Phase formation processes and synthesis of solid solutions in Ca-R-Nb-M-O systems 17 D O I: 1 0. 15 82 6/ ch im te ch .2 02 0. 7. 1. 03 Levina A. A., Tadevosyan N. O., Petrova S. A., Buyanova E. S., Morozova M. V. Chimica Techno Acta. 2020. Vol. 7, no. 1. P. 17–25. ISSN 2409–5613 A. A. Levinaa, N. O. Tadevosyana, S. A. Petrovab, E. S. Buyanovaa, M. V. Morozovaa a Ural Federal University, 19 Mira St., 620002, Ekaterinburg, Russia b Institute for Metallurgy, Ural Branch of the Russian Academy of Sciences, 101 Amundsen St., 620016, Ekaterinburg, Russia email: anastasia.levina@urfu.ru Phase formation processes and synthesis of solid solutions in Ca–R–Nb–M–O systems During the study of the phase formation process in Ca–R–Nb–M–O systems (R = La, Bi, M = Mo, W), an attempt was made to obtain single-phase compounds of CaRNbMO8 composition by the standard ceramic technique. In addition, sam- ples based on LaNbO4, CaWO4, BiNbO4 were also synthesized by the standard ceramic technique. The phase composition of the samples was studied by XRD analysis. The electrical conductivity of the obtained solid solutions and potential composite materials was investigated by impedance spectroscopy. Keywords: sheelite; fergusonite; solid solutions; electrical conductivity Received: 18.02.2020. Accepted: 12.03.2020. Published: 31.03.2020. © Levina A. A., Tadevosyan N. O., Petrova S. A., Buyanova E. S., Morozova M. V., 2020 Introduction Currently, there is  an  active search for new materials that can be used as electrodes and electrolytes of solid ox- ide fuel cells, oxygen sensors and mem- branes of electrochemical devices. Modern technology uses well-established materi- als with highly symmetric (usually cubic) structure such as  fluorite or perovskite, where ionic conductivity is  based on the migration of oxygen vacancies [1–3]. Recently, it has been demonstrated that structures with lower symmetry achieve high oxygen ion conductivity, which is due to  the  presence of  oxygen ions in  inter- stitial positions in  materials with both ionic and mixed conductivity [4]. Such materials include a  number of  complex oxides that have sheelite or fergusonite structure and correspond to the formula ABO4 (A = Me 2+/Me3+, B = Me5+/Me6+). Thus, LaNbO4 is  characterized by  insuf- ficiently high values of electrical conductiv- ity (at 1000 °C σ ≈ 5.5 · 10–5 Ohm–1 · cm–1), but some solid solutions based on it show an increase in conductivity by 1–3 orders of magnitude both with an acceptor [5] and a donor doping [6]. In addition, new materials with promis- ing characteristics can be found based on the  results of  studies on the  preparation of complex oxide solid solutions. For ex- ample, Vu et al. [7] investigated the phase diagram La2O3-WO3-Nb2O5 and to  date discovered and studied a  new composi- tion La3NbWO10, which is  characterized by  a  relatively high oxygen conductivity due to the transport of oxygen ions. 18 Deepa et al. [8] studied the system Ca– Ce–Nb–M–O (where M = Mo or W). All samples Ca2CeNbM2O12, CaCeNbMO8, and CaCe2Nb2MoO12 have cubic structure with a  space group (SG)  I41/a. Electri- cal conductivity increases with increas- ing cerium concentration, which is  due to the variable valence of cerium. Excess oxygen is released into the atmosphere up- on heating, leaving electron in the lattice, which is further responsible for the con- ductivity of the compound [8]. Thus, the establishment