Pressure-induced phase transitions in ${\mathrm{LiNH}}_4$${\mathrm{SO}}_4$

Raman spectra of ${\mathrm{LiNH}}_4$${\mathrm{SO}}_4$ in frequency range from 600 to 1200 ${\mathrm{cm}}^{\mathrm{-}1}$ were measured from atmospheric pressure up to a pressure at 82 kbar. Two distinct phases are identified in the ranges below 13 kbar (β phase) and above 35 kbar (δ phase). In the range between 13 and 35 kbar the spectrum changes qualitatively in a continuous manner, both as a function of time for fixed pressure and as a function of pressure when the latter is rapidly changed. The results suggest a dynamical reorientation of the ${\mathrm{SO}}_4$${\mathrm{}}^{2\mathrm{-}}$ ions taking place in this pressure interval.