Sheelite-related strontium molybdates: synthesis and characterization 189 Z. A. Mikhaylovskayaa*, E. S. Buyanovaa, S. A. Petrovab, А. А. Nikitinaa a Ural Federal University, 19 Mira St., Ekaterinburg, 620002, Russia b Institute for Metallurgy, Ural Branch of the Russian Academy of Sciences, 101 Amundsen St., Ekaterinburg, 620016, Russia E-mail: zozoikina@mail.ru Sheelite‑related strontium molybdates: synthesis and characterization The present research is devoted to the cationic-deficient SrMoO4-based sheelite-related complex oxides. The doping with bismuth to A sublattice and codoping with bismuth and vanadium (to A and B sublattices, respectively) were discussed. The X-Ray powder diffraction and infrared spectroscopy were used to investigate structural characteristics of the complex oxides. In Sr1–1.5xBixMoO4, a superstructural ordering was observed. Conductivity and dielectric loss of ce- ramic samples are measured using alternating current. Keywords: sheelite; strontium molybdates; dielectric materials. Received: 25.10.2018. Accepted: 17.12.2018. Published: 31.12.2018. © Mikhaylovskaya Z. A., Buyanova E. S., Petrova S. A., Nikitina А. А., 2018 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 4. 03 Introduction Scheelite-type complex oxides are quite interesting research objects be- cause flexibility of  substitutions in  these systems leads to variety of their composi- tions, structure types and properties. Ideal scheelite-related oxides have a general for- mula ABO4 and consist of A n+ cations and (BO4) n– anions. A-site ions are coordinated with eight oxygen ions, and B-site ions are coordinated with four oxygen ion. Each site can be occupied simultaneously by dif- ferent ions with various oxidation states; additional interstitial positions and vacan- cies lead to the deviation from the general formula. A lot of  scheelite-type complex oxides are used as materials for scintilla- tion detectors, lasers [1, 2], ionic conduc- tors [3], phosphors [4], photocatalysts [5], and microwave dielectrics [6]. The regula- tion of the desired properties of sheelite-re- lated materials can be provided by varying the quantity of the dopant, its nature, the ratios between dopants and the presence of additional vacancies or interstitial posi- tions in the structure. For example, a sub- stitution of A-site ions with Me+3 in ABO4 complex oxide can be described in  two ways: (1) a formation of A1–xMe +3 xBO4+x / 2 phases, where electroneutrality is pro- vided by  the interstitial oxygen ions; (2) a  formation of  A+21−1.5xMe +3 xФ0.5xMoO4 (or A+21−1.5xMe +3 xMoO4) phases containing cationic vacancies Ф. The first way of sub- stitution was detected for  Pb(Mo / W)O4 [7, 8] and Ca(Mo / W)O4 [9] parent com- pounds. The second way was described for  rare-earth substituted Ca(Mo / W)O4, Sr(Mo / W)O4 and Cd(Mo / W)O4 [10–12]. Mikhaylovskaya Z. A., Buyanova E. S., Petrova S. A., Nikitina А. А. Chimica Techno Acta. 2018. Vol. 5, No. 4. P. 189–195. ISSN 2409–5613 190 As a result, cationic vacancies Ф and their ordering are additional structural fac- tors influencing physico-chemical prop- erties. Another way of  the substitution of A positions is codoping with Me+3 and Me+1 or Me+5 ions, which leads to the for- mulae A1–x Me +1 0.5xMe +3 0.5xBO4 [13] and A1–xMe +3 xB1–xMe +5 xO4, respectively [14]. The present research is devoted to  the Bi-doped strontium molybdate SrMoO4. The existence of  Sr1–1.5xBixMoO4 series was shown by  Sleight and coauthors [15], who synthesized the