of mechanisms of formation and study of the characteris- tics of complex oxides in the systems calci- um — bismuth (lanthanum) — niobium — molybdenum (tungsten) — oxygen is very relevant and is in line with current trends in the search for new materials for various functional applications and technologies for their production. Experimental All samples were synthesized using a standard ceramic technique with several heating stages and intermediate grinding after every 50–100 °C of  heating. Initial components were taken in stoichiometric quantities to  obtain products according to the equations 1–6: CaCO3 + R2O3 + Nb2O5 + MO3 = = CaRNbMO8, (1) La2O3 + Bi2O3 + Nb2O5 + WO3 = = La1–xBixNb1–yWyO4±δ, (2) CaCO3 + R2O3 + WO3 = = Ca1–uRuWO4+δ, (3) CaCO3 + WO3 + Nb2O5 = = CaW1–zNbzO4–δ, (4) Bi2O3 + CaCO3 + Nb2O5 = = Bi1–vCavNbO4–δ, (5) Bi2O3 + Nb2O5 + WO3 = = BiNb1–wWwO4+δ, (6) where R = La or Bi, M = Mo or W; x, y, u, z, v, w are the quantities of the dopant ele- ment. Stoichiometric amounts of dried pre- cursors were weighed and mixed in an ag- ate mortar as dispersion in ethanol. Pow- ders were heated at  500–1000 °C (up to 1400 °C for LaNb1–yWyO4+δ samples) for ~8 hours at each stage. The phase composition of the powders was controlled by  means of  X-ray pow- der diffraction in the range of 5–75° of 2θ (D8 ADVANCE diffractometer (Bruker, Germany), Cu Kα radiation, β-filter, PSD VANTEC1). The phase composition and structure of the compounds was examined by comparing XRD patterns with the PDF2 database entries. Surface morphology and local chemical composition of the powders and ceramic specimens annealed at 1200–1270 °С were determined by scanning electron micros- copy (SEM) using JEOL JSM 6390LA (Jeol, Japan) microscope. For conductivity measurements the ce- ramic pellets of  10 mm in  diameter and 2.5 mm thickness were used. The flat sur- faces of pellets were covered with Pt. Im- pedance spectra were obtained in two-elec- trode measurement cell on Elins Z-3000 impedance spectrometer, over the  fre- quency ranges 3 MHz to  10 Hz at  stabi- lized temperatures from 850 °C to 250 °C in the cooling run. Impedance spectra were treated with “ZView” software. Using these data, the temperature dependences of elec- trical conductivity (σ) were plotted in Ar- rhenius coordinates lgσ – 1000/T. 19 Results and discussion For a series of Ca–R–Nb–M–O at a ra- tio of metal components 1:1:1:1 the analysis of  phase formation during the  synthesis and search for the proposed single-phase compositions of the CaRNbMO8 type com- plex oxides was carried out. According to the XRD data, the samples after a series of annealing are not single-phase, but have a variable phase composition throughout the  temperature range. In  each system, compounds based on complex oxides with the general formula ABO4 (RNbO4, CaMO4) are predominantly formed. Qual- itative change in  the  phase composition with temperature is presented in Table 1. Thus, the expected compounds of the total composition CaRNbMO8 in the analyzed systems at  the  applied temperature and time conditions of synthesis were not yet detected. In the La1–xBixNbO4 series within sin- gle-phase regions with increasing bismuth concentration, the  lattice is  compressed at x = 0.0–0.3 (monoclinic, SG I2/b) and expands at  x = 0.775–1.0 (triclinic, SG P-1). A  disproportionately modulated structure was found for the LaNb1–yWyO4+δ solid solutions, previously