We have studied the temperature and pressure dependence of the elastic constants of ammonium fluoroberyllate [(NH${}_4)$${}_2$BeF${}_4$ or AFB] using Brillouin scattering. Combining the results of the Brillouin-scattering study with those of the measurements of the isothermal compressibility and the volume thermal expansion allows the determination of the anharmonicities, which play an important role in producing the observed anomalies in certain elastic constants at the normal to incommensurate and commensurate to incommensurate phase transitions. Furthermore, we have compared the results of our high-pressure Brillouin-scattering study with the results of an ultrasonic study at high pressure, and have discussed our results based on theoretical models developed to explain the elastic anomalies observed in K${}_2{\mathrm{SeO}}_4$.

• Received 19 June 1997

DOI:https://doi.org/10.1103/PhysRevB.57.5693