RAMAN STUDIES ON THE PHASE TRANSITIONS OF (NH₄)₂SnBr₆ CRYSTALS

We report here the structural phase transitions of the crystal (NH₄)₂SnBr₆ investigated by Raman scattering at temperatures ranging from 10K to room temperature. Two phase transitions occurring at 150 and ll0K are found. Based on the group theory, it is proposed that the crystal undergoes a second-order phase transition at 150 K, resulting from a ferro-distortion with symmetry \varGamma⁴⁺. The change of structure is confirmed to be O⁵_h to C⁵_4h, which is assigned to the rotary of SnBr₆^2- ion groups around the axis of. Furthermore the crystal undergoes an order-disorder phase transition at ll0K which is related with the reorientation of the ammonium ion group. It is noticed that the change of the vibrational modes at 77K does not show any phase transition.