Defect structure and chemistry of (Ca x Sr 1-x ) n+1 Ti n 0 3n+1 layer perovskites

Abstract

The Ruddlesden-Popper structural family is constructed from ordered intergrowths of rocksalt type (AX) layers with perovskite (ABX ₃ ) blocks of varying widths, yielding phases with the general formula n ABX ₃ AX where 1 < n < ∞ In this article, the characterization of (Ca _x Sr _1-x ) _n+1 Ti _n O _3n+1 layer perovskites by powder X-ray and neutron diffraction and high resolution electron microscopy is described. These phases undergo a phase transition from tetragonal to orthorhombic symmetry at x = 0.65. Structure solutions of neutron diffraction data are presented for the end-member phases Sr ₃ Ti ₂ O ₇ (I4/mmm), Ca ₃ Ti ₂ O ₇ (Ccm2 ₁ ) and Ca ₄ Ti ₃ O ₁₀ (Pcab). Refinement of the mixed alkaline earth preparation (Ca _0.85 Sr _0.15 ) ₄ Ti ₃ O ₁₀ showed that Sr partitioned preferentially to the perovskite blocks rather than the rocksalt layers. Ordered and disordered intergrowths of rocksalt layers were found on the{101} _orth = {100} _tet perovskite planes with extensive disorder most prevalent in samples annealed for short periods (less than 24 hours). Evidence is presented for the existence of V"A and V"," point defects in the intermediate structures that precede the formation of ordered layer sequences. These data are discussed in terms of existing thermodynamic and structural information. For generalized Ruddlesden Popper phases the relation between chemistry and symmetry is reviewed and factors contributing to the stability of different n -members enunciated. For completeness, the (Ca _x Sr _1-x ) _n+1 Ti _n O _3n+1 perovskites are considered as the structural prototypes of the oxide superconductors being a subset of an extended n ABX ₃ mAX Ruddlesden-Popper family.