The perovskite structure type is one of the most regularly observed structure types in condensed matter sciences, as well as in advanced materials research and applications. This is due to their wide array of proprieties: superconducting, magnetoresistance,…. The double perovskite with general formula A₂BB’O₆ can be represented as a three-dimensional network of alternating BO₆ and B’O₆ octahedra, sharing the oxygens at the corners of the octahedra, with the A-cations occupying the interstitial spaces between the octahedra. The family of antimony double perovskite oxides with double perovskite has attracted a considerable attention because of the magnetic properties of some of its members, such as Sr₂FeSbO₆ [1,2].

The Sr_2-xCa_xLnSbO6 (Ln=La,Sm) (0≤x≤1) materials have been elaborated by the standard solid state reaction method. Rietveld analysis of laboratory x-ray powder diffraction data at room temperature, shows that these materials are double perovskite oxides and that should be represented by Sr_2-xLn_x[Ln_1-xCa_x]SbO₆ for 0≤x≤1, and by [Sr_1-xCa_xLn]CaSbO6 for 1≤x≤2, general formulas. In the lanthanum compounds, the A and B sites are totally occupied by La³⁺ and Ca²⁺ cations, respectively. In the samarium containing compounds, the Sm³⁺ and Ca²⁺ are partially disordered at the A and B. At room temperature, all these materials were found to have a monoclinic unit cell with a primitive space group P2₁/n. The high-temperature powder laboratory x-ray powder diffraction analysis has revealed that the most general temperature evolution of the crystal structures in these materials shows a double phase-transition sequence: from monoclinic (S.G. P2₁/n) to rhombohedral (S.G. R-3) and, then, to cubic symmetry (S.G. Fm-3m), with increasing of temperature. The transition temperatures increase as the size of the cation in the A-site decreases. Thus, the mechanism of these phase transition is related to the mistmatch between the size of the A cation and the cuboctahedral space between the BO6 and B’O6 octahedra.

[1] E.J. Cussen, J.F. Vente, P.D. Battle and T.C. Gibb (1997), J. Mater. Chem. 7 459-463. [2] N. Kashima, K. Inoue, T. Wada and Y. Yamaguchi (2002), Appl. Phys. A 74 S805-S807

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