High pressure Brillouin scattering study of ammonium fluorberyllate
Abstract
We have studied the temperature and pressure dependence of the elastic constants of ammonium fluorberyllate {(NH 4) 2BeF 4 or AFB} using Brillouin scattering. We report here results of the temperature and pressure dependence of the elastic constant C55, which shows anomalies both at the normal to incommensurate and incommensurate to commensurate ferroelectric phase transition at ambient pressure. Application of high hydrostatic pressure at constant temperature results in an increase of the elastic constant when approaching the phase transition to the incommensurate phase from the commensurate phase. This is contrary to the behaviour of C55 when this transition is approached at constant pressure with increasing temperature. We interpret the fact that (∂ ln{C 55}/{∂T}) p has the same sign and is much larger than -( {β}/{κ T})(∂ ln{C 55}/{∂p}) T, where β and κT are the volume thermal expansion and isothermal compressibility, respectively, as an indication that anharmonicities (cubic or quartic, or both) play an important role in producing the observed anomaly of C55 at the commensurate to incommensurate phase transition.
- Publication:
-
Solid State Communications
- Pub Date:
- January 1997
- DOI:
- 10.1016/S0038-1098(96)00498-X
- Bibcode:
- 1997SSCom.101..139S