Ammonium Ion Motion and Phase Transitions in (ND₄)₂TeCl₆ Studied by Raman Scattering

The internal vibrations of the ammonium ion in (ND₄)₂TeCl₆ have been studied by Raman scattering in the temperature range 4.2-300K. Splittings of A₁-, E-, and T₂-modes have been observed at T_c2 = 48 K and T_c3 = 28 K. These splittings were well interpreted by group theoretical correlation method. The reason for the transitions at T_c2 and T_c3 may be the decrease of the force constant in the deuterated ammoniumion.