Synthesis, crystal structure, and electronic properties of double orthovanadate Sr2Bi2/3 (VO4)2

Complex oxides A 3 M 2 O 8 (A = Sr, Ba; M = P, As, V) with the palmierite structure (space group R -3 m (no. 166), Z = 3) [1‐6] are considered as promising ceramics, ionic conductors, and membrane materials. In the structures of these compounds, the A cations occupy 3 a and 6 c positions with coordination numbers (CN) of 6 and 10, respectively. The M cations occupy 6 c positions (CN = 4) and the O atoms are in 6 c and 18 h positions. The A 3 M 2 O 8 oxides are quite stable with respect to heterovalent replacement in the cationic (A, M) and anionic sublattices; for example, Ba 3 (W,Nb) 2 O 8 , Ba 3 Mo 2 O 6 N 2 , and Ba 3 W 2 O 6 N 2 were obtained [7, 8]. The double orthovanadates A 2 Ln 2/3 (VO 4 ) 2 with the palmierite structure were found in the A 3 (VO 4 ) 2 –LnVO 4 binary systems, where A = Ca, Sr, Ba; Ln = La, Nd [9, 10]. In this group of compounds, 6 c positions are statistically occupied by two sorts of cations, 0.3333 Ln 3+ and 0.5 A, while 3 a positions contain only one sort of cation (A). These compounds can be considered as the limiting compositions of solid solutions formed upon replacement of A atoms in the orthovanadates A 3 (VO 4 ) 2 , where A = Ca, Sr, Ba. Thus, if the valences are balanced, one can expect the formation of stable compounds upon any heterovalent substitution in the sublattices, which may be either complete or defective. However, no data on the A 3 M 2 O 8 oxides with heterovalent substitutions in the 3 a position have been reported. In this work, we report the results of synthesis and study of the structural and electronic properties of a new orthovanadate Sr 2 Bi 2/3 (VO 4 ) 2 in which Sr → Ba heterovalent substitution in the 3 a position is implemented for the first time. The compound was prepared by solid-phase synthesis from special purity grade bismuth and vanadium(V) oxides and strontium carbonate. The reactants were mixed and subjected to stepwise annealing in the temperature range 550–1100°e with a temperature rise by 100°e every 12‐15 h. The samples were pressed into