ADP imputerty effects on the KDP soft-mode

Abstract

Impurity effects on the phase transition in $KH_2PO_4$ (KDP) crystal were studied by Raman scattering and dielectric constant measurements. Antiferroelectric impurities of $NH_4H_2PO_4$ (ADP) are chosen to search for any possible cooperative effects of impurities such as "the spin glass state" of the random magnetic system where the ferromagnetic are mixed with the antiferromagnetic impurities crystals. The dielectric anomaly was observed occuring at the same temperature $T_c$ as for the pure KDP crystal for all the crystals of various ADP impurity concentrations within the range of concentrations less than 0.2\% even though the value of the dielectric constant at $T_c$ was reduced as the impurity concentration increased, indicating the same temperature $T_c$ of the ferroelectric phase transitions for all the crystals studied. However, the softening temperature $T_o$ of the soft-modes obtained from the analysis of the soft-mode Raman Spectra was observed to be raised even above the ferroelectric transition temperature $T_c$ as the ADP impurity concentration increased. $T_o$>$T_c$, first observed in this work, does indeed imply a possible decoupling of the proton ordering from the ferroelectric lattice polarization. The $\vec{q}$-dependent coupling interations of Kobayashi between the protonic softmode and the lattice vibrations were extended to be applied in the mixed crystal to show that the softening temperature $T_o$ should be raised as the antiferroelectric impurity concentration increased. From the experimental data and the two oscillator coupled mode analysis we have obtained :\\ \\ \begin{center}\begin{tabular}{lllll} $\underline{X_o\;(\%)}$ & $\qquad$ & $\underline{T_c\;(^\circ\!K)}$ & $\qquad$ & $\underline{T_o\;(^\circ\!K)}$\\ 0 & & 123 & & 117.7\\ 0.02 & & 123 & & 122.6\\ 0.10 & & 123 & & 130.0\\ 0.50 & & 123 & & 132.0\\ 2.00 & & 123 & & 145.2\\ \end{tabular} \end{center} where $X_o$ (\%) represents the ADP impurity concentration of a starting soluti...