Polarization Relaxation and Susceptibility in the Ferroelectric Transition Region of KD2PO4

Previously published results on the propagation characteristics of sound waves in K${\mathrm{D}}_2$P${\mathrm{O}}_4$ are analyzed phenomenologically to give the electric susceptibility and the polarization relaxation rate above and below the transition temperature. These results are then compared with the predictions of a modified Slater-Takagi order-disorder model of the phase transition in this crystal. The comparison of susceptibility and relaxation rate as deduced in the sound-propagation experiment with analogous data obtained from direct dielectric and other experiments is moderately good. The comparison with the predictions of the order-disorder model are probably as good as can be expected in view of the simplicity of the model. In particular, the high ratios of slopes in the temperature dependence of the relaxation rate and the reciprocal susceptibility above and below the transition temperature are correctly given using the measured values of certain critical parameters of the order-disorder model and reasonable values of other parameters which are not separately measured.