ABO4 type scheelite phases in (Ca/Sr)MoO4 - BiVO4 - Bi2Mo3O12 systems: synthesis, structure and optical properties Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(2), № 20218204 eISSN 2411-1414; chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.2.04 1 of 7 ABO4 type scheelite phases in (Ca/Sr)MoO4 - BiVO4 - Bi2Mo3O12 systems: synthesis, structure and optical properties Z.A. Mikhaylovskaya a,b,* , E.S. Buyanova b , S.A. Petrova c , A.V. Klimova a,b a: Zavaritsky Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, 15 Ak. Vonsovskogo st., Ekaterinburg, 620016, Russia b: Ural Federal University, 19 Mira st., Ekaterinburg, 620002, Russia c: Institute for Metallurgy of the Ural Branch of the Russian Academy of Sciences, 101 Amundsena st., Ekaterinburg, 620016, Russia * Corresponding author: zozoikina@mail.ru This article belongs to the regular issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract The cation deficient complex oxides of (Ca/Sr)MoO4 - BiVO4 - Bi2Mo3O12 triple system are promising photocatalysts and pigments. Compounds with general formula of Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4 were synthesized by convention solid state technique in the range of 550-720 °C. Two wide regions of the solid solutions (ordinary and superstructured scheelite-type phases re- spectively) were found for each system. The diffuse scatering of ho- mogeneous samples was investigated in the range of 190-1100 nm. Energy gaps calculated with linear approximation of Kubelka-Munk function decreases with bismuth and vanadium content. Keywords strontium bismuth molybdate calcium bismuth molybdate Kubelka-Munk method energy gap Received: 25.03.2021 Revised: 12.04.2021 Accepted: 28.04.2021 Available online: 29.04.2021 1. Introduction Materials based on CaMoO4 and SrMoO4 are of interest for science and technology as catalysts and photocatalysts, scintillation detectors, solid-state lasers, pigments as well as for using in photoluminescent and microwave devices due to a wide variety of functional properties [1-8]. The latter strongly depend on not only the nature of dopants [2-3] but also on synthesis techniques employed [2,3,8] and a place of doping. The doping in different sublattice (Ca/Sr or Mo) and varying parameters of synthesis (tem- peratures, irradiation time, pH, speed of mixture, etc.) may cause distortion of MoO4 polyhedra. This distortion, in turn, affects such physical and chemical properties as photocatalytic activity [8-10], optical and luminescence properties [2-3,11], conductivity [12], etc. The most common way of substitution in ABO4 complex oxides with scheelite structure is doping A-sublattice by trivalent Me 3+ cations [3-4,6-7,13]. In this case, three basic charge compensation mechanisms are possible in ABO4 scheelites: (1) formation of oxide ion interstitials ( A1-xMxBO4+x/2) [12]; (2) co-substitution on A or B sites by subvalent cations (A1-2xMe 3+ xMe + xO4 or A1-xMe 3+ xB1-xMe 5+ xO4) [16-17,6]; (3) formation of cation vacancies (A1-3xM2xФxBO4) [13-17]. Mechanism (3) was re- ported for rare earth molybdates Ln2Mo3O12 (Ln2/3MoO4, x=1/3) with scheelite-type structure [14] and for a small number of completely investigated A1−3xM2xФxMoO4 series where 0 < x ≤ 1/3 [13,17]. But cation vacancies (Ф) and their ordering can influence not only structure, but also the physical and chemical properties of the molybdates, and, therefore, we pay close attention to them in the pre- sent work. The simultaneous use of