Dielectric Studies of NaNH₄(SO₄)₁-x(SeO₄)_x.2H₂O on tlie Phase Transition

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 a:=0.3. The space group in the ferroelectric phase of NaNH4 SO4. 2H2 O was identified to be P2i from the phase diagram.