. It is seen that, despite the same calcination condition, the average crystallite size of La2O2SO4:Tb3+ (x = 1.0) is much larger than that of Gd2O2SO4:Tb3+ (x = 0). This is due to the fact that the hydroxyls and carbonate anions in the precursor of La2O2SO4:Tb3+ can be removed at lower temperatures,23 which allowed more growth of the La2O2SO4:Tb3+ crystallites. Meanwhile, the crystallite sizes of the other samples are smaller than those of the x = 0 and 1.0 ones. This could be due to higher degrees of lattice distortion by La3+ substitution, which caused more broadening of the XRD peaks. FIGURE 2 XRD patterns (A) and lattice parameters and cell volume (B) of the powders obtained by calcining the precursors at 950°C in air.  FE-SEM observation of the three typical [(Gd1-xLax)0.99Tb0.01]2O2SO4 powders (x = Page 10 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review110, 0.5, 1.0) indicated that they similarly contain porous agglomerates of primary particles/crystallites (Figure 3A-C) but the La2O2SO4:Tb3+ sample appeared coarser. BET analysis found that the x = 0, 0.5 and 1.0 powders have specific surface areas (SBET) of ~11.43, 10.7 and 7.12 m2/g. The sphere-equivalent particle size Ds (nm) of a powder can be derived via equation Ds = 6000/(SBET×D), where D is the theoretical density (g/m3) of the material. The x = 0, 0.5 and 1.0 compositions were calculated to have D values of 6.80, 6.15 and 5.50 g/m3 with the monoclinic unit cell and their cell volumes (Figure 2B) and, therefore, have Ds values of 77, 90 and 153 nm, respectively. The Ds of La2O2SO4:Tb3+ (x = 1.0) is much larger than those of the other two, and this conforms to the results of FE-SEM observation (Figure 3A-C). We also analyzed the particle size distribution of these three powders via laser diffraction, and the results are exhibited in Figure 3D-F. It is seen that they favorably have a unimodal size distribution in each case and have peak sizes of ~250, 215 and 257 nm for x = 0, 0.5 and 1.0, respectively.FIGURE 3 FE-SEM morphologies (A-C) and particle size distributions (D-F) for the three typical [(Gd1-xLax)0.99Tb0.01]2O2SO4 powders of x = 0 (A, D), x = 0.5 (B, E) and x = 1.0 (C, F).Page 11 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review12FIGURE 4 The photoluminescence spectra of (Gd1-xLax)2O2SO4:Tb3+, where (A) is for excitation (PLE, λem = 545 nm) and (B) and (C) are for emission (PL) under 228 and 275 nm excitation, respectively. Part (D) presents the CIE color coordinates for the luminescence under 228 nm excitation. The insets in parts (B) and (C) are relative intensities of the 417 and 545 nm emissions, where the 545 nm emission intensity of the x = 0 sample was normalized to 1.Figure 4A shows the excitation spectra (PLE) of the series of [(Gd1-xLax)0.99Tb0.01]2O2SO4 phosphor powders, which were obtained by detecting the 545 nm green emission of Tb3+. The double split peaks centered at ~211 and 228 nm were assigned to the low-spin and high-spin 4f8-4f75d1 transitions of Tb3+, respectively. Host excitation was not considered herein since the materials did not show light absorption above 200 nm (bandgap >6.2 eV, Figure S2). The bandgap energy of either La2O2SO4 or Gd2O2SO4 has not been reported before to best of our knowledge. The 8S7/2-6IJ and 8S7/2-6PJ intra-4f7 transitions of Gd3+ were also observed at ~275 and 311 nm32 for the Gd3+-containing samples, respectively. The presence of Gd3+ excitation implies the Page 12 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review13occurrence of Gd3+→Tb3+ energy transfer.33 It is clear that the 4f8-4f75d1 excitation of Tb3+ monotonically gained intensity with increasing La3+ content. This could be due to gradually stronger ionicity of the host lattice, since the electronegativity of La (χ = 1.1) is