mechanisms (2) and (3) have not been described yet. For this reason the pre- sent work is devoted to the synthesis and characterization Bi and V co-doped CaMoO4 and SrMoO4 obtained by mech- anisms (2) and (3). Previously, Sleight et al. [18] reported the x = 0.04 compositions of (Ca/Sr)1−3xBi2xФxMoO4 series with the te- tragonal scheelite structure (sp. g. I41/a). Sleight assumed that solid solution limit could apparently go all the way to 0.333. The structure, microwave dielectric properties, conductivity and photocatalytic activity of Ca1−3xBi2xФxMoO4 series were investigated in [13,19]. Guo et al. [13] synthesized series of solid solutions Ca1−3xBi2xФxMoO4 (0.005 ≤ x ≤ 0.20), using a conventional ceramic method, and examined their microstructure and microwave dielectric properties. The 0 ≤ x ≤ 0.15 compo- sitions were found to be single-phase and to have scheelite structure with cationic vacancies [13,19] (the structural model of Sr0.88Bi0.08MoO4 [18] was used). It was shown that Bi-doped samples exhibit improved values of the mi- http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.2.04 http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0002-0050-9216 Chimica Techno Acta 2021, vol. 8(2), № 20218204 ARTICLE 2 of 7 crowave quality factor (Qf). Vibrational spectroscopy re- sults revealed large distortions of MoO4 and BiO8 polyhe- dra [13], with a strong correlation between substitutions in the cation (A n+ ) sublattice and microwave dielectric properties of the Ca1−3xBi2xФxMoO4 series. Later, the pow- der X-ray and neutron diffraction patterns for composi- tions with 0.15 < x ≤ 0.225 were shown to exhibit a te- tragonal supercell with asup ≈ √5a, csup ≈ c where a and c are the tetragonal scheelite cell parameters [19]. This su- perstructural ordering results in the additional reflections on XRPD patterns detected by Guo et al. [13] and provided by ordering of Bi atoms and cationic vacancies [19]. Sam- ples described with supercell showed maximal photocata- lytic activity due to complex microstructure at grain sur- face and also showed a decrease of total conductivity as compared to samples with normal scheelite structure [19]. The conductive properties and structure of Sr1−3xBi2xФxMoO4 compositions (0.025 ≤ x ≤ 0.225) were described in [20]. The superstructural ordering was also observed for 0.15 ≤ x ≤ 0.4. In Fourier-transformed infra- red spectra of Sr1−3xBi2xФxMoO4, the general shifting of absorption bands caused by deformation of MoO4 tetrahe- dra was observed. An increase of overall electrical conduc- tivity with x was also observed. The activation energy de- creased with x insignificantly (from ~1.2 to ~1.1eV), indi- cating that the charge carriers and conduction mechanism in Sr1−3xBi2xФxMoO4 generally were the same as in the par- ent compound SrMoO4. Probably, the increase of conduc- tivity was caused by the increase in the oxygen ion mobili- ty provided by the distortion of MoO4 polyhedra [20]. Synthesis of Sr1-xBixMo1-xVxO4 results in the two-phase samples consisting of BiVO4 (monoclinic) and SrMoO4 phases [20]. In contrast, Ca1-xBixMo1-xVxO4 single-phase solid solutions are observed for 0 ≤ x ≤ 0.9 [6]. One possi- ble reason of such discrepancy is that the dopants influ- ence differently the composition and structure of stronti- um and calcium molybdates. The simultaneous presence of Bi and V in Ca1-xBixMo1-xVxO4 oxide leads to the simultane- ous expansion and contraction of its unit cell due to the replacement