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

Abstract

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−xFe_0.2M_xO_3−δ (M = Nb, Ta; 0 ≤ x ≤ 0.1) was studied for ferroelastic SrCo_0.8Fe_0.2O_2.5 perovskite-related oxide with mixed ion-electron conductivity (MIEC). According to “3−δ lg pO₂–T” phase diagrams constructed for SrCo_0.8−xFe_0.2M_xO_3−δ oxides (M = Nb, Ta; 0 ≤ x ≤ 0.05), the doping expands the stability region of the cubic paraelastic phase P to lower pO₂ and decreases the temperature of the P–ВМ phase transition. With the help of in situ high-temperature X-ray diffraction carried out in the isostoichiometric mode at 3−δ = 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. Аt 0 ≤ x ≤ 0.05, a sharp P–BM phase transition leads to the formation of submicron lamellar 90° 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 Mössbauer 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.