smaller than that of Gd (χ = 1.2), which improved the efficiency of excitation through decoupling Tb3+ electrons with host lattice. Figure 4B shows the emission spectra (PL) obtained under 228 nm excitation of the powders, where the 5D3,4-7FJ (J = 3-6) luminescence of Tb3+ can be readily identified. The intensity of PL successively increased with increasing La3+ content, following the trend observed from the 4f8-4f75d1 excitation intensity, and the 545 nm main emissions of the x = 0.15 and 1.0 compositions are ~2.5 and 4.2 times as strong as that of Gd2O2SO4:Tb3+ (x = 0, the inset of Figure 4B). The luminescence of Tb3+ is mediated by the host Gd3+ ions in this work (except for the case of x =1.0), which involves relaxation of the 228 nm excitation energy to the 6IJ/6PJ levels of Gd3+ followed by energy transfer to the 5D3,4 states of Tb3+. In this regard, the enhanced PL by La3+ doping is mainly owing to reduced consumption of the 228 nm excitation energy by Gd3+, which includes energy migration among the Gd3+ ions themselves, non-radiative relaxation, and 6PJ → 8S7/2 emission (~312 nm). Noteworthy is that the luminescence from 5D3 energy level is quite strong (bule/green intensity ratio I417/I545 = 0.41, I438/I545 = 0.34). This implies that Tb3+(5D3) + Tb3+(7F6) = Tb3+(5D4) + Tb3+(7F0) cross relaxation, which quenches 5D3 emission, is not significant. The powders were assayed from their PL spectra to have CIE color coordinates of around (0.26, 0.40), corresponding to a cyan color (Figure 4D). We also analyzed the luminescence through exciting the host Gd3+ ions with 275 nm UV light Page 13 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review14(8S7/2-6IJ transition), and the results are presented in Figure 4C. It is seen that the PL spectra are essentially identical to those of Figure 4B, but the intensities of both the 5D3-7FJ and 5D4-7FJ emissions first increase with increasing La3+ doping up to x = 0.3 and then steadily decrease. Such a phenomenon may be explained by considering that (1) replacing Gd3+ with a proper amount of La3+ (x up to ~0.3) reduces the number density of Gd3+ ions, which alleviates the aforesaid energy consumption by Gd3+ and, therefore, enhances Gd3+ → Tb3+ energy transfer, and (2) a high enough La3+ content (x > ~0.3) may elongate the separation distance of neighboring Gd3+ and Tb3+ ions to such an extent that the efficiency of Gd3+→Tb3+ energy transfer decreases.Page 14 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review15FIGURE 5 Temperature-dependent PL spectra (A-C) and relative intensities of the 417 and 545 nm emissions as a function of measurement temperature (D-F) for the x = 0 (A, D), x = 0.5 (B, E) and x = 1.0 (C, F) typical phosphors. The insets in (D-F) show the plots for determination of the activation energy of thermal quenching.   The thermal stability of luminescence was analyzed for the three typical phosphors of x = 0, 0.5 and 1.0, and the results are displayed in Figure 5. It was observed from the temperature-dependent PL spectra that raising the measurement temperature from 25 to 250°C did not induce any new emission and did not alter the position of each existing peak (Figure 5A-C). Of particular interest is that both the 545 nm green and 417 nm blue emissions showed high stability against temperature increase, and respectively retained over 90% and 80% of their room temperature intensities at 150°C (Figure 5D-F), a temperature frequently used to evaluate whether a phosphor is suitable for high power LED application. Especially, the Gd2O2SO4:Tb3+ phosphor (x = 0) almost has no Page 15 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review16intensity loss up to 150°C for the 545 nm emission, and the 417 nm emission kept as high as ~92.6% of its room temperature intensity at 150°C. Such an excellent thermal stability implies that thermal ionization of Tb3+ (promotion of Tb3+ electrons into conduction band) hardly took place owing to the wide bandgap (above ~6.2 eV) and high structure stiffness (low phonon-energy) of (Gd, La)2O2SO4. Monoclinic RE2O2SO4 (space group: C2/c) has a unique layered crystal structure, which is constructed by alternative stacking of [RE2O2]2+ main layers and interlayer SO42-. 