of calcium with bismuth and molybdenum with vanadium, respectively; as a result, the unit cell vol- ume changes slightly [6]. In contrast, in Sr1-xBixMo1-xVxO4 doping with both Bi and V leads to the contraction of unit cell and by that means makes this oxide unstable. It can be assumed that such contraction decreases the distance be- tween [BO4] n- (B = Mo,V) clusters and, consequently, in- creases the repulsion between them. As a result, decompo- sition of Sr1-xBixMo1-xVxO4 oxide is observed [20]. In gen- eral, Bi- and (Bi+V) doped CaMoO4 and SrMoO4 show the decreasing melting point temperature and, as a conse- quence, lower sintering temperature, as well as increasing conductivity, catalytic and photocatalytic activity. In addi- tion, the shift of absorbance bands from UV to violet and blue parts of spectra was observed, and the energy gap decreases [6,22]. The last mentioned characteristics are important factors for such technical areas as photocatalyt- ic oxidation under visible light or pigment technology. Thus, the priority goal of the present work was to show the effects of bismuth and vanadium co-doping on the UV-VIS spectral characteristics and Eg values of the Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4 complex oxides. 2. Experimental Samples of Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4 (0 < x ≤ 0.4, 0 < y ≤ 0.5) were synthesised by conventional solid state methods from SrCO3 (99.5%) or CaCO3 (99.5%), Bi2O3 (99.9%), V2O5 (98.5%) and MoO3 (99.0%) as starting materials. Stoichi- ometric amounts of dried precursors were weighed and mixed in an agate mortar as dispersions in ethanol. Mixed powders were then pelletized and calcined in steps at 550– 720 C with duration about 10 hours at each step followed by regrinding and re-pelletizing. The overall time of calci- nation was about 30 hours. X-ray powder diffraction (XRD) data were obtained using a Bruker Advance D8 diffractometer with a VANTEC1 detector (Ni filtered Cu Kα radiation, θ/θ geometry). XRD data were collected in the 2θ range of 6–120°, with steps of 0.02103° and an effective scan time of 200 s per step. The reflection spectra were obtained in the range of 190–1100 nm by using a spectrophotometer Evolution 300 (Termo Sci) equipped with an integrating sphere. The ab- sorption coefficient curves were derived from reflection curves using Kubelka-Munk model. Energy gaps for direct interband transitions were calculated with linear approx- imation of Kubelka-Munk function [21]. 3. Results and Discussion As-prepared samples of nominal composition Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4 (0 < x ≤ 0.5, 0 < y ≤ 0.5) were found to contain one phase for y ≤ 0.05 (x = 0) and y ≤ 0.2 (x = 0) for CaMoO4- and SrMoO4-based systems, respectively, although the solubili- ty limit of vanadium in Ca1−yBiyMo1-yVyO4 is reported to be y = 0.9 [6,22]. BiVO4 was also detected in CaMoO4-based samples with x = 0 and 0.2 < y ≤ 0.5. Presence of BiVO4 was also found in Ca1−yBiyMo1-yVyO4 (0.4 < y ≤ 0.9) [22], but this fact was disregarded by authors during solid solu- tion limits discussion. At low x concentration the sheelite structure are observed. But for oxides with higher x values the additional peaks in the XRD patterns are evident. These oxides were found to have the tetragonal supercell with asup ≈ √5a, csup ≈ c, where a and c are the tetragonal scheelite cell parameters, like it was found earlier for Ca1−3xBi2xMo1O4 [19] and Sr1−3xBi2xMo1O4 [20]. The