Structures of Langbeinite-Type K2Mn2(SO4)3 in Cubic and Orthorhombic Phases

Structures of cubic and orthorhombic phases of langbeinite-type K₂Mn₂(SO₄)₃ have been determined at 20°C and -90°C by X-ray diffraction. Thermal parameters of oxygen atoms in the cubic phase are remarkably anisotropic and show unusually high values, whereas those in the orthorhombic phase are rather normal. The change in bond lengths and angles at the transition clarifies a rigid body motion of SO₄ group. It is concluded that the principal structural change at the cubic-orthorhombic transition is characterized by ordering of sulfate groups.