Polar and vibrational properties of dimethylammonium cation and orientational ordering in the ferroelectric phase transition in a dimethylammoniumsulphate hexahydrate (DMAAS) ferroelectric-ferroelastic crystal

The equilibrium geometry, charge distribution, dipole moment, and frequenci es of internal vibrations of a free dimethylammonium (DMA) cation are deter mined by ab initio calculations. The theoretically calculated frequencies a re compared with the experimental Raman-scattering data on the lattice dyna mics of ferroelectric DMAAS crystals. The atomic charges and the dipole mom ents determined provided the calculation of the reorientation energy of a D MA-cation in the lattice of a DMAAS crystal. The temperature of the ferroel ectric phase transition and the spontaneous polarization in the ferroelectr ic phase are calculated in the approximation of a self-consistent molecular field. The calculated data are in good accordance with the experiment.