Coupled anion and cation ordering in Sr3RFe4O10.5 (R=Y, Ho, Dy) anion-deficientperovskites

The Sr₃RFe₄O_10.5 (R=Y, Ho, Dy) anion-deficient perovskites were prepared using a solid-state reaction in evacuated sealed silica tubes. Transmission electron microscopy and ⁵⁷Fe Mössbauer spectroscopy evidenced a complete A-cations and oxygen vacancies ordering. The structure model was further refined by ab initio structure relaxation, based on density functional theory calculations. The compounds crystallize in a tetragonal a≈2√2a_p≈11.3 Å, с≈4с_p≈16 Å unit cell (a_p: parameter of the perovskite subcell) with the P4₂/mnm space group. Oxygen vacancies reside in the (FeO_5/4□_3/4) layers, comprising corner-sharing FeO₄ tetrahedra and FeO₅ tetragonal pyramids, which are sandwiched between the layers of the FeO₆ octahedra. Smaller R atoms occupy the 9-fold coordinated position, whereas the 10-fold coordinated positions are occupied by larger Sr atoms. The Fe sublattice is ordered aniferromagnetically up to at least 500 K, while the rare-earth sublattice remains disordered down to 2 K.