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 (NH 4 ) x H 2 PO 4 ) have been investigated. Dielectric constants ε 11 and ε 33 and elastic compliance ( s 45°- Z ) on the 45°– Z -cut bars were measured. The crystals with \(0{\leqq}x{\lesssim}0.17\) show temperature dependence of the dielectric constant ε 33 and elastic compliance s 45°- Z similar to that of pure KH 2 PO 4 . Occurrence of the spontaneous polarization in these crystals was observed by pyroelectric charge method. The crystals with \(0.85{\lesssim}x{\leqq}1\) show dielectric and elastic properties similar to those of pure NH 4 H 2 PO 4 , 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 ε 33 and s 45°- Z similar to the temperature dependence of ε 33 in Rb 0.66 (NH 4 ) 0.34 H 2 PO 4 crystal which is reported to show glassy dielectric behavior.