Structural glass phase in ferroelectric-anferroelectric mixed crystal

Abstract

Random mixtures of KDP-type ferroelectrics and antiferroelectrics were studied to investigate the frustrated cooperative phenomena associated with competing interaction in random systems. We have studied $(KDP)_{1-x}(ADP)_x$ crystals at low temperatures, KDP impurity effects on the antiferroelectrics side, to obtain a partial phase diagram complementary to the $\mbox{\overline{A}$DP impurity effects on the ferroelectric side. Decreasing transition temperature due to the competing interaction on both sides is explanined in terms of the Prelovsek-Blinc theroy. The solid solution between two different KDP-type antiferroelectrics were also studied, where phosphates and arsentates were randomly mixed to form a good-quality mixed crystal. The transition temperature was found to fail between the temperatures of the end components and a strong first-order transition was retained. We studied the dielectric properties of $(RDP)_{0.61}(ADP)_{0.39}$ crystals along the [100]-and [110]-directions in the temperature range from 300 K down to 10 K. The possibility of ferroelectricity along the [110]-direction suggested in a recent theoretical phase diagram of $(RDP)_{1-x}(ADP)_x$ was found questionable. Tetragonal structure of glass phase at low temperatures was confirmed in accordance with the previous results of X-ray diffraction and Raman scattering. Raman spectra of $(RDP)_{0.61}(ADP)_{0.39}$ was studied in detail to find glass phase anomalise of $B_2$ symmetry soft-mode and $NH_4^+$-local modes in systematic comparisions with those of pure KDP, ADP, RDP and RDA crystals. We found that there were symmetry breaking effects (local dynamical symmetry lowering of internal vibrations and two mode behavior of lattice vibration of $B_2$ symmetry) even at room temperature.