complex ox- ide Sr0.88Bi0.08MoO4 (tetragonal symmetry, Sp. Gr. I41 / a) and described its good cata- lytic properties [15]. Sr1–1.5xBixMoO4 family has not been described yet, while Bi-doped calcium molybdates have been intensively researched as  microwave dielectric [16] or pigments [14]. The basic characteristic and structure types of  SrMoO4 are simi- lar to those of CaMoO4, so the Bi-doped strontium molybdates should possess similarly promising properties. In addi- tion, Bi-doping of  strontium molybdates is expected to lead to decrease of the unit cell, resulting from the significant chang- es in  B-sublattice. Therefore, the objects of  the present work are Sr1–1.5xBixMoO4, Sr1–xBixMo1–xVxO4, Sr1–1.5xBixMo1–yVyO4–d solid solutions and their structure and properties. Experimental Synthesis of Sr1–1.5xBixMoO4 (0 ≤ x ≤ 0.45), Sr 1–xBi xMo 1–xV xO 4 (0 ≤ x ≤ 0.4), Sr1–1.5xBixMo1–yVyO4–d (0 < x ≤ 0.4, 0 < y ≤ 0.2) were synthesised by  conventional solid state methods from SrCO3 (98.5 %), Bi2O3 (99.9 %), V2O5 (98.5 %) and MoO3 (99.0 %). Stoichiometric amounts of dried precur- sors were weighed and mixed in an agate mortar as dispersion in ethanol. Mixtured powders were pelletized and heated at 550– 650 °C with regrinding and repelletizing. Time of each heating was ~10 hours, the total time of heating was ~30 hours. X-ray powder diffraction data were ob- tained on a DRON-3 with Cu Kα mono- chromatic radiation in the range of 5–75° of 2θ. IR FT spectrometry measurements were carried out at  Nicolet 6700 with attenua ted total reflection attachment. Density of powder samples was measured by hydrostatic weighting. For conductivity measurements, the ceramic pellets of 10 mm in diameter and ca. 2.5 mm thickness were covered by  Pt. Impedance spectra were obtained in two-electrode measure- ment cell on  LCR-819 and Elins Z-3000 impedance spectrometers, over the fre- quency ranges 1 Hz and 3 MHz to 10 Hz, respectively, at  stabilised temperatures from ca. 25 °C to ca. 625 °C. Data presented correspond to the second cooling run. Data were modelled using equivalent electrical circuits with the Zview software (Version 2.6b, Scribner Associates, Inc.). Results and discussion Synthesis of Sr1–1.5xBixMoO4 yielded sam- ples with the structure of Sr0.88Bi0.08MoO4 [15], up to x = 0.2 (Sp.gr. I41 / a). At 0.2 < x < 0.4 additional peaks in the small angle range are evident; for  the x > 0.45 com- position additional peaks in  the pattern can be identified as Bi3MoO12. We supposed that 0.2 < x ≤ 0.4 compositions have su- perstructural ordering caused by ordering of cationic vacancies (Fig. 1). As a  result, in  the XRPD data for  0.2 < x ≤ 0.4 compositions all peaks can be successfully indexed using a  te- tragonally ordered supercell asup=√5asub, 191 csup= csub (where sup and sub subscripts de- note the super- and subcell, respectively) in  I41 / a  space group (Fig. 2). Composi- tional dependence of the unit cell param- eters for Sr1–1.5xBixMoO4 compositions are shown in Fig. 2, where the linear chemical compression is caused by the substitution of the bigger cation with the smaller one (ionic radii r Sr2+ = 1.26  Å, r Bi3+ = 1.17  Å [17]). The measurements of density showed that experimental density is equal to the theoretical (X-ray) one to within the 2–3 % of absolute values. Synthesis of Sr1–xBixMo1–xVxO4 results in  the two-phase samples that consist of BiVO4 (monoclinic) and SrMoO4 phases. In contrast, Ca1–xBixMo1–xVxO4 solid so- lutions are observed for 0 ≤ x ≤ 0.9 [14]. One possible reason was that the dopants influence differently the composition and structure of strontium and calcium molyb- dates. The simultaneous presence of Bi and V in Ca1–xBixMo1–xVxO4 leads to the expan- sion of the unit cell of a complex oxide due to the replacement of calcium with bismuth and compression of the unit cell due to the replacement of molybdenum with vanadi- um; as a result, the unit cell changes slightly [14] (ionic radii r Ca2+ = 1.12 Å, r V 5+ = 0.54 Å, r Mo6+ = 0.41 Å [17]). In contrast, in Sr1–xBix- Mo1–xVxO4 both of Bi and V lead to com- pression of the unit cell, making it unstable. It can be assumed that such compression of the unit cell leads to the decrease of the distance between [BO4] n– (B = Mo, V) clusters and, consequently, to the increase of repulsion between them. As a result, the destruction of Sr1–xBixMo1–xVxO4 system is observed. Creating an oxygen deficiency in  the crystal lattice provides distortion in BO4 tetrahedra and formation of [BO4]– [BO3] n–-type bonds through common oxy- gen atoms. This can be realized by changing Fig. 1. X-ray diffraction profiles for selected Sr1–1.5xBixMoO4 compositions. Arrows and asterisks indicate superlattice and Bi2Mo3O12 reflections, respectively Fig. 2. Compositional variation of unit cell parameters in Sr1–1.5xBixMoO4; super (a’,b’) and sub (a,b) — cells in Sr1–1.5xBixMoO4 series (inset) 192 the composition of strontium molybdate to Sr1–1.5xBixMo1–yVyO4–d. It was expected that low concentration of  bismuth would not provide a  proper compression of  the unit cell and single- phase samples would not be observed even at small y. But high concentration of bis- muth can provide an efficient compression of the unit cell and the possibility of doping with vanadium. In fact, we observed such trend. For x = 0.1 in Sr1–1.5xBixMo1–yVyO4–d no single-phase samples was obtained, for  x = 0.2 only y = 0.05 composition is single phase, for x = 0.3 and x = 0.4 single- phase compositions were observed at y = 0.05–0.1 and 0.05–0.2, respectively. In this research, the maximum concentration of vanadium was not determined exactly, but the general trend is clear. In the Table 1, the unit cell parameters of  the single-phase samples are shown. When x is fixed and y increases or when y is fixed and x increases, a general com- pression of the unit cell is observed. The presence of vanadium in the structure leads to  the absence of  cationic ordering, and no additional peaks in  X-ray diffraction profiles of x = 0.3–0.4 are observed. Unfor- tunately, it has proved impossible to accu- rately refine the oxide ion positions in the unit cell using a  Rietveld approach, due to dominance of the X-ray scattering by the cations in this system and only neutron dif- fraction can refute or confirm the specified theory about the [BO4]–[BO3] n–-type bond formation. Powders of  the Sr1–1.5xBixMoO4 com- positions were characterized by  IR FT spectroscopy (Fig. 3). Several adsorp- tion bands in the range of 950–500 сm–1 were detected. According to  Basiev [18] sheelite-related compound ABO4 consists of [MoO4] 2– clusters and isolated A2+ ions and, as a result, characteristic absorption bands can be assigned only to the vibra- tions in [MoO4] 2– clusters [19]. Strong ab- sorption bands in the range 940–550 cm– 1 are related to  O–Mo–O stretches of  the Table 1 Unit cell parameters of Sr1–1.5xBixMo1–yVyO4 (0.1 < x < 0.4, 0.05 < y < 0.2) compositions Composition а, Å b, Å c, Å V, Å3 Sr0.7Bi0.2Mo0.95V0.05O4 5.367 5.367 