J. Appl. Cryst. (2014). 47, 745-754 [ doi:10.1107/S1600576714004907 ]
Abstract: The oxide series La2-xSrxCoTiO6 (0 x 1.0) belong to the perovskite family with general formula ABO3. 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 group P21/n) or orthorhombic (space group Pnma) 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 of B ions in the monoclinic symmetry, which is lost for x 0.30. As the Sr content increases, a better matching of the A-O and B-O distances occurs. This is produced by an elongation of the A-O distance as La3+ is replaced by the larger ion Sr2+, and the shortening of the B-O distance due to the oxidation of Co2+ to Co3+ induced by the aliovalent substitution. As a result, the cuboctahedral A-site cavity becomes less and less distorted; the A ion 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 the BO6 octahedra remain active but those associated with the in-phase tilting become negligible, anticipating for x 0.6 a transition to a new structure with tilting scheme either (a0a0c-) (space group I4/mcm) or (a-a-a0) (space group Imma) or (a-a-a-) (space group Rc).
CCDC references: 989515, 989516, 989517, 989518, 989519, 989520, 989521, 989522, 989523 and 989524
Keywords: perovskites; phase transitions; neutron diffraction; synchrotron X-ray diffraction; AMPLIMODES; cobaltite.
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