B 1-to- B 2 structural transitions in rock salt intergrowth structures

The rock salt (B1) structure of binary oxides or chalcogenides transforms to the CsCl (B2) structure under high pressure, with critical pressures P s depending on the cation to anion size ratio (Rc/R a). We investigated structural changes of A2MO3 (A = Sr, Ca; M = Cu, Pd) comprising alternate 7-fold B1 AO blocks and corner-shared MO2 square-planar chains under pressure. All of the examined compounds exhibit a structural transition at Ps = 29-41 GPa involving a change in the A-site geometry to an 8-fold B2 coordination. This observation demonstrates, together with the high pressure study on the structurally related Sr3Fe2O5, that the B1-to-B2 transition generally occurs in these intergrowth structures. An empirical relation of Ps and the Rc/Ra ratio for the binary system holds well for the intergrowth structure also, which means that Ps is predominantly determined by the rock salt blocks. However, a large deviation from the relation is found in LaSrNiO3.4, where oxygen atoms partially occupy the apical site of the MO4 square plane. We predict furthermore the occurrence of the same structural transition for Ruddlesden-Popper-type layered perovskite oxides (AO)(AMO3) n, under higher pressures. For investigating the effect on the physical properties, an electrical resistivity of Sr2CuO3 is studied. © 2011 American Chemical Society.