23,24,28 In such a structure, each [SO4] tetrahedron provides three O atoms to form REO7 one-capped trigonal prim that serves as the building block of the main layer. The strong pillaring effect of interlayer SO42- may restrain thermal vibration of the RE containing main layers to render a high structure rigidity. It was observed that the 417 nm emission lost intensity slightly faster than the 545 nm one with increasing temperature in each case, which could be due to thermal relaxation of some of the 5D3 electrons to the 5D4 level of Tb3+. Besides, the thermal stability of luminescence tends to decrease with increasing La3+ content, and this might be due to the lattice expansion (Figure 2B; lattice softening) caused by La3+ doping. The activation energy of thermal quenching (Ea) can be derived from the following equation34: I(T) = I0/[1 + cexp(-Ea/(kT)]                 (1)where I0 and I(T) are the luminous intensities at room temperature and measurement temperature T (in Kelvin), respectively, k is the Boltzmann constant (8.617 × 10-5 eV), and c is a pre-exponential constant. The insets in Figure 5D-F show Ln(I0/I–1) versus 1/(kT) transformation of the experimental data, and linear fitting yielded Ea values of Page 16 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review17~0.492, 0.140 and 0.105 eV for the 545 nm emission and ~0.156, 0.155 and 0.148 eV for the 417 nm emission of the x = 0, 0.5 and 1.0 samples, respectively. It should be noted that the Ea of the 545 nm emission of Gd2O2SO4:Tb3+ (x = 0) was determined with the temperature range where thermal quenching of luminescence took place (175-250°C).FIGURE 6 The XRD patterns (A) and lattice parameters and theoretical densities (B) of the powders calcined at 950°C in H2.Figure 6A shows the XRD patterns of the products after calcination at 950°C in hydrogen, where it can be found that the diffraction peaks can be indexed with those of the hexagonal structured Gd2O2S (P-3m1 space group; JCPDS No. 26-1422) in each case and the peaks gradually shifted toward lower diffraction angles with increasing La3+ doping (x value). Analysis of the XRD patterns with the Jade 6 software confirmed the formation of solid solution, since lattice constants a and c (a = b) and cell volume V almost linearly expanded with increasing x. Broadening analysis of the XRD peaks Page 17 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review18with Scherrer formula found that the x = 0, 0.15, 0.3, 0.5, 0.65, 0.8 and 1.0 powders have average crystallite sizes of ~30, 24.9, 24.2, 21.5, 17.7, 15.5 and 42.2 nm, respectively. Such a tendency of size variation follows that of the (Gd, La)2O2SO4:Tb3+ series, but in each case the average crystallite size of (Gd, La)2O2S:Tb3+ is smaller than that of (Gd, La)2O2SO4:Tb3+. This is due to disintegration of the (Gd, La)2O2SO4:Tb3+ crystallites by the occurrence of (Gd, La)2O2SO4:Tb3+ + 4H2 → (Gd, La)2O2S:Tb3+ + 4H2O reduction reaction and reconstructive phase transition. FIGURE 7 FE-SEM morphologies (A-C) and particle size distributions (D-F) for the (Gd1-xLax)2O2S:Tb3+ powders of x = 0 (A, D), x =0.5 (B, E) and x=1.0 (C, F). FE-SEM analysis showed that the three typical (Gd1-xLax)2O2S:Tb3+ powders, like their (Gd1-xLax)2O2SO4:Tb3+ counterparts (Figure 3A-C), are porous agglomerates of the primary particles/crystallites (Figure 7A-C). Laser