Dielectric Studies on the Phase Transitions of {(NH₄)₃H(SO₄)₂}_1-x{(ND₄)₃D(SO₄)₂}_x System –Ferroelectricity in (NH₄)₃H(SO₄)₂–

Abstract

Dielectric constant along the direction perpendicular to the (001) plane in crystals of {(NH 4 ) 3 H(SO 4 ) 2 } 1- x {(ND 4 ) 3 D(SO 4 ) 2 } x system was measured for various x values in a temperature range from liquid nitrogen temperature to room temperature. The crystal has two ferroelectric phases in the range of x ≥0.9 and one ferroelectric phase in the range of x <0.9. It is found from the phase diagram that the normal crystal (NH 4 ) 3 H(SO 4 ) 2 undergoes a ferroelectric phase transition at about -210°C, besides four non-ferroelectric transitions reported previously. Saturated value of spontaneous polarization estimated as a function of x is about 1.4 µC/cm 2 for (NH 4 ) 3 H(SO 4 ) 2 and about 2.5 µC/cm 2 for (ND 4 ) 3 D(SO 4 ) 2 in the lowest ferroelectric phase. There is practically no isotope effect on non-ferroelectric transitions, in contrast with large isotope effect on ferroelectric ones.