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Structure evolution with Sr content of the perovskite‐like materials La 2− x Sr x CoTiO 6 (0 ≤ x ≤ 0.5)
Author(s) -
GómezPérez Alejandro,
PérezFlores Juan Carlos,
Ritter Clemens,
Boulahya Khalid,
Castro German R.,
GarcíaAlvarado Flaviano,
Amador Ulises
Publication year - 2014
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s1600576714004907
Subject(s) - orthorhombic crystal system , octahedron , monoclinic crystal system , crystallography , perovskite (structure) , tetragonal crystal system , crystal structure , ion , chemistry , space group , materials science , physics , x ray crystallography , diffraction , organic chemistry , optics
The oxide series La 2− x Sr x CoTiO 6 (0 ≤ x ≤ 1.0) belong to the perovskite family with general formula AB O 3 . 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 P 2 1 / 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 La 3+ is replaced by the larger ion Sr 2+ , and the shortening of the B —O distance due to the oxidation of Co 2+ to Co 3+ 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 B O 6 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 ( a 0 a 0 c − ) (space group I 4/ mcm ) or ( a − a − a 0 ) (space group Imma ) or ( a − a − a − ) (space group R c ).