diffraction particle sizing confirmed that the three powders also have unimodal size distributions, which are centered at ~256, 197 and 233 nm for x = 0 (Figure 7D), x = 0.5 (Figure 7E) and x = 1.0 (Figure 7F), respectively. Such a size distribution would be beneficial to the construction of a uniform phosphor screen for scintillation and cathodoluminescence Page 18 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review19and to the sintering of high-quality ceramic scintillators. BET analysis found that the x = 0, 0.5 and 1.0 powders have specific surface areas of ~17.3, 16.7 and 15.7 m2/g, which correspond to sphere-equivalent average particle sizes of ~47, 55 and 66 nm, respectively, according to the theoretical densities shown in Figure 6B.FIGURE 8 TEM morphologies (A-C) and lattice images (D-F) for the (Gd1-xLax)2O2S:Tb3+ powders of x = 0 (A, D), x = 0.5 (B, E) and x = 1.0 (C, F). Part (G) shows the results of elemental mapping for the x = 0.5 sample, and the insets in parts (D-F) are the corresponding SAED patterns.TEM analysis was performed for the three typical (Gd1-xLax)2O2S:Tb3+ samples to understand finer microstructure, and the results are shown Figure 8A-F. Low magnification observation showed that a number of the primarily crystallites were glued together to form small clusters (Figure 8A-C), which explains why the particle size revealed by laser diffraction is much larger than the average crystallite size assayed via Scherrer equation. The individual crystallites (~20-50 nm) tend to be faceted, implying Page 19 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review20a high degree of crystallization, as also revealed by the clear lattice fringes (Figure 8D-F) and the spot-like selected area electron diffractions (SAED, the insets in Figure 8D-F). Analyzing the lattice fringes found that the (100) and (101) crystal planes of Gd2O2S:Tb3+ (x = 0) have d-spacings of ~0.329 and 0.292 nm, respectively, which are smaller than the ~0.336 nm for (100) and ~0.300 nm for (101) of (Gd0.5La0.5)2O2S:Tb3+ (x = 0.5). This indicates lattice expansion by La3+ doping. Elemental mapping with the (Gd0.5La0.5)2O2S:Tb3+ midway composition showed that the constituent elements of Gd, La, Tb, O and S are all uniformly distributed across the crystallites (Figure 8G). The results thus provided firm evidence for the formation of solid solution. Besides, the (101), (102), (110) and (103) planes of the La2O2S:Tb3+ (x = 1) end composition can be clearly identified via lattice imaging and SAED analysis, and the measured d-spacings of ~0.310, 0.2480, 0.200 and 0.194 nm are close to the 0.3128, 0.2470, 0.2024 and 0.1938 nm reported in the standard diffraction file of La2O2S (JCPDS No. 27-0623), respectively.Page 20 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review21FIGURE 9 PLE (A), PL (B) and UV-vis absorption (C) spectra and the CIE color coordinates of luminescence (D) for the series of (Gd1-xLax)2O2S:Tb3+ phosphors.Figure 9A shows the PLE spectra of the series of (Gd1-xLax)2O2S:Tb3+ phosphors (λem = 545 nm), where a broad band up to ~325 nm is clearly seen in each case. The band is overlapped from host excitation (centered at ~272 nm; promotion of electrons from the valence band to the conduction band of (Gd1-xLax)2O2S) and 4f8 → 4f75d1 inter-configurational transition of Tb3+ (at ~290 nm).35 The appearance of 4f8 → 4f75d1 excitation as a sub-band implies that the centroid of the 5d energy level of Tb3+ is close to the bottom of the conduction band of (Gd1-xLax)2O2S. The 8S7/2-6IJ and 8S7/2-6PJ transitions of Gd3+, which occurred at ~275 and 311 nm in (Gd1-xLax)2O2SO4:Tb3+, respectively, were buried in the wide and strong excitation band and are not identifiable. Noteworthy is that the intensity of host excitation steadily gained intensity and