Structural phase transitions in the ordered double perovskite Sr ₂MnTeO₆

The crystal structure of the ordered double perovskite Sr ₂MnTeO₆ has been refined at ambient temperature from high resolution neutron and x-ray powder diffraction data in the monoclinic space group P12₁/n1 with a = 5.7009(1)Å, b = 5.6770(1) Å, c = 8.0334(1) Å and β= 90.085(1)°. This represents a combination of in-phase (+) and out-of-phase (-) rotations of virtually undistorted MnO ₆ and TeO₆ octahedra in the (- - +) sense about the axes of the ideal cubic perovskite. High temperature x-ray powder diffraction shows three structural phase transitions at approximately 250,550 and 675°C, each corresponding to the disappearance of rotations about one of these axes. The first transition was analysed by differential scanning calorimetry and showed a thermal hysteresis with an enthalpy of 0.55 J g^-1. We propose the (P12₁/n1 → 112/m1 → 14/m → Fm3̄m) sequence of structural transitions which has not been previously reported for a double perovskite oxide.