mentioned by Li et al. [9] when describing the  proper- ties of  the  LaNb0.92W0.08O4.04 compound. In the present study, after prolonged expo- sure of samples at T = 1400 °С (32 hours), additional peaks on the diffraction patterns (see Fig. 1) were detected only for the com- positions with y = 0.10 and above. Solid so- lutions are formed throughout the studied concentration range, and at y≤0.15, a mon- Table 1 Phase composition of Ca–R–Nb–M–O systems at different sintering stages Т, oC R = Bi, M = Mo R = La, M = Mo R = Bi, M = W R = La, M = W 500 CaCO3, Nb2O5, Bi2MoO6, Bi2Mo3O12 CaCO3, La2O3, Nb2O5, Mo4O11, LaNbO4, La4Mo2O11, La2(MoO4)3, CaMoO4 CaCO3, Nb2O5, WO3, Bi2O3, CaNb2O6 CaCO3, WO3, La2WO6, Nb2O5, CaWO4 600 CaCO3, Nb2O5, Bi2MoO6, CaMoO4 CaCO3, Nb2O5, La2Mo2O9, CaMoO4 WO3, Ca3Bi8O15, CaNb2O6, CaWO4 WO3, La2WO6, La3NbO7, CaWO4 700 BiNbO4, Nb2O5, Bi2MoO6, CaMoO4 CaO, Nb2O5, La2Mo2O9, CaMoO4, La2MoO6 BiNbO4, CaNb2O6, CaWO4 La2WO6, La3NbO7, CaWO4 800 BiNbO4, CaMoO4 Nb2O5, La3NbO7, CaMoO4, La2MoO6 BiNbO4, CaWO4 La2WO6, La3NbO7, CaWO4 900 BiNbO4, CaMoO4 CaNb2O6, La3NbO7, CaMoO4, La2MoO6 BiNbO4, CaWO4 CaNb2O6, La3NbO7, CaWO4, La14W8O45 1000 BiNbO4, CaMoO4 CaNb2O6, La3NbO7, LaNbO4, CaMoO4, La2MoO6 BiNbO4, CaWO4 CaNb2O6, CaWO4, La14W8O45 20 oclinic phase with a  SG I2/b is  formed; at y > 0.15 the tetragonal phase with SG. I41/a is formed. Reflections on the diffrac- tion patterns of double substituted samples (La1–xBixNb1–yWyO4±δ) correspond to  ad- ditional phases listed in Table 2. All samples of  Ca1–uBiuWO4+δ and CaW1–zNbzO4–δ systems are not single- phase. The main phase in Ca1–uRuWO4+δ sys- tem is based on calcium tungstate CaWO4 with tetragonal structure (SG I41/a). Ho- mogeneity range of the Ca1–uLauWO4+δ solid solution is limited by the La concentration u = 0.0–0.05. The  following phases were additionally detected in  Ca1–uRuWO4+δ and CaW1–zNbzO4–δ samples with sub- sequent substitution to  a  small extent: La22W9O60, R2WO6, La0.14WO3, La2(WO4)3. In  CaW1–zNbzO4–δ samples, the  second phase of Ca2Nb2O7 (monoclinic, SG P21) was found and its concentration increased with the degree of substitution of tungsten in the B-sublattice. The unit cell parameters of the samples were calculated. The values vary slightly within the error of determina- tion (Fig. 2), so the formation of the solid solution even within the range u = 0.0 – 0.05 is still under discussion. To resolve this is- sue, an additional XRD is required with an increase in the exposure time. The  change in  the  phase composi- tion in the BiNbO4-based system is pre- sented with BiNb 0.95W 0.05O 4.025 and Bi0.95Ca0.05NbO3.975 as an example. Tungsten- containing sample at 500 °С, in addition to  the  initial phases, contains Bi14W2O27 (tetragonal, SG I41/a). An  increase in  temperature from 700 °С to  800 °С leads to the formation of the orthorhom- Fig. 1. XRD patterns of samples LaNb1–yWyO4+δ (additional reflections are marked) Table 2 Phase composition of the La1–xBixNb1–yWyO4+δ samples Dopant concentration Crystal structure x=0.1, y=0.1 LaNbO4, monoclinic, I2/b LaNbO4, monoclinic, I2/a La0.33NbO3, orthorhombic, Pmmm x=0.1, y=0.2 LaNbO4, tetragonal, I41/a x=0.2, y=0.1 LaNbO4, monoclinic, I2/a La0.33NbO3, orthorhombic, Pmmm x=0.2, y=0.2 x=0.3, y=0.1 x=0.3, y=0.2 x=0.4, y=0.1 BiNbO4, triclinic, P-1 x=0.4, y=0.2 LaNbO4, monoclinic, I2/a x=0.5, y=0.1 BiNbO4, triclinic, P-1 x=0.5, y=0.2 LaNbO4, monoclinic, I2/a 