Phase transitions and microstructure of ferroelastic MIEC oxide SrCo0.8Fe0.2O2.5 doped with highly charged Nb/Ta(V) cations

The effect of compositional disorder generated by partial isomorphic substitution of cobalt by ferroactive highly charged cations Nb/Ta(V) on the phase transition "perovskite-brownmillerite" (P-BM), crystal structure and microstructure of low-and high-temperature phases SrCo(0.8-x)Fe(0.2)MxO(3-delta) (M = Nb, Ta; 0 <= x <= 0.1) was studied for ferroelastic SrCo0.8Fe0.2O2.5 perovskite-related oxide with mixed ion-electron conductivity (MIEC). According to "3-delta lg pO(2)-T" phase diagrams constructed for SrCo(0.8-x)Fe(0.2)MxO(3-delta) oxides (M = Nb, Ta; 0 <= x <= 0.05), the doping expands the stability region of the cubic paraelastic phase P to lower pO(2) and decreases the temperature of the P-CN phase transition. With the help of in situ high-temperature X-ray diffraction carried out in the isostoichiometric mode at 3-delta = 2.5, for the first time the P-BM phase transition was established to involve the formation of an intermediate tetragonal phase T (space group P4/mmm) with dynamically disordered tetrahedral chains. It has been shown that the microstructure of low-temperature phases is determined by the ferroelastic nature of the P-BM phase transition. At 0 <= x <= 0.05, a sharp P-BM phase transition leads to the formation of submicron lamellar 90 degrees domains, whereas at 0.05 < x <= 0.1 diffuse phase transformation results in tweed texture composed of nanosized coherently joined domains with brownmillerite and perovskite structures. Based on in situ high temperature Mossbauer and X-ray diffraction data, it was suggested that dynamic nanostructuring occurs in the studied MIEC materials at temperatures above the BM-P phase transition similar to that observed in the related class of relaxor ferroelectrics. Nanosized brownmillerite domains appear as a result of fluctuations in the paraelastic lattice; with decreasing temperature they freeze into a static nanotexture observed by high resolution transmission electron microscopy.