TEMPERATURE AND CONCENTRATION DEPENDENCES OF THE LA[110] BRILLOUIN SHIFT IN THE STRUCTURAL GLASS RB1-X(NH4)XH2PO4

Temperature studies of the LA[110] Brillouin shift have been conducted on the mixed ferroelectric-antiferroelectric system, Rb1-x(NH4)xH2PO4 for two ammonium concentrations (x = 0.28 and 0.42). The shift undergoes a softening around 100 K, followed by a hardening of the mode below 50 K. This is due to both a linear (piezoelectric) and a quadratic coupling of this acoustic mode to the polarization fluctuations. The quadratic coupling results from the large electrostriction in these materials and becomes the dominant mechanism at low temperature. This is related to the frustration of the ammonium ions below 100 K. This temperature behavior is consistent with the results previously reported by Courtens et al. on other concentrations of the Rb1-x(NH4)xH2PO4 system.