Pressure and Temperature Dependences of the Dielectric Properties and Raman Spectra of RbH2PO4

Abstract

The pressure (0-21 kbar) and temperature [(4-300)°K] dependence of the static dielectric properties of rubidium dihydrogen phosphate were investigated. Among the important results of these measurements are the large decrease in the transition temperature with pressure leading to the eventual vanishing of the ferroelectric state at all temperatures for $P\ge 15.2$ kbar and the decreases with pressure of the Curie constant and dielectric constant in the paraelectric phase. The pressure (0-4 kbar) and temperature [(140-300)°K] dependences of the Raman-active phonons in the range 0-1000 ${\mathrm{cm}}^{-1\mathrm{}}$ were also investigated. The mode Grüneisen parameter $\gamma$ for each of these modes was determined and the pressure dependences of the mode frequencies permitted the modes to be separated into internal and external vibrations relative to the P${\mathrm{O}}_4^{-3\mathrm{}}$ ions. From the coupled-mode spectra for the $B_2$ symmetry phonons, the pressure dependence for the "soft" mode as well as the interaction between this mode and another mode of the same symmetry were determined. These data, together with the dielectric results, were examined within both the coupled proton-tunneling-optic-mode model of Kobayashi and the coupled-mode description discussed by Scott and Wilson. Evaluation of the pressure dependences of the parameters of the Kobayashi model using normally accepted approximations leads to internal inconsistencies in this model. The approximations appropriate to the present measurements and the resulting indeterminancy of the model parameters are briefly discussed.