Phase Transitions in Mixed Crystal System K_1−x(NH₄)_xH₂PO₄

Abstract

Mixed crystals of potassium dihydrogen phosphate and ammonium dihydrogen phosphate (K_1−x(NH₄)_xH₂PO₄) have been investigated. Dielectric constants ε₁₁ and ε₃₃ and elastic compliance (s_45°−Z) on the 45°−Z-cut bars were measured. The crystals with 0≤x\lesssim0.17 show temperature dependence of the dielectric constant ε₃₃ and elastic compliance s_45°−Z similar to that of pure KH₂PO₄. Occurrence of the spontaneous polarization in these crystals was observed by pyroelectric charge method. The crystals with 0.85\lesssimx≤1 show dielectric and elastic properties similar to those of pure NH₄H₂PO₄, except for weak rounding-offs in the anomalies of dielectric constant for smaller x. It was confirmed by X-ray diffraction that the crystal with x=0.85 undergoes an antiferroelectric phase transition. The crystal with x=0.25 shows broad and round-off anomalies in ε₃₃ and s_45°−Z similar to the temperature dependence of ε₃₃ in Rb_0.66(NH₄)_0.34H₂PO₄ crystal which is reported to show glassy dielectric behavior.