Raman scattering study of temperature induced phase transitions in crystalline ammonium heptafluorozirconate, (NH4)3ZrF7

Abstract This paper reports on a Raman spectroscopy investigation of phase transitions in (NH 4 ) 3 ZrF 7 crystal. Raman spectra were obtained and analyzed in a wavenumber range from 3800 to 15 cm −1 and in the temperature range from 7 to 360 K. The anomalies caused by a series of subsequent structural phase transitions have been analyzed. A soft phonon mode restoring in the distorted phase was found. The spectral changes observed in the current study are similar to those typically found in phase transitions near a tricritical point. The spectral changes in the middle wavenumber range, 700–150 cm −1 , indicate that the phase transitions are due to structural ordering of pentagonal ZrF 7 3− bipyramids. Spectral data also show that a phase transition near 225 K is accompanied by an increase of the unit cell volume. In addition, spectral changes in the range of internal vibrations of ammonium ions indicate that the ion's motion slows down with decrease of temperature.