Pressure dependence of the static dielectric properties of K(H1-xDx)2PO4 and RbH2PO4

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Abstract
The effects of hydrostatic pressure on the static dielectric constant, εc, Curie-Weiss constant, C, saturation polarization, Ps, and transition temperature, Ts, of the ferroelectrics K(H1-xDx)2PO4, and RbH2PO4 were investigated. The pressure dependence of Tc is known from earlier work and the present work extends the results to other deuterium concentrations, x; however, the results on C and Ps are largely new. An important feature of the results is the strong dependence of the pressure derivatives of these quantities on deuteration. The results are analyzed in terms of the order-disorder and the coupled proton-phonon models. The order-disorder model provides a good account of the results for the heavily deuterated (x ≃ 1) crystals but not for the lightly deuterated ones. For the latter crystals the coupled proton-phonon model appears satisfactory. In fact the use of known microscopic parameters and their pressure derivatives in this model provides an essentially quantitative account of the pressure dependences of Tc C and Ps of KH2PO4. The measurements on two deuterated crystals with x = 0.35 and 0.82 crystals were extended to quite high pressure to examine the possible vanishing of the ferroelectric state as we have observed earlier for KH2PO4 and RbH2PO4. For the x = 0.35 sample the results indicate that the ferroelectric state vanishes above ≃ 3.8 GPa (= 38 kbar) whereas for the x = 0.82 sample a much higher pressure is needed.