areas of solid solutions (ordinary scheelite and superstructured scheelite) and inveatigated fields of triple phase diagram are shown at Fig. 1. It is seen that continuation of solid Chimica Techno Acta 2021, vol. 8(2), № 20218204 ARTICLE 3 of 7 Fig. 1 The areas of scheelite-type solid solutions (red), superstructured scheelite solid solutions (blue) and tree-phases areas (gray) in CaMoO4 - BiVO4 - Bi2Mo3O12 (a) and SrMoO4 - BiVO4 - Bi2Mo3O12 (b) ternary diagram. White regions are unstudied areas (x > 0.5, y > 0.5). solutions areas can be expected for {0.5 < x ≤ 0.7, 0.5 < y ≤ 0.6} values. Compositions without solid solu- tions areas contain additional Bi3MoO12 and monoclinic BiVO4. Previously, free refinement of the occupancies in su- perstructured Ca1−3xBi2xMo1O4 solid solutions showed the 4b site positions to be fully occupied by bismuth, the 16f site positions are partially occupied by both Ca 2+ and Bi 3+ [19]. In the present work, the refinement of the occu- pancies of 4b and 16f sites revealed the same pattern of doping. It can be clearly observed that high concentration of bismuth only (i.e. x+y values) does not lead to the super- structural ordering, but relatively high concentration of cationic vacancies accompanying high concentrations of bismuth provide the ordering of bismuth at 4b position. Samples with superlattice ordering contain plains which include only bismuth and molybdenum atoms (Fig. 2) and the formation of such planes can be associated with pre- ferred orientation of 6S 2 lone pair electrons of bismuth along z axis. High concentration of cationic vacancies in 16f Sr/Bi position provides required geometrical factors of favourable orientation of 6S 2 lone pair and, as a result, the mentioned planes are formed. Fig. 2 Planes including Bi 4b positions show the ordering of bis- muth atoms in tetragonal supercell. “SrBi” 16f positions are mixed with Sr, Bi and Ф. The dependencies of the unit cell parameters for Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4 compositions are shown in Fig. 3. The compression and enlargement are caused by substitution by smaller of big- ger cation, respectively (ionic radii 𝑟Ca2+ = 1.12 Å, 𝑟Sr2+ = 1.26 Å, 𝑟Bi3+ = 1.17 Å [23]). The ionic radius of vanadium is smaller than that of molybdenum (𝑟MoIV 6+ = 0.41 Å, 𝑟VIV 5+ = 0.355 Å), and V- doping impedes expansion of the unit cell in the case of Ca1−1.5x-yBix+yФxMo1-yVyO4 and promotes the decrease of cell volume in the case of Sr1−1.5x-yBix+yФxMo1-yVyO4. In both cases, the superstructural ordering is a “growth factor” of с-parameter. It leads to the sharp rise of c-parameter or local plateau on c-parameter dependence in the cases of Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4, re- spectively. Probably, it is caused by stereochemical activi- ty of 6S 2 lone pair of bismuth in the planes parallel to (xoy) plane. Typical diffuse scattering spectra of (Ca/Sr)1−1.5x-yBix+yФxMo1-yVyO4 compositions are similar (Fig. 4). The scattering in the range of ~500–1100 nm is close to 100%. The spectra for CaMoO4 and SrMoO4 con- tain a broad band in the range of wavelengths ~200– 300 nm that corresponds to electronic transition within the MoO4 2- complex [24]. For Bi-doped and (Bi+V) -doped samples this band is located in higher wavelengths (see Fig. 4). The separate absorbance bands that correspond to transitions in the vanadium-oxygen complex are not ob- served. The band gap calculations for (Ca/Sr)1−1.5x-yBix+yФxMo1-yVyO4 were