Dielectric Properties of a New Ferroelectric, NaNH4SO4·2H2O, near Its Curie Temperature

Dielectric properties of a new ferroelectric, NaNH 4 SO 4 ·2H 2 O, near its Curie temperature are studied. The crystal has three successive phases, i.e. a ferroelectric (III), another ferroelectric (II), and a paraelectric phase (I). The II-III phase transition is accompanied with a small discontinuous change in spontaneous polarization. The dielectric constant along the c axis ( ε c .) above the Curie temperature follows the Curie.Weiss law, ε c = ε 0 + C /( T - T c ), with a rather normal magnitude of the Curie-Weiss constant, C . Two different shapes of ε c vs T curves are observed and the result is discussed with the help of pyroelectric measurements.