Structure evolution with Sr content of the perovskite-like materials La2−xSrxCoTiO6(0 ≤x≤ 0.5)

Abstract

The oxide series La2−xSrxCoTiO6(0 ≤x≤ 1.0) belong to the perovskite family with general formulaABO3. The evolution of the room-temperature structure as a function of the Sr content was studied using complementary techniques by applying the symmetry-adapted modes formalism (AMPLIMODES). In the compositional range presented in this article (0 ≤x≤ 0.5), the compounds adopt distorted perovskite structures of monoclinic (space groupP21/n) or orthorhombic (space groupPnma) symmetry, both with octahedral tilting scheme (a−a−c+) (out of phase along two perovskite main directions and in phase along the third direction). The main difference between these structures is the existence of rock-salt order ofBions in the monoclinic symmetry, which is lost forx≥ 0.30. As the Sr content increases, a better matching of theA—O andB—O distances occurs. This is produced by an elongation of theA—O distance as La3+is replaced by the larger ion Sr2+, and the shortening of theB—O distance due to the oxidation of Co2+to Co3+induced by the aliovalent substitution. As a result, the cuboctahedralA-site cavity becomes less and less distorted; theAion tends to occupy its ideal positions, increasing its coordination and giving rise to a more symmetrical structure. In the whole compositional range, the symmetry-adapted atomic displacements (modes) responsible for the out-of-phase tilting of theBO6octahedra remain active but those associated with the in-phase tilting become negligible, anticipating forx≥ 0.6 a transition to a new structure with tilting scheme either (a0a0c−) (space groupI4/mcm) or (a−a−a0) (space groupImma) or (a−a−a−) (space groupR\overline 3c).