Ferroelasticity of SrCo_0.8Fe_0.2O_3–δperovskite-related oxide with mixed ion–electron conductivity

Abstract

A group-theoretical analysis was carried out to determine the possible orientation states of domains formed as a result of the 'perovskitebrownmillerite' phase transition in SrCo 0.8 Fe 0.2 O 2.5 oxide with mixed ionelectron conductivity (MIEC). The results of the theoretical analysis agree with the experimental data obtained in the study of the SrCo 0.8 Fe 0.2 O 2.5 microstructure by means of transmission electron microscopy. Brownmillerite SrCo 0.8 Fe 0.2 O 2.5 (BM) has a lamellar texture composed of 90 twins 60-260 nm in size; the h010i BM and h101i BM directions are linked through twinning in accordance with the predictions of the group-theoretical analysis. The presence of twins and their switching under mechanical load provide evidence that the perovskite-brownmillerite phase transition in SrCo 0.8 Fe 0.2 O 2.5 is ferroelastic. Comparative analysis of the phenomena observed for ferroelectrics and MIEC oxides indicates their similarity based on the common nature of ferroelectricity and ferroelasticity, and allows us to suppose that nonstoichiometric SrCo 0.8 Fe 0.2 O 3À with compositional disorder may be considered (in terms of its microstructural features) a 'relaxor ferroelastic'.

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

“…34,35 Previously we demonstrated that the P-BM phase transition in MIEC oxide SrCo 0.8 Fe 0.2 O 2.5 is ferroelastic: a change of point symmetry is accompanied by the formation of 90 twins, which may be reoriented by external mechanical stress. 36,37 An increase in the compositional disorder due to the oxygen nonstoichiometry in SrCo 0.8 Fe 0.2 O 2.5AEx is accompanied by the phenomena similar to those observed in relaxor ferroelectrics (in terms of its microstructural features), namely the formation of nanosized brownmillerite domains in the low-temperature (ferroelastic) phase.…”

“…To emphasize the microstructural similarity with relaxor ferroelectrics, we proposed to use a term "relaxor ferroelastic" for MIEC ferroelastics with high degree of compositional disorder, in which the phase transition is smeared and the low-temperature (ferroelastic) phase has nanosized texture. 36 It is evident that the resemblance in the progress of phase transitions and microstructural features of low-temperature phases for ferroelectrics and ferroelastics (see Fig. 10-12) is related to the common nature of ferroelectricity and ferroelasticity based on the crystal lattice distortion.…”

“…Previously we pointed out that similar microstructural phenomena were described for ferroelectrics. 36 In the absence of compositional disorder, ferroelectrics are characterized by the presence of a sharp phase transition and the formation of micron-sized domains in the low-temperature ferroelectric phase. The introduction of compositional disorder by isomorphous substitution in the cation sublattice of the oxides leads to the relaxor state.…”

“…S1a †), which agrees with the size of twins observed by transmission electron microscopy. 36,37 So, the study of the progress of ferroelastic transition P-BM in SrCo 0.8 Fe 0.2 O 2.5 carried out for the rst time in the isostoichiometric mode revealed the formation of an intermediate tetragonal phase T.…”

“…As a result, six types of 90 BM domains are formed, which agrees with previous data. 36,37 Doping causes decrease of the domain size and change of texture from lamellar to tweed-like (Fig. 11).…”