11.935 343.78 Sr0.7Bi0.2Mo0.9V0.1O4 5.363 5.363 11.961 344.02 Sr0.55Bi0.3Mo0.95V0.05O4 5.353 5.353 11.896 340.87 Sr0.55Bi0.3Mo0.9V0.1O4 5.348 5.348 11.896 340.24 Sr0.55Bi03Mo0.8V0.2O4 5.335 5.335 11.907 338.90 Sr0.4Bi0.4Mo0.95V0.05O4 5.342 5.342 11.826 337.48 Sr0.4Bi0.4Mo0.9V0.1O4 5.326 5.326 11.885 337.13 Sr0.4Bi0.4Mo0.8V0.2O4 5.318 5.318 11.868 335.64 Fig. 3. IR FT spectra of the Sr1–1.5xBixMoO4 compositions 193 MoO4 tetrahedron. Additional absorp- tion band near 425–400 cm–1 can also be assigned to the deformational vibrations of O–Mo–O bands. In the IR FT spectra, the general shifting of absorption bands is observed. The same trend was observed for Ca1–1.5xBixMoO4 [16]; in both cases it is caused by the deformation of MoO4 tetra- hedra resulting from the presence of cati- onic vacancies. C onduc t ivity me asurements of Sr1–1.5xBixMoO4 ceramic showed very high resistivity of samples (Fig. 4). A slight in- crease of conductivity is observed for the samples with superstructural ordering (0.2  < x < 0.45) and two-phase samples (x > 0.45). Changing of  dielectric loss tangent (tgδ) was measured at the range of 303– 903  K at  cooling at  the fixed frequency of 1 kHz using the parallel Rp + С model (series connected Rs and Ls were shown to be negligible). The tgδ vs temperature curves of Sr1–1.5xBixMoO4 compositions are shown at Fig. 5. The acceptable dielectric losses (tgδ < 0.1) of Sr1–1.5xBixMoO4 com- positions were observed for temperatures below ~573 K, while tgδ decreases with x values until x = 0.3. Then, at x > 0.3, tgδ increases, probably because of structural ordering of the samples. For Sr1–1.5xBixMo1–yVyO4–d composi- tions we observed a  significant growth in  conductivity in  comparison with Sr1–1.5xBixMoO4. As a  result, in  the range of  ~303–573  K the tgδ rises by  approxi- mately one order (Fig. 6). It is consistent with the increase of oxygen ion conductiv- ity associated with structural changes. The tgδ vs frequency dependences of all compositions of  substituted SrMoO4 in- dicate that effective dielectric properties are observed at frequency above 1 MHz, i.e. in  the microwave range, as  well Fig. 4. Arrhenius plots for selected Sr1–1.5xBixMoO4 compositions Fig. 5. The tgδ vs temperature curves of Sr1–1.5xBixMoO4 compositions at 1 kHZ Fig. 6. The tgδ vs temperature curves of several Sr1–1.5xBixMo1–yVyO4–d compositions at 1 kHZ 194 as CaMoO4 — based dielectric materials [16] (the example is given in Fig. 7). Conclusions Thus, the present research demonstrates the existence of cationic-deficient sheelite- related complex oxides of Sr1–1.5xBixMoO4 series and Sr1–1.5xBixMo1–yVyO4–d series. In Sr1–1.5xBixMoO4, superstructural or- dering is observed for  0.2 < x ≤ 0.4. For Sr1–1.5xBixMo1–yVyO4–d series a changing in ox- ygen sublattice is assumed. Both of the com- plex oxide series show dielectric properties at temperatures below 503 K and frequen- cies above 1 MHz. Conductivity and tangent of dielectric loss for Sr1–1.5xBixMo1–yVyO4–d compositions is greater than for  Sr1– 1.5xBixMoO4. It is consistent with the increase of oxygen ion conductivity associated with mentioned structural changes. Acknowledgements This work was financially supported by grant of Russian Foundation for Basic Re- search, project № 16-33-60026. References 1. Mikhailik VB, Kraus H, Miller G, Mykhaylyk MS, Wahl D. Luminescence of CaWO4, CaMoO4, and ZnWO4 scintillating crystals under different excitations. J. Appl. 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