High pressure Brillouin scattering and energy dispersive X-ray diffractionstudy of (NH4)(2)ZnCl4

We have performed a high pressure Brillouin scattering and energy dispersiv e X-ray diffraction study on (NH4)(2)ZnCl4 in the temperature range from ro om temperature to 150 degrees C. At ambient pressure three phase transition s occur in this temperature range. Of special interest are the phase transi tions at 91 degrees C and 133 degrees C because an incommensurate phase has been detected in this temperature range. We compare the results of our hig h pressure Brillouin scattering study with the ones obtained from other stu dies of the elastic constants. Our results for the anomaly in C-33 observed at the commensurate-incommensurate transition at 133 degrees C at ambient pressure contradict the results of the other Brillouin scattering studies b ut agree well with the results of an ultrasonic study. This result seems to indicate that critical fluctuations of the order parameter at the phase tr ansition are reduced by pressure. Values of the calculated adiabatic bulk m odulus, B-S, and the experimentally determined isothermal bulk modulus, B-T , for various materials of the A(2)BX(4) family, are compiled and discussed . (C) 2000 Elsevier Science Ltd. All rights reserved.