PRESSURE AND TEMPERATURE-DEPENDENCE OF THE ELASTIC-CONSTANTS OF AMMONIUM FLUOROBERYLLATE

We have studied the temperature and pressure dependence of the elastic constants of ammonium fluoroberyllate [(NH4)(2)BeF4 or AFB] using Bri llouin scattering. Combining the results of the Brillouin-scattering s tudy with those of the measurements of the isothermal compressibility and the volume thermal expansion allows the determination of the anhar monicities, which play an important role in producing the observed ano malies in certain elastic constants at the normal to incommensurate an d commensurate to incommensurate phase transitions. Furthermore, we ha ve compared the results of our high-pressure Brillouin-scattering stud y with the results of an ultrasonic study at high pressure, and have d iscussed our results based on theoretical models developed to explain the elastic anomalies observed in K2SeO4.