a State Key Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Academia Sinica, Shanghai 200083, China; b Bohl Institute of Soild State Physics, Tongji University, Shanghai 200092, China; c Institute of Experimental Physics, Ruhr University, W-4630 Bochum, Germany
Abstract 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 $\varGamma$4+. The change of structure is confirmed to be O5h to C54h, 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.
Received: 31 August 1994
Accepted manuscript online:
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