the 4f8 → 4f75d1 excitation of Tb3+ gradually lost intensity against host excitation with increasing La3+ content. Figure 9B shows the PL spectra of (Gd1-xLax)2O2S:Tb3+, which were taken under host excitation at 272 nm. It is clear that, as observed from the (Gd1-xLax)2O2SO4:Tb3+ series in Figure 4B, each of the 5D4-7FJ (J = 3-6) emissions of Tb3+ monotonically gained intensity at a higher La3+ content, following the tendency observed from that of host excitation, and the 545 nm main emission (5D4-7F5 transition) Page 21 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review22of La2O2S:Tb3+ (x = 1.0) is ~2.5 times as strong as that of Gd2O2S:Tb3+ (x = 0). The 5D3-7FJ luminescence of Tb3+, on the contrary, gradually lost intensity with increasing La3+ substitution, and was completely quenched when x reached ~0.5 (the inset in Figure 9B). The series of phosphors were analyzed to have quantum yields (QYs) of ~31.6, 30.9, 32.8, 32.6, 39.3, 47.0 and 58.6% for x = 0, 0.15, 0.3, 0.5, 0.65, 0.8 and 1.0, respectively. Noteworthy is that the QYs of the (Gd1-xLax)2O2SO4:Tb3+ series cannot be credibly determined, since the most effective excitation wavelength of 228 nm is very close to the shorter end of the corrected spectral region (220-850 nm). It is also seen from Figure 9D that the (Gd1-xLax)2O2S:Tb3+ phosphors emit a green color in each case, but the CIE color coordinates slightly drifted toward the green corner with increasing x, especially for the x = 0-0.5 samples, owing to the gradually weaker 5D3-7FJ emissions. To understand the effect of La3+ on excitation and emission, UV-vis spectroscopy was conducted on the series of (Gd1-xLax)2O2S:Tb3+ phosphors and the results are shown in Figure 9C. It is seen that the capability of UV absorption (up to ~325 nm, peaking at ~254 nm) is steadily stronger with increasing La3+ content. This indicates that La2O2S has a larger absorption coefficient than Gd2O2S, though data are not available from the literature, and enhanced UV absorption is the main reason for the gradually higher intensities of host excitation and 5D4-7FJ luminescence. Though credible determination of bandgap energy (Eg) is difficult with the absorption spectra, owing to the mixing in of Tb3+ transition (Figure 9A), theoretical analysis indicated that Gd2O2S has an Eg of ~4.2-4.6 eV36,37 and La2O2S has a smaller value of ~2.91 Ev.38 It can then be inferred that the Eg of (Gd, La)2O2S will decrease with increasing La3+ content, owing to Page 22 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review23downward shifting of the bottom of the conduction band (CB). This will in turn causes the 5d energy level and 5D3 state of Tb3+ to move closer to the edge of CB. As a result, 5D3 electrons may move into CB more easily under thermal fluctuation, and this explains the gradually weaker and eventually quenched 5D3-7FJ luminescence with increasing La3+ content (Figure 9B). The lack of 5D3 emissions in La2O2S:Tb3+ crystal was indeed reported to be due to the burying of 5D3 level in CB.39FIGURE 10 Temperature-dependent PL spectra (A-C; λex = 272 nm) and determination of the activation energy of thermal quenching for the 545 nm emission of the x = 0, 0.5 and 1.0 phosphors (D). The insets in parts (A)-(C) show relative intensity of the 545 nm emission as a function of measurement temperature.Figure 10A-C exhibits the temperature-dependent PL spectra for the three (Gd1-xLax)2O2S:Tb3+ phosphors of x = 0, 0.5 and 1.0. Though new emission did not appear in the measured temperature range of 25-250°C, the 5D4-7FJ luminescence was steadily weakened with increasing temperature, and the 545 nm emission retained ~68% (x = Page 23 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review240), 70% (x = 0.5) and 44% (x = 1.0) of its room temperature