The pressure dependence of the ferroelectric mode and molecular vibrations of the ferroelectric arsenates

Light scattering measurements are reported of the pressure dependence (0-8 kbar), at 295K, of all the Raman-active modes of vibration in RbH 2 AsO 4 and of the coupled-mode spectra for the B 2 -symmetry phonons in KH 2 AsO 4 , RbH 2 AsO 4 , CsH 2 AsO 4 and their deuterated isomorphs. The pressure dependence of the ferroelectric mode and its interaction with a B 2 optical phonon is determined from fits of the low-frequency y(xy)x spectra to a real-coupled oscillator system. The results reveal a strong dependence on pressure for the ferroelectric mode damping for all the arsenates which causes the mode to become under-damped at quite low pressure. The results are used to discuss the pressure dependence of some of the microscopic parameters characterising the pseudospin model of Kobayashi (1968).