Reactivity and Structural Defects in Glaserite‐Type Iron‐Vanadates

Abstract

We investigated (a) the redox reaction of FeO and Ag2O in the presence of BaCO3 and V2O5 under inert atmosphere and (b) the oxidation of FeO in air in the presence of BaCO3, Na2CO3, and V2O5. The reaction products are members of the glaserite‐type of compounds: Ba(Ag□)Fe[VO4]2 [a = 5.47397(4) Å, c = 7.1780(1) Å, P3]; BaNa1.63Fe0.79[VO4]2 [a = 9.672(1) Å, b = 5.5860(6) Å, c = 14.1505(4) Å, β = 90.042(9)°, C2/c]; and Ba(Ba,Na)Na[VO4]2 [a = 5.8478(2) Å, c = 7.3835(3) Å, P3m1]. The latter presents an example for a statistical distribution of differently charged ions with differences in ionic radii (Ba2+ and Na+) on the B site of the general formula AB2M[VO4]2. The iron compounds exhibit differences in the defect concentration and localization of voids. For Ba(Ag□)Fe[VO4]2 the Fe3+‐triangular lattice remains intact and the Ag substructure is depleted by 50 % through the redox reaction (a). In contrast, for BaNa1.63Fe0.79[VO4]2 defects occur on both sites (B = Na+ and M = Fe3+) with a total of 0.58 defects per formula unit. Extended 57Fe‐Mössbauer studies on a series of Fe‐glaserite compounds relate structural findings from Rietveld refinements of X‐ray diffraction data with defect concentrations, substitution variants, and charge distributions.