Dielectric Studies of NaNH₄(SO₄)_1−x(SeO₄)_x·2H₂O on the Phase Transition

Abstract

Dielectric constants of a NaNH₄(SO₄)_1−x(SeO₄)_x·2H₂O system along the c axis under stress-free and clamped conditions were measured in a temperature range from −210°C to 30°C. Examinations of the pyroelectric current were also carried out. The dielectric constants under stress-free and clamped conditions followed the Curie-Weiss law above the phase transition temperature. It was therefore suggested that the phase transition of the NaNH₄(SO₄)_1−x(SeO₄)_x·2H₂O system is caused predominantly by dielectric instability. A phase diagram of the NaNH₄(SO₄)_1−x(SeO₄)_x·2H₂O system was established from the results of dielectric measurements. It was found that the phase transition temperature of the NaNH₄(SO₄)_1−x(SeO₄)_x·2H₂O system exhibits a minimum value of −200°C at x=0.3. The space group in the ferroelectric phase of NaNH₄SO₄·2H₂O was identified to be P2₁ from the phase diagram.