High pressure Brillouin scattering study of ammonium fluorberyllate

✍ Scribed by A. Simon; M. Takesada; K. Weishaupt; M. Gelfand; A. Onodera; H.D. Hochheimer

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 530 KB
• Volume: 101
• Category: Article
• ISSN: 0038-1098
• DOI: 10.1016/s0038-1098(96)00498-x

✦ Synopsis

We have studied the temperature and pressure dependence of the elastic constants of ammonium fluorberyllate ((NH4)2BeF4 or AFB} using Brillouin scattering. We report here results of the temperature and pressure dependence of the elastic constant Cs5, 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 Css when this transition is approached at constant pressure with increasing temperature. We interpret the fact that (a ln C,/aT) has the same sign and is much larger than -(&)( a In C&~P)~, where p" and KT 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 C5s at the commensurate to incommensurate phase transition.