New Structural Distortions in Osmate Perovskite Na1–xKxOsO₃ Synthesized under High Pressure

Most ABO3 oxides crystallize in the perovskite structure. In response to the degree of bonding mismatch of A–O versus B–O in the structure, the perovskite can adopt a total of 15 tilting systems of BO6 octahedra. Depending on the charge configurations, i.e., A3+B3+O3, A2+B4+O3, and A1+B5+O3, these complex oxides undergo distinct pathways within the tilting systems as the bond length mismatch is changed by either chemical substitution or temperature or pressure. The report of orthorhombic NaOsO3 and the newly synthesized nearly cubic KOsO3 lead to an opportunity for studying the structural distortions in A1+B5+O3 which has only been studied in the d0 systems of AMO3 (A = alkaline, M = Nb and Ta). Here, we report the new structural sequence from a cubic perovskite phase to distorted phases as the temperature decreases in the solid solution of Na1–xKxOsO3 by synchrotron X-ray powder diffraction; these distorted phases do not belong to the 15 tilting systems. In comparison with the distorted perovskite phases found in the d0 systems of A1+B5+O3 perovskites, the phase transitions with decreasing temperature found in Na1–xKxOsO3 are likely caused by the instabilities of their electronic structures.