High pressure Brillouin scattering and energy dispersive X-ray diffraction study of (NH4)2ZnCl4 Original Research Article

We have performed a high pressure Brillouin scattering and energy dispersive X-ray diffraction study on (NH4)2ZnCl4 in the temperature range from room temperature to 150°C. At ambient pressure three phase transitions occur in this temperature range. Of special interest are the phase transitions at 91°C and 133°C because an incommensurate phase has been detected in this temperature range. We compare the results of our high pressure Brillouin scattering study with the ones obtained from other studies of the elastic constants. Our results for the anomaly in C33 observed at the commensurate–incommensurate transition at 133°C at ambient pressure contradict the results of the other Brillouin scattering studies but agree well with the results of an ultrasonic study. This result seems to indicate that critical fluctuations of the order parameter at the phase transition are reduced by pressure. Values of the calculated adiabatic bulk modulus, BS, and the experimentally determined isothermal bulk modulus, BT, for various materials of the A2BX4 family, are compiled and discussed.