2016 年 58 巻 6 号 p. 261-266
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 Ps 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.