Vibrational spectra and phase transitions in ferroelectric-ferroelastic langbeinites: K2Mn2(SO4)3, (NH4)2Cd2(SO4)3and Tl2Cd2(SO4)3

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Abstract
Complete vibrational spectra (Raman, infrared and far-infrared) have been recorded for the ferroelastic, ferroelectric langbeinites, K2Mn2(SO4)3, (NH4)2Cd2(SO4)3 and Tl2Cd2(SO4)3. From the polarised single-crystal Raman spectra the vibrational modes, corresponding to the 'internal' vibrations of SO42-ion, were identified and assigned to the various symmetry species of the para phase factor group T for all the crystals. Single-crystal Raman spectra at various temperatures (down to liquid nitrogen temperature) were recorded to study the phase transitions. No soft modes could be observed near the phase transitions. On the other hand abrupt changes in the slopes of full width at half maximum (FWHM) versus temperature and peak height versus temperature curves, corresponding to the Raman-active SO4 'internal' vibrations, were seen near the phase transitions. This indicates strongly the probability of an order-disorder phase transition mechanism involving the SO42- ions. Further cell doubling due to the antiferrodistortive transition to the ferroelectric phases (P21 and P1) in Tl2Cd2(SO4)3, which should be associated with Raman line splittings, could not be observed.