21 bic phase BiNbO4 (SG Pnna), Bi5Nb3O15 (tetragonal, SG P4/mmm), Bi2WO6 (orthorhombic, SG Pcan). At  850 °С the  reflections of  Bi5Nb3O15 disappear. The  calcium-containing sample under- goes changes from the initial composition (Bi2O3, Nb2O5) to  BiNbO4 (orthorhom- bic, SG Pnna), Bi5Nb3O15 (tetragonal, SG P4/mmm), CaNb2O6 (orthorhombic, SG Pcan). The formation of a new phase CaBi2Nb2O9 with orthorhombic structure (SG Pbcn) was found for the compositions Bi1–vCavNbO4–δ (v = 0.15 – 0.30). From the concentration dependence of the unit cell parameters (Fig. 3) it can be seen that a significant change in the lattice param- eters occurs when the dopant content in- creases from 0.00 to 0.05, which may indi- cate the formation of a solid solution in this concentration range. Further increase of the Ca and W content in the samples practically does not result in any changes in the values of the unit cell parameters. The  particle size of  the  powdered samples is in the range of 1.0–14 μm (for example LaNb0.9W0.1O4+δ  — Fig.  4); for the sintered monoclinic briquettes the val- ues increase to 5.0–20 μm. In the region of tetragonal phase existence, the particle size range of  sintered samples is  wider 1.0–20 μm. The  SEM scan (Figs. 4–5) shows that the grains are tightly adjacent to each other only in the monoclinic phase of LaNb1–yWyO4+δ compositions. The general shape of the temperature dependences of conductivity for LaNbO4- based samples is linear and is typical for ionic conductors (Figs. 6–9). In addition, small change of  slope is  seen on the  de- pendencies which may be due to the pres- ence of slight structural phase transitions (at  a.c. 600–700 °С); these are typical for lanthanum niobate. In bismuth-con- taining samples conductivity increases within the  monoclinic phase existence Fig. 2. The unit cell parameters of Ca1–uRuWO4+δ and CaW1–zNbzO4–δ systems, calculated according to the tetragonal structure of CaWO4 (SG I41/a) Fig. 3. The unit cell parameters of BiNbO4-based systems calculated in orthorombic structure of BiNbO4 (SG Pnna) 22 range and decreases within the  triclinic phase with increasing x. A similar trend is observed for LaNb1–yWyO4+δ: at y ≤ 0.15 with increasing dopant concentration, the conductivity values increase sharply, exceeding those of the parent compound by three orders of magnitude at the maxi- mum point, and then gradually decrease Fig. 6. Temperature dependencies of electrical conductivity La1–хBixNbO4 Fig. 7. Temperature dependencies of electrical conductivity LaNb1–yWyO4+δ a b Fig. 4. Micrographs of the sample LaNb0.9W0.1O4+δ obtained by scanning (a) the surface or (b) the cross-section of the briquettes in the secondary (left) and reflected electrons (right) a b Fig. 5. Micrographs of the sample LaNb0.8W0.2O4+δ obtained by scanning (a) the surface or (b) the cross-section of the briquettes in the secondary (left) and reflected electrons (right) 23 Fig. 8. Temperature dependencies of electrical conductivity La1–хBixNb0.9W0.1O4+δ Fig. 9. Temperature dependencies of electrical conductivity La1–хBixNb0.8W0.2O4+δ Table 3 Activation energy values of samples based on LaNbO4 Composition Ea (Tlow), eV Ea (Thight), eV LaNbO4 0.91 1.21 La0.9Bi0.1NbO4 1.00 1.00 La0.8Bi0.2NbO4 0.88 0.88 La0.7Bi0.3NbO4 1.18 0.88 La0.225Bi0.775NbO4 1.33 1.08 (1.76 *) La0.2Bi0.8NbO4 0.88 1.05 La0.1Bi0.9NbO4 0.89 1.06 LaNb0.9W0.1O4+δ 1.24 1.24 LaNb0.85W0.15O4+δ 1.44 