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, samples 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.


Introduction
Currently, there is an active search for new materials that can be used as electrodes and electrolytes of solid oxide fuel cells, oxygen sensors and membranes of electrochemical devices. Modern technology uses well-established materials with highly symmetric (usually cubic) structure such as fluorite or perovskite, where ionic conductivity is based on the migration of oxygen vacancies [1][2][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 interstitial 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 ABO 4 (A = Me 2+ /Me 3+ , B = Me 5+ /Me 6+ ). Thus, LaNbO 4 is characterized by insufficiently high values of electrical conductivity (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 promising characteristics can be found based on the results of studies on the preparation of complex oxide solid solutions. For example, Vu et al. [7] investigated the phase diagram La 2 O 3 -WO 3 -Nb 2 O 5 and to date discovered and studied a new composition La 3 NbWO 10 , which is characterized by a relatively high oxygen conductivity due to the transport of oxygen ions. Deepa et al. [8] studied the system Ca-Ce-Nb-M-O (where M = Mo or W). All samples Ca 2 CeNbM 2 O 12 , CaCeNbMO 8 , and CaCe 2 Nb 2 MoO 12 have cubic structure with a space group (SG) I41/a. Electrical conductivity increases with increasing cerium concentration, which is due to the variable valence of cerium. Excess oxygen is released into the atmosphere upon heating, leaving electron in the lattice, which is further responsible for the conductivity of the compound [8].
Thus, the establishment of mechanisms of formation and study of the characteristics of complex oxides in the systems calcium -bismuth (lanthanum) -niobiummolybdenum (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: Stoichiometric amounts of dried precursors were weighed and mixed in an agate mortar as dispersion in ethanol. Powders were heated at 500-1000 °C (up to 1400 °C for LaNb 1-y W y O 4+δ samples) for ~8 hours at each stage.
The phase composition of the powders was controlled by means of X-ray powder 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 microscopy (SEM) using JEOL JSM 6390LA (Jeol, Japan) microscope.
For conductivity measurements the ceramic pellets of 10 mm in diameter and 2.5 mm thickness were used. The flat surfaces of pellets were covered with Pt. Impedance spectra were obtained in two-electrode measurement cell on Elins Z-3000 impedance spectrometer, over the frequency ranges 3 MHz to 10 Hz at stabilized 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 electrical conductivity (σ) were plotted in Arrhenius coordinates lgσ -1000/T.

Results and discussion
For a series of Ca-R-Nb-M-O at a ratio 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 CaRNbMO 8 type complex 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 ABO 4 (RNbO 4 , CaMO 4 ) are predominantly formed. Qualitative change in the phase composition with temperature is presented in Table 1. Thus, the expected compounds of the total composition CaRNbMO 8 in the analyzed systems at the applied temperature and time conditions of synthesis were not yet detected.
In the La 1-x Bi x NbO 4 series within single-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 LaNb 1-y W y O 4+δ solid solutions, previously mentioned by Li et al. [9] when describing the properties of the LaNb 0.92 W 0.08 O 4.04 compound. In the present study, after prolonged exposure of samples at T = 1400 °С (32 hours), additional peaks on the diffraction patterns (see Fig. 1) were detected only for the compositions with y = 0.10 and above. Solid solutions are formed throughout the studied concentration range, and at y≤0.15, a mon-  Table 2.
All samples of Ca 1-u Bi u WO 4+δ and CaW 1-z Nb z O 4-δ systems are not singlephase. The main phase in Ca 1-u R u WO 4+δ system is based on calcium tungstate CaWO 4 with tetragonal structure (SG I41/a). Homogeneity range of the Ca 1-u La u WO 4+δ solid solution is limited by the La concentration u = 0.0-0.05. The following phases were additionally detected in Ca 1-u R u WO 4+δ and CaW 1-z Nb z O 4-δ samples with subsequent substitution to a small extent: La 22 W 9 O 60 , R 2 WO 6 , La 0.14 WO 3 , La 2 (WO 4 ) 3 . In CaW 1-z Nb z O 4-δ samples, the second phase of Ca 2 Nb 2 O 7 (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 determination (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 issue, an additional XRD is required with an increase in the exposure time.
The change in the phase composition in the BiNbO 4 -based system is presented with BiNb 0.95 W 0.05 O 4.025 and Bi 0.95 Ca 0.05 NbO 3.975 as an example. Tungstencontaining sample at 500 °С, in addition to the initial phases, contains Bi 14 W 2 O 27 (tetragonal, SG I41/a). An increase in temperature from 700 °С to 800 °С leads to the formation of the orthorhom-   -Fig. 4); for the sintered monoclinic briquettes the values 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  shows that the grains are tightly adjacent to each other only in the monoclinic phase of LaNb 1-y W y O 4+δ compositions.
The general shape of the temperature dependences of conductivity for LaNbO 4based samples is linear and is typical for ionic conductors (Figs. 6-9). In addition, small change of slope is seen on the dependencies which may be due to the presence of slight structural phase transitions (at a.c. 600-700 °С); these are typical for lanthanum niobate. In bismuth-containing samples conductivity increases within the monoclinic phase existence   (Table 3) were calculated according to the plots of the conductivity temperature dependencies; the average value is around 1.1 eV, which is consistent with the values of activation energies typical for ionic conductors.
In Ca 1-u Bi u WO 4+δ and CaW 1-z Nb z O 4-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. Therefore, the measurement error in the temperature range below 500 °С is very large, which does not allow to uniquely determine the values of -lg(σ) for all samples. The values of electrical conductivity of samples Ca 1-u Bi u WO 4+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 BiNbO 4 -based series, the conductivity of these compositions as composite materials were evaluated. In comparison with the matrix composition BiNbO 4 (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 nominal composition BiNb 0.9 W 0.1 O 4.05 (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 equalization of the conductivity values to 8 · 10 -8 Om -1 · cm -1 for all samples containing tungsten.

Conclusions
This work demonstrates the processes of phase formation in Ca-R-Nb-M-O. In each system, compounds based on complex oxides of the general formula ABO 4