carried out by using Kublenka-Munk theory and Tauc relation. Fig. 5 shows the Tauc plot for some compositions. The graph consists of non-linear and linear regions. The value of Eg can be ob- tained by drawing a tangent on the linear part. The point of inflection on the X-axis provides the value of band gap for the prepared powders. Chimica Techno Acta 2021, vol. 8(2), № 20218204 ARTICLE 4 of 7 Fig. 3 The unit cell parameters depending on x and y parameters in Ca1−1.5x-yBix+yФxMo1-yVyO4 (a,c,e) and Sr1−1.5x-yBix+yФxMo1-yVyO4 (b,d,f) solid solutions Chimica Techno Acta 2021, vol. 8(2), № 20218204 ARTICLE 5 of 7 Fig. 4 The diffuse scattering spectra for some Ca1−1.5x-yBix+yФxMo1-yVyO4 (a) and Sr1−1.5x-yBix+yФxMo1-yVyO4 (b) compositions Fig. 5 Tauc plots for some Ca1−1.5x-yBix+yФxMo1-yVyO4 (a) and Sr1−1.5x-yBix+yФxMo1-yVyO4 (b) compositions Calculated values of the band gap (Eg) for Ca1−1.5x-yBix+yФxMo1-yVyO4 were found to be 3.83–2.64 eV for {x = 0, y = 0} – {x = 0.1, y = 0.5}compositions. Eg val- ues for Sr1−1.5x-yBix+yФxMo1-yVyO4 were 4.25–2.87 eV for {x = 0, y = 0} – {x = 0.2, y = 0.3} compositions. It is seen that the band gap value decreases with x and y (Fig. 6), and it reduces not only with bismuth content (x+y value) but with vanadium content (y) as well. For CaMoO4-based solid oxides, Eg value slowly decreases with x and rapidly decreases with y, while for SrMoO4-based solid oxides we observed the opposite trends (Fig. 6). In general, the ener- gy gap in (Ca/Sr)1−1.5x-yBix+yФxMo1-yVyO4 is reduced by addi- tional bands of Bi 6p electrons and by modification of states of Mo 4d electrons caused by distortion of MoO4 polyhedra. Bands of Bi 6p electrons lead to the significant decrease of Eg even for low concentration of bismuth for both series of molybdates. In the case of distortion of MoO4 polyhedra for SrMoO4-based solid oxides, the suffi- cient contraction of the unit cell is observed, while for CaMoO4-based solid oxides the cell volume grows. There- fore, the distortion of MoO4 polyhedra caused by the for- mation of cation vacancies (represented by x while y = 0) is generally smaller for CaMoO4-based compounds than that for SrMoO4-based compounds. Strongly distorted MoO4 polyhedra in SrMoO4-based compounds do not change significantly at vanadium doping (y > 0), but weakly distorted MoO4 polyhedra in CaMoO4-based com- pounds show a sharp alteration at the same time. Mean- while, some special effects for superstructured phases are not observed. For the purpose of Eg minimization, there- fore, the best compositions are found to be {x = 0.1–0.2, y = 0.4–0.5} and {x = 0.2–0.425, y = 0.1–0.2} for CaMoO4- Chimica Techno Acta 2021, vol. 8(2), № 20218204 ARTICLE 6 of 7 Fig. 6 The Eg changing in Ca1−1.5x-yBix+yФxMo1-yVyO4 (a) and Sr1−1.5x-yBix+yФxMo1-yVyO4 (b) series based and SrMoO4-based compounds, respectively. These compositions are expected to be the most effective photo- catalysts in (Ca/Sr)1−1.5x-y Bix+yФxMo1-yVyO4 series because of absorption of energy in both UV and vis (i.e. blue) parts of spectra. 