intensity at 150°C. While 5D3-7FJ luminescence is hardly observable for the x = 0.5 and 1.0 samples, that of the x = 0 sample gradually lost intensity at a higher temperature and was almost completely quenched at ~125°C owing to thermal promotion of 5D3 electrons to the conduction band as aforesaid. Comparing Figure 10A-C with Figure 5D-F showed that the (Gd1-xLax)2O2S:Tb3+ phosphors have a significantly faster rate of luminescence quenching than (Gd1-xLax)2O2SO4:Tb3+ in each case. This is primarily owing to the smaller bandgap and more covalent chemical bonds of (Gd1-xLax)2O2S. Though 7-fold coordinated in both the types of compounds, (Gd, La)3+ is bonded with seven O atoms in (Gd1-xLax)2O2SO4 but with four O and three S atoms in (Gd1-xLax)2O2S.24,28 The much smaller electronegativity of S (χ = 2.58 for S and χ = 3.44 for O) makes (Gd1-xLax)2O2S remarkably more covalent (less ionic) than (Gd1-xLax)2O2SO4, which enhances electron-host coupling and lowers the thermal stability of luminescence through raising the possibility/intensity of photon-phonon interaction. It is also seen from the insets of Figure 10A-C that La2O2S:Tb3+ (x = 1.0) has the lowest thermal stability among the three (Gd1-xLax)2O2S:Tb3+ phosphors. Aside from the considerations from cell volume (lattice stiffness), bandgap and bond covalency, thermal promotion of 5D4 electrons into the conduction band could also be a possible reason for the severe quenching of La2O2S:Tb3+, since the 5D4 excited state of Tb3+ lies only ~0.4 eV below the bottom of CB, as roughly estimated with the theoretical bandgap energy of La2O2S (2.91 eV)38 and the emission wavelength of 5D4 → 7F6 transition (492 nm, 2.52 eV). With the experimental data shown in Figure 9A-C, the activation energies (Ea) of thermal Page 24 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review25quenching were assayed by linear fitting of the Ln[I0/I(T)-1] versus 1/(kT) plots to be ~0.336, 0.298 and 0.311 eV for the x = 0, 0.5 and 1.0 phosphors, respectively. Though Ea values of ~0.31 eV40 and 0.25 eV41 were reported for Gd2O2S:Tb3+ (x = 0), our literature survey failed to find Ea data for the x = 0 and 0.5 phosphors.4. ConclusionThe series of precursors obtained by coprecipitation have the general chemical formula of [(Gd1-xLax)0.99Tb0.01]2(OH)4-2m(CO3)m(SO4)∙nH2O (x = 0-1.0), which yielded [(Gd1-xLax)0.99Tb0.01]2O2SO4 by calcination in air at 950°C for 1 h. Calcining [(Gd1-xLax)0.99Tb0.01]2O2SO4 in hydrogen at 950°C for 2 h then produced [(Gd1-xLax)0.99Tb0.01]2O2S. Both the series of powders are nanocrystalline solid solutions having high specific surface area and unimodal particle-size distribution. The [(Gd1-xLax)0.99Tb0.01]2O2SO4 nanophosphors showed strong 5D3,4-7FJ (J = 3-6) luminescence under 228 nm excitation (4f8 → 4f75d1 transition of Tb3+), which steadily gained intensity at a high La3+ content. The 545 nm main emission of these phosphors has a high thermal stability and kept over 90% of its room-temperature intensity at 150°C. The series of [(Gd1-xLax)0.99Tb0.01]2O2S nanophosphors showed strong 5D4-7FJ but much weaker 5D3-7FJ luminescence under 272 nm excitation (host excitation). The intensity of 5D4-7FJ luminescence successively increased while that of 5D3-7FJ decreased with increasing La3+ content, and the emission from 5D3 was completely quenched when x reached ~0.5. [(Gd1-xLax)0.99Tb0.01]2O2S has a much lower thermal stability than its [(Gd1-xLax)0.99Tb0.01]2O2SO4 counterpart, and that of La2O2S:Tb3+ is the lowest among the typical compositions of x = 0, 0.5 and 1.0. The observed results of luminescence Page 25 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review26were explained by considering bandgap, crystal structure, excitation absorption, and the character of chemical bonds.Declaration of Competing Interest 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.AcknowledgmentThis work is supported in part by the National Natural Science Foundation of China (Grant No. 52172112, 51972047).References1. Liu Q, Pan HM, Chen X, Li XY, Liu X, Li W, et al. Gd2O2S:Tb scintillation ceramics fabricated from high sinterability nanopowders via hydrogen reduction. Opt Mater. 