STRUCTURAL STUDY OF PHASE-TRANSITION IN LANGBEINITE-TYPE K2MN2(SO4)(3) CRYSTALS

Accurate crystal structure of K2 Mn-2 (SO4)(3) was determined at several temperatures in the high-temperature phase. It turns out from the split-atom calculation that the SO4 ion takes two reorientational arrangements as has been implied in the early studies. Thermal vibrations of all the constituent atoms become large with approaching the phase transition temperature. We conclude that the coexistence of the disordered motions and the soft vibrations arises from the short Mn-O distances which govern the driving mechanism of the phase transition.