Vibrational spectra and phase transitions in columnar M I3[M′III(SO4)3] compounds

Compounds of general formula M[M′III(SO4)3] exhibit a columnar structure and, at room temperature, belong to the C, C and C symmetry space groups with, respectively, M1T1, M′III In; MIT1, M′IIIFe; and MI Cs, M′III Fe. The Raman spectra of powder samples have been recorded in the temperature range 77–600 K. These vibrational studies indicate the role played by the SO4 groups and MI cations in the phase transitions. All the compounds follow a space group phase sequence in accordance with the general point group relationship 2/m 3 3m 3m.