Investigation of the Local Atomic Structure of Ordered Medium-entropy Perovskite PbSc1/4Fe1/4Nb1/4Ta1/4O3 by Mössbauer and XAFS Spectroscopy

The local atomic structure of the medium-entropy perovskite PbSc1/4Fe1/4Nb1/4Ta1/4O3, which exhibits local cationic ordering of the 1 : 2 type, was studied. Mössbauer spectroscopy revealed two types of Fe3+ ion environment: octahedral (FeO6, ~44%) and with a coordination number of 5 (FeO5, ~56%), indicating the presence of oxygen vacancies in the sample. Based on the analysis of XANES and EXAFS spectra of Fe-, Nb K-edges and Ta-, Pb L3-absorption edges, it has been found that Nb5+ and Ta5+ ions occupy relatively symmetrical octahedral positions, while the Fe3+ environment exhibits significant static heterogeneity due to chemical disorder and defects. Based on the obtained results, a model of the structure of medium-entropy perovskite PbSc1/4Fe1/4Nb1/4Ta1/4O3 has been proposed, in which the global cubic symmetry coexists with nanoscale ordered regions of two types: enriched with trivalent ( [Pb(B_2/3^3 + B_1/3^5 + )O₃]^1/3 - ) and pentavalent ( [Pb(B_1/3^3 + B_2/3^5 + )O₃]^1/3 + ) cations, the charge of which is compensated by oxygen and lead vacancies, respectively.