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The evolution of the room‐temperature structure of the oxide series La2−xSrxCoTiO6 (0.6 ≤ x ≤ 1.0) was studied as a function of the Sr content using different diffraction techniques and applying the symmetry‐adapted modes formalism (AMPLIMODES). The title compounds adopt perovskite‐like structures of rhombohedral symmetry with an octahedral tilting scheme (a−a−a−) with either space group or R32. The latter symmetry is observed in those cases (for x ≃ 0.6) where additional rock‐salt‐like ordering of La and Sr is produced in the perovskite A sites. Two composition‐driven phase transitions are observed in the whole series La2−xSrxCoTiO6 (0.0 ≤ x ≤ 1.0). Using the concept of internal pressure, the effect of doping with Sr on the structure can be properly discussed. Both phase transitions seem to be of first order since they can be associated with discontinuities either in the entropy or in the structure. The first transition (P21/n → Pnma) occurs as the B cations become totally disordered. Along the whole compositional range the modes responsible for the out‐of‐phase tilting of BO6 octahedra remain active, but those associated with the in‐phase octahedral tilting vanish for x ≥ 0.6, this being associated with the second transition (Pnma → ). Finally, for x = 1.0 the three pseudo‐cubic cell parameters become very similar, pointing to a transition to a cubic structure which could be obtained by applying pressure or raising the temperature.

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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).

The best performances are obtained for low x due to a compromise between sufficiently high amount of defects, but not so high to induce defect clustering.