2019;94:299-304.2. Wang XJ, Meng QH, Li MQ, Wang XJ, Wang ZH, Zhu Q, et al. 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X-ray scintillator Gd2O2S:Tb3+ materials obtained by a rapid and cost-effective microwave-assisted solid-state Page 26 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960https://xueshu.baidu.com/s?wd=author%3A%28L%20Hern%C3%A1ndez-Adame%29%20&tn=SE_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight%3Dpersonhttps://xueshu.baidu.com/s?wd=author%3A%28A%20M%C3%A9ndez-Blas%29%20&tn=SE_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight%3Dpersonhttps://xueshu.baidu.com/s?wd=author%3A%28J%20Ruiz-Garc%C3%ADa%29%20&tn=SE_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight%3Dpersonhttps://xueshu.baidu.com/s?wd=author%3A%28JR%20Vega-Acosta%29%20&tn=SE_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight%3Dpersonhttps://xueshu.baidu.com/s?wd=author%3A%28FJ%20Medell%C3%ADn-Rodr%C3%ADguez%29%20&tn=SE_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight%3Dpersonhttps://xueshu.baidu.com/s?wd=author%3A%28G%20Palestino%29%20&tn=SE_baiduxueshu_c1gjeupa&ie=utf-8&sc_f_para=sc_hilight%3DpersonFor Peer Review27synthesis. 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Controllable synthesis of monodispersed middle and heavy rare earth oxysulfide nanoplates based on the principals of HSAB theory. Acta Chim Sin. 2013;71:360-6.26. Sklensky AF, Buchanan RA, Maple TG, Bailey HN. Quantum Utilization in X-Ray Intensifying Screens. IEEE Trans Nucl Sci. 1974;21:685-91.27. Yap SV, Ranson RM, Cranton WM, Koutsogeorgis D. Decay time characteristics of La2O2S: Page 27 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review28Eu and La2O2S:Tb for use within an optical sensor for human skin temperature measurement. Appl Optics. 2008;47:4895.28. Wang XJ, Li J-G, Molokeev MS, Zhu Q, Li XD, Sun XD. Layered hydroxyl sulfate: Controlled crystallization, structure analysis, and green derivation of multi-color luminescent (La,RE)2O2SO4 and (La,RE)2O2S phosphors (RE = Pr, Sm, Eu, Tb, and Dy). 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J Mater Chem. 2012;22:15183-9.Page 28 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960For Peer Review1Supporting InformationSystematic synthesis of (Gd1-xLax)2O2SO4:Tb3+ and (Gd1-xLax)2O2S:Tb3+ nanophosphors for remarkably enhanced luminescenceZhenqi Songa, Fan Lia, Sihan Fenga, Zhiyuan Pana, Qi Zhua,*, Xudong Suna,b, Ji-Guang Lic,*aKey Laboratory for Anisotropy and Texture of Materials and School of Materials Science and Engineering, Shenyang, Liaoning, 110819, ChinabFoshan Graduate School of Northeastern University, Foshan, Guangdong, 528311, ChinacResearch Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, JapanCorresponding authorDr. Qi ZhuNortheastern UniversityShenyang, ChinaE-mail: zhuq@smm.neu.edu.cnTel: +86-24-8367-2700Dr. Ji-Guang LiNational Institute for Materials ScienceTsukuba, JapanE-Mail: li.jiguang@nims.go.jpTel: +81-29-860-4394Page 29 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960mailto:zhuq@smm.neu.edu.cnmailto:li.jiguang@nims.go.jpFor Peer Review2Fig. S1. FE-SEM morphologies of the three typical precursors of x = 0 (A), 0.5 (B) and 1.0 (C).Fig. S2. UV-vis absorption spectra of the series of (Gd, La)2O2SO4:Tb3+ powders.Page 30 of 30Journal of the American Ceramic SocietyJournal of the American Ceramic Society123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960