Impurity effects on phase transition anomaliesin 4KH_2PO_4$ crystal

Abstract

The ferroelectric phase transition in $KH_2PO_4$ (KDP), $KDP_{1-x}ADP_x(K_{1-x}(NH_4)_xH_2PO_4)$ and $KDP_{1-x}KDA_x(KH_2P_{1-x}As_xO_4)$ crystals has been studied using dielectric measurements and Raman scattering for varing concentrations x of isomorphic impurities. In the dielectric constant measurements we observed the linear dependence of the transition temperature $T_c$ on the impurity concentration x, characterized by $dT_c/dx\simeq -250K$ for $KDP_{1-x}ADP_x$ and $dT_c/dx\simeq -60K$ for $KDP_{1-x}KDA_x$ mixed crystals. Such a negative temperature shift is well understood in the virtual crystal approximation (VCA). The VCA solution for the transition temperature in the pseudo-spin model given by Lage and Stinchcombe was extended for out problems, and the numerical solutions giving the observed values were obtained as $\mu_{ADP}=0.36＼mu_{KDP}$ and \mu_{KDA} =0.88\mu_{KDP}$ with $\Omega_{KDP}=\Omega_{KDA}=\Omega_{ADP}=300K$. The physical significances of the approximations used and the resultant numerical values are critically discussed. In the dielectric loss measurements we observed, for the domain freezing temperature $(T_f(x))$, $dT_f/dx\simeq -850K$ for $KDP_{1-x}ADP_x$ and $dT_f/dx\simeq -500K$ for $KDP_{1-x}KDA_x$ crystals. These results are also interpreted using the VCA solutions. The domain freezing phenomena of KDP type crystals was examined from the viewpoint of the roughening transition of the interfacial layer between domains and a qualitative explanation was given for more pronounced impurity effect on $T_f$ than $T_c$ using the roughening mechanism. In the high concentration regime of x≥0.05 for $KDP_{1-x}KDA_x$ crystals, we observed that the anomalous domain dynamics disappeared. The polarization reversal current measurements using the step-function field E≤185 V in $KDP_{1-x}ADP_x$ crystals were also applied to observe the domain freezing. And the field dependence of the freezing temperature $T_f$ was observed to show $T_f$ signifi...