High temperature induced phase transitions in SrCaCoTeO6 and SrCaNiTeO6 ordered double perovskites

The new double perovskite SrCaCoTeO6 and SrCaNiTeO6 have been synthesized and characterized. The crystal structures at room temperature have been obtained from the refinement of the X-ray powder diffraction. Both materials have been found to be monoclinic ordered double perovskites described with the P21/n space group. For both materials, three phase transitions have been highlighted as a function of temperature with the following phase transition sequence: P21/n (a−a−c+) → I2/m (a−a−c0) → I4/m (a0a0c−) → Fm3¯m (a0a0a0), at about 500, 700 and 950K for SrCaCoTeO6 and 375, 675 and 875K for SrCaNiTeO6. [Display omitted] Two new double perovskites SrCaCoTeO6 and SrCaNiTeO6 have been successfully synthesized via the conventional solid state reaction method. The crystal structures and phase transitions of both materials have been investigated using conventional X-ray powder diffraction method at room- and high-temperatures. The Rietveld analysis has shown that both compounds crystallize at room temperature in a primitive monoclinic perovskite-type structure with P21/n (No. 14) space group and 1:1 ordered arrangement of the B-site cations (Co(Ni)/Te). The high temperature analysis has shown that these materials present temperature induced phase transitions following the next phase transition sequence: P21/n(a-a-c+)→I2/m(a-a-c0)→I4/m(a0a0c-)→Fm3¯m(a0a0a0).