Phase transitions in (ND4)2BeF4 crystal

The results of a Raman study of deuterated ammonium fluoberyllate single crystals are presented. It is shown that deuterating the sample has no significant effect on the behavior of the Raman lines at the ferroelectric phase transition (Tc1 = 180°K), indicating that the determinative role in the establishment of order is played by the BeF4− − ion. Nonlinear resonance between different lattice vibrations is confirmed, and it is probably this that is responsible for the phase transition. A new phase transition at Tc2 = 100°K, occurring with a change in the symmetry of the lattice, is discovered.

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