Raman spectra and phase transitions in some langbeinites

Double sulfates of mono- and divalent metals belong to a family of compounds with a M2+ M22+(S04), overall formula which mainly crystallize in the structural type K2Mg2(SO4)3 (langbeinite). In the high-symmetry phase the langbeinite structure described by the space group T4(P213) with four molecules per unit cell.1 By now, there have been synthesized 26 compounds of this family on 11 of which the phase transitions have been discovered. In some crystals a sequence of transitions has been observed.2