RAMAN-SCATTERING AND X-RAY-DIFFRACTION STUDY OF A PHASE-TRANSITION IN(ND4)(2)TEBR6

Raman spectra of (ND4)(2)TeBr6 were recorded in the temperature range 5-300 K, A continuous minimum of the Raman shift and a continuous maxi mum of the linewidth of the A(1g) mode of the TeBr62- octahedron were observed at T-c = 68 K. Also, the T-2g mode of the TeBr62- octahedron continuously split into two lines below T-c. X-ray diffraction measure ments at 10 K indicate that the low-temperature phase has a primitive tetragonal structure, and an antiferro-rotation of the TeBr62- octahed ra is associated with the transition. This behaviour indicates a displ acive phase transition at T-c.