1.44 LaNb0.8W0.2O4+δ 0.60 1.50 LaNb0.75W0.25O4+δ 0.41 1.53 LaNb0.7W0.3O4+δ 0.92 1.44 La0.9Bi0.1Nb0.9W0.1O4+δ 1.28 1.10 La0.8Bi0.2Nb0.9W0.1O4+δ 1.22 1.12 La0.7Bi0.3Nb0.9W0.1O4+δ 1.28 1.13 La0.6Bi0.4Nb0.9W0.1O4+δ 1.28 1.08 La0.5Bi0.5Nb0.9W0.1O4+δ 1.26 1.04 La0.9Bi0.1Nb0.8W0.2O4+δ 1.28 1.28 La0.8Bi0.2Nb0.8W0.2O4+δ 1.25 1.25 La0.7Bi0.3Nb0.8W0.2O4+δ 1.26 1.26 La0.6Bi0.4Nb0.8W0.2O4+δ 1.21 1.21 La0.5Bi0.5Nb0.8W0.2O4+δ 1.19 1.19 * The value of Ea in the high-temperature region on the graph of the temperature dependence of the conductivity of the solid solution La0.225Bi0.775NbO4. 24 within the  tetragonal phase. Samples La1–xBixNb1–yWyO4±δ show high conduc- tivity comparable to the maximum values of the LaNb1–yWyO4+δ conductivity. The ac- tivation energy values (Table 3) were cal- culated according to the plots of the con- ductivity temperature dependencies; the average value is around 1.1 eV, which is consistent with the values of activation energies typical for ionic conductors. In Ca1–uBiuWO4+δ and CaW1–zNbzO4–δ series, conductivity increases slightly with increasing dopant concentration. As can be seen in Figs. 10–11 the total conductivity value for all samples is rather low. There- fore, the measurement error in the tem- perature range below 500 °С is very large, which does not allow to  uniquely de- termine the values of –lg(σ) for all sam- ples. The values of electrical conductivity of samples Ca1–uBiuWO4+δ are higher than that of  the  matrix by  no more than one order of magnitude. Despite the  absence of  single-phase samples in  the  BiNbO4-based series, the  conductivity of  these compositions as  composite materials were evaluated. In comparison with the matrix composi- tion BiNbO4 (σ = 7.77 · 10 –7 Ohm–1 · cm–1, at T = 800 °С) it was possible to increase the conductivity for the sample of the nom- inal composition BiNb0.9W0.1O4.05 (at T = 800 °С it is  9.61 · 10–6 Ohm–1 · cm–1). When the  temperature decreases to 500 °С, it is possible to observe an equali- zation of the  conductivity values to 8 · 10–8 Om–1 · cm–1 for all samples contain- ing tungsten. For calcium doped bismuth niobates, no significant conductivity changes occur. The highest value shows the sample with nominal composition Bi0.85Ca0.15NbO3.925 at T = 800 °С σ = 1.41 · 10–6 Ohm–1 · cm–1, and the smallest sample of Bi0.9Ca0.10NbO3.95 σ = 2.71 · 10–7 Ohm–1 · cm–1 at T = 800 °С. Conclusions This work demonstrates the  pro- cesses of  phase formation in  Ca-R-Nb- M-O. In each system, compounds based on complex oxides of  the  general for- mula ABO4 (LaNbO4, BiNbO4, CaMO4) are predominantly formed. A  number of  samples based on LaNbO4, BiNbO4, CaWO4 substituted with bismuth, tung- sten, calcium, lanthanum and niobium were obtained. Solid solutions are formed in  the  range x = 0.0–0.3 and x = 0.775– 1.0 for La1–xBixNbO4, y = 0.0–0.3 for Fig. 10. Temperature dependencies of electrical conductivity Ca1–zBizWO4+δ Fig. 11. Temperature dependencies of electrical conductivity CaW1–yNbyO4–δ 25 LaNb1–yWyO4+δ, u = 0.0 – 0.05, z = 0.0–0.05 for Ca1–uLauWO4+δ and CaW1–zNbzO4–δ. A significant increase in the conductivity is observed mainly for lanthanum niobates substituted with tungsten. Acknowledgements This work was financially supported by grant of Russian Foundation for Basic Re- search, project № 18-33-00921. References 1. Goodenough JB. Oxide-Ion Electrolytes. 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