Interplay between chemical strain, defects and ordering in Sr1-xLaxFeO3 materials

Chemical lattice strain in nonstoichiometric oxides: an overview

“…All Fe n + ions were assumed to be in the HS state in the chemical expansion model of Sr 1– x La x FeO 3– δ ( x = 0, 0.6). 13 The model yielded similar chemical expansion coefficients for La 0.6 Sr 0.4 FeO 3– δ and SrFeO 3– δ , β ≈ 0.0197, and the calculated ε chem ( δ ) dependences coincided with the experimental data 136–140 for both oxides up to δ ≈ 0.2. 13 At the same time, Bae et al 19,20 demonstrated recently that chemical expansion of Sr 1– x La x FeO 3– δ ( x = 0.2, 0.5) is less than that predicted by the model based on eqn (32) assuming high-spin-only Fe n + ions, and can be described with this model only if some Fe n + ions are in the LS state.…”

“…B-site substitution with ions that do not change their oxidation state, such as Al or Ga, causes different chemical expansion changes in different ferrite perovskites. For example, β of La 0.8 Sr 0.2 Fe 0.7 Ga 0.3 O 3– δ is greater than that of La 0.8 Sr 0.2 FeO 3– δ , 19,218 while replacing Fe in SrFeO 3– δ with Al (or Ti) leads to the opposite effect of decreasing β ; 13,112,219 in both cases the high-temperature crystal lattice symmetry after the substitution remained practically unchanged. 13,112,215,218,219 Though they have lower chemical expansion than SrFeO 3– δ , SrFe 1– x Ti x O 3– δ oxides, which have been extensively studied lately as promising cobalt-free mixed ion–electron conductors (MIECs), 220 still possess fairly high β , above that of most La 1– x Sr x Co 1– y Fe y O 3– δ .…”

“…For example, β of La 0.8 Sr 0.2 Fe 0.7 Ga 0.3 O 3– δ is greater than that of La 0.8 Sr 0.2 FeO 3– δ , 19,218 while replacing Fe in SrFeO 3– δ with Al (or Ti) leads to the opposite effect of decreasing β ; 13,112,219 in both cases the high-temperature crystal lattice symmetry after the substitution remained practically unchanged. 13,112,215,218,219 Though they have lower chemical expansion than SrFeO 3– δ , SrFe 1– x Ti x O 3– δ oxides, which have been extensively studied lately as promising cobalt-free mixed ion–electron conductors (MIECs), 220 still possess fairly high β , above that of most La 1– x Sr x Co 1– y Fe y O 3– δ . 112,201 As compared with SrFe 0.35 Ti 0.65 O 3– δ , SrFe 0.35 Sn 0.65 O 3– δ is characterized by lower chemical expansion coefficient values, and the opposite trends in both TEC and β : the TEC of SrFe 0.35 Sn 0.65 O 3– δ decreases with δ , and β decreases with temperature.…”

“…Among Sr 1– x La x FeO 3– δ , only for SrFeO 3– δ at δ > 0.3 did the in situ high-temperature XRD and dilatometric data show much higher chemical strain, with β ≈ 0.0574. 13 Such disagreement between the model and the measurement results was explained by oxygen vacancy ordering, typical of highly nonstoichiometric SrFeO 3– δ (ref. 13) as well as similar oxides such as SrFe 1– x Ti x O 3– δ , 141,142 impacting ε chem values.…”

“…13 Such disagreement between the model and the measurement results was explained by oxygen vacancy ordering, typical of highly nonstoichiometric SrFeO 3– δ (ref. 13) as well as similar oxides such as SrFe 1– x Ti x O 3– δ , 141,142 impacting ε chem values. This conclusion is qualitatively supported by the DFT simulation results, showing increasing oxygen vacancy polaron size with increasing δ in Sr 1– x La x FeO 3– δ due to the vacancy ordering.…”