Raman investigation of the phase transition in (NH4)2(BeF4)

A Raman investigation of the low frequency spectrum (0 - 200 cm-1) of ammonium fluoroberyllate is presented in the temperature range 10 to 260K. The anomalous behaviours of the very low frequency spectra and of some modes are discussed in connection with the ferroelectric transition and the disordered character of the paraelectric phase.