4. Conclusions Compounds with general formulae of Ca1−1.5x-yBix+yФxMo1-yVyO4 and Sr1−1.5x-yBix+yФxMo1-yVyO4 were synthesized by conventional solid state technique in the temperature range of 550–720 °C. The areas of solid solutions (ordinary scheelite and superstructured scheel- ite) were determined. It was observed that relatively high concentration of cationic vacancies accompanying high concentrations of bismuth provide the ordering of bismuth at 4b position. The diffuse scatering spectra of homogene- ous samples contain a broad band in the range of ~200– 450 nm. Energy gaps, calculated with linear approxima- tion of Kubelka-Munk function, decrease with bismuth content. The compositions with minimal Eg are found to be {x = 0.1–0.2, y = 0.4–0.5} and {x = 0.2–0.425, y = 0.1–0.2} for CaMoO4-based and SrMoO4-based compounds, respec- tively. These compositions are expected to be the most effective photocatalysts in (Ca/Sr)1−1.5x-y Bix+yФxMo1-yVyO4 series. Acknowledgements The XPRD data were obtained in Ural-M center of Institute for Metallurgy, Ural Br. of RAS This work is supported by grant of RSF, project № 20-73-10048. References 1. Frank M, Smetanin SN, Jelínek M, Vyhlídal D, Kopalkin AA, Shukshin VE, Ivleva LI, Zverev PG, Kubeček V. Synchronous- ly-pumped all-solid-state SrMoO4 Raman laser generating at combined vibrational Raman modes with 26-fold pulse short- ening down to 1.4 ps at 1220 nm. Opt Laser Technol. 2019;111:129-33. doi:10.1016/j.optlastec.2018.09.045 2. Kunzel R, Umisedo NK, Okuno E, Yoshimura EM, Marques AP. Effects of microwave-assisted hydrothermal treatment and beta particles irradiation on the thermoluminescence and op- tically stimulated luminescence of SrMoO4 powders. Ceram Int. 2020;46(10):15018-26. doi:10.1016/j.ceramint.2020.03.032 3. Yu H, Shi, X, Huang L, Kang X, Pan D. Solution-deposited and low temperature-annealed Eu 3+/ Tb 3+- doped CaMoO4/SrMoO4 luminescent thin films. J Lumin. 2020;225:117371. doi:10.1016/j.jlumin.2020.117371 4. Elakkiya V, Sumathi S. Low-temperature synthesis of envi- ronment-friendly cool yellow pigment: Ce substituted SrMoO4. Mater Lett. 2019;263:127246. doi:10.1016/j.matlet.2019.127246 5. Mikhailik VB, Elyashevskyi Yu, Kraus H, Kim HJ, Kapustianyk V, Panasyuk M. Temperature dependence of scintillation properties of SrMoO4. Nucl Instrum Methods Phys Res A. 2015;792:1-5. doi:10.1016/j.nima.2015.04.018 6. Guo HH, Zhou D, Pang LX, Qi ZM. Microwave dielectric prop- erties of low firing temperature stable scheelite structured (Ca,Bi)(Mo,V)O4 solid solution ceramics for LTCC applica- tions. J Eur Ceram Soc. 2019;39(7):2365-73. doi:10.1016/j.jeurceramsoc.2019.02.010 7. Yu-Ling Y, Xue-Ming L, Wen-Lin F, Wu-Lin L, Chuan-Yi T. Co- precipitation synthesis and photoluminescence properties of (Ca1−x−yLny)MoO4: xEu 3+ (Ln =Y, Gd) red phosphors. J Alloys Compd. 2010;505(1):239-42. doi:10.1016/j.jallcom.2010.06.037 8. Zhu Y, Zheng G, Dai Z, Zhang L, Ma Y. Photocatalytic and luminescent properties of SrMoO4 phosphors prepared via hydrothermal method with different stirring speeds. J Mater Sci Technol. 2017;33(1):23-39. doi:10.1016/j.jmst.2016.11.019 9. Yao Z-F, Zheng G-H, Dai Z-X, Zhang L-Y. Synthesis of the Dy: SrMoO4 with high photocatalytic activity under visible light irradiation. Appl Organomet Chem. 2018;32(8):e4412. doi:10.1002/aoc.4412 https://doi.org/10.1016/j.optlastec.2018.09.045 https://doi.org/10.1016/j.ceramint.2020.03.032 https://doi.org/10.1016/j.jlumin.2020.117371 https://doi.org/10.1016/j.matlet.2019.127246 https://doi.org/10.1016/j.nima.2015.04.018 https://doi.org/10.1016/j.jeurceramsoc.2019.02.010 https://doi.org/10.1016/j.jallcom.2010.06.037 https://doi.org/10.1016/j.jmst.2016.11.019 https://doi.org/10.1002/aoc.4412 Chimica Techno Acta 2021, vol. 8(2), № 20218204 ARTICLE 7 of 7 10. Wang Y, Xu H, Shao C, Cao J. Doping induced grain size re- duction and photocatalytic performance enhancement of