Pressure-Induced Transitions in Square Planar Coordinate Metal Oxides

Abstract

A spin state transition in a square planar coordinated iron was found in SrFeO₂ and Sr₃Fe₂O₅ under high pressure. This transition occurs as a consequence of enhanced Fe-Fe direct interactions between adjacent face-to-face FeO₄ square planar units. We also found a pressure-induced structural transition in Sr₃Fe₂O₅, which is viewed as a B1（NaCl structure） to B2（CsCl structure） structural transition. Such a B1-B2 structural transition generally occurs in intergrowth system consisting of rock-salt blocks and square-planar blocks, as found in A₂MO₃ （A＝Ca, Sr; M＝Cu, Pd）. An empirical relation between the structural transition pressure P_s and the ionic radius for the binary system holds well for the intergrowth structure, which indicates that the square-planar block is highly compressible along the out-of-plane direction.