SrMoO4:Bi 3+ . Appl Surf Sci. 2017;392:649-57. doi:10.1016/j.apsusc.2016.09.09 11. Vidya S, John A, Solomon S, Thomas J. Optical and dielectric properties of SrMoO4 powders prepared by the combustion synthesis method. Adv Mater Res. 2012;1:191-204. doi:10.12989/amr.2012.1.3.191 12. Cheng J, Liu C, Cao W, Qi M, Shao G. Synthesis and electrical properties of scheelite Ca1-xSmxMoO4+d solid electrolyte ce- ramics. Mater Res Bull. 2011;46(2):185-9. doi:10.1016/j.materresbull.2010.11.019 13. Guo J, Randall CA, Zhou D, Zhang G, Zhang C, Jin B, Wang H. Correlation between vibrational modes and dielectric proper- ties in (Ca1−3xBi2xx)MoO4 ceramics. J Eur Ceram Soc. 2015;35(3):4459-64. doi:10.1016/j.jeurceramsoc.2015.08.020 14. Pang L-X, Sun G-B, Zhou D. Ln2Mo3O12 (Ln = La, Nd): A novel group of low loss microwave dielectric ceramics with low sintering temperature. Mater Lett. 2011;65(2):164-6. doi:10.1016/j.matlet.2010.09.064 15. Esaka T. Ionic conduction in substituted scheelite-type ox- ides. Solid State Ionics. 2000;136-137(1-2):1-9. doi:10.1016/s0167-2738(00)00377-5 16. Yang X, Wang Y, Wang N, Wang S, Gao G. Effects of co-doped Li + ions on luminescence of CaWO4:Sm 3+ nanoparticles. J Ma- ter Sci Mater Electronics. 2014;25:3996-4000. doi:10.1007/s10854-014-2119-4 17. Jiang P, Gao W, Cong R, Yang T. Structural investigation of the A-site vacancy in scheelites and the luminescence behav- ior of two continuous solid solutions A1-1.5xEux 0.5xWO4 and A0.64–0.5yEu0.24Liy 0.12–0.5yWO4 (A = Ca, Sr; = vacancy). Dalton Trans. 2015;44(13):6175-83. doi:10.1039/c5dt00022j 18. Sleight JAW, Aykan K. New nonstoichiometric molybdate, tungstate, and vanadate catalysts with the scheelite-type structure. J Solid State Chem. 1975;13(4):231-6. doi:10.1016/0022-4596(75)90124-3 19. Mikhaylovskaya ZA, Abrahams I, Petrova SA, Buyanova ES, Tarakina NV, Piankova DV, Morozova MV. Structural, photo- catalytic and electroconductive properties of bismuth- substituted CaMoO4. J Solid State Chem. 2020;291:121627. doi:10.1016/j.jssc.2020.121627 20. Mikhaylovskaya ZA, Buyanova ES, Petrova SA, Nikitina АА. Sheelite-related strontium molybdates: synthesis and charac- terization. Chimica Techno Acta 2018;5(4):189-95. doi:10.15826/chimtech.2018.5.4.03 21. Kay MI, Frazer BC, Almodovar I. Neutron diffraction refine- ment of CaWO4. J Chem Phys. 1964;40(2):504-506. doi:10.1063/1.1725144 22. Yao W, Ye J. Photophysical and photocatalytic properties of Ca1-xBixVxMo1-xO4 solid solutions. J Phys Chem B. 2006;110:11188-95. doi:10.1021/jp0608729 23. Shannon RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. 1976;A32:751–67. doi:10.1107/S0567739476001551 24. Verma A, Sharma SK. Rare-earth doped/codoped CaMoO4 phosphors: A candidate for solar spectrum conversion. Solid State Sci. 2019;96:105945. doi:10.1016/j.solidstatesciences.2019.105945 https://doi.org/10.1016/j.apsusc.2016.09.09 https://doi.org/10.12989/amr.2012.1.3.191 https://doi.org/10.1016/j.materresbull.2010.11.019 https://doi.org/10.1016/j.jeurceramsoc.2015.08.020 https://doi.org/10.1016/j.matlet.2010.09.064 https://doi.org/10.1016/s0167-2738(00)00377-5 https://doi.org/10.1007/s10854-014-2119-4 https://doi.org/10.1039/c5dt00022j https://doi.org/10.1016/0022-4596(75)90124-3 https://doi.org/10.1016/j.jssc.2020.121627 https://doi.org/10.15826/chimtech.2018.5.4.03 https://doi.org/10.1063/1.1725144 https://doi.org/10.1021/jp0608729 https://doi.org/10.1107/S0567739476001551 https://doi.org/10.1016/j.solidstatesciences.2019.105945