Ferroelectricity and isotopic effect in NH4HSeO4

OPTICAL AND ELECTRIC MANIFESTATIONS OF INCOMMENSURATE PHASE IN NH_{4}HSeO_{4} CRYSTALS

“…The experimental results on the temperature dependences of optical birefringence of AHSe [9,10] are rather ambiguous. The temperature dependences of dielectric permittivity ε ′ (T ) of AHSe were studied earlier [1][2][3][4][5]. One maximum of this dependence has been found at 250 K, which was explained by the transition from the IC to the ferroelectric phase.…”

“…The crystal under study is characterized by a weak ferroelectric (FE) PT (∆P s 0.5 · 10 −2 C/m 2 ) at 250 K [1,2]. Therefore, this PT manifests itself in comparatively small anomalies in the temperature dependences of different physical parameters, that complicates the experimental studies of the PT.…”

“…The phase III is an incommensurate (IC) one [7] and is limited by the temperature T 34 = T c = 250 K from the lower side. The AHSe crystal exhibits the ordered ferroelectric-ferroelastic phase IV in the temperature range between 100 and 250 K. The phase V below T 45 = 100 K is nonferroelectric [1]. The PTs at 100, 250, and 406 K in AHSe are regarded as c B.V.Andriyevsky , Z.Czapla , S.Dacko , O.Ya.Myshchyshyn being of a discontinuous type (first order).…”

“…Below the melting temperature T m = 427 K, the crystal possesses a superionic conductive phase I. Paraelectric (PE) monoclinic phase II is characterized by spatial symmetry C 3 2 and by piroelectric polarization along the b-axis [1]. The lower limit of this phase is ascribed by many investigators to the temperature T 23 = T i = 262 K [1][2][3][4][5][6].…”

“…The PTs at 100, 250, and 406 K in AHSe are regarded as c B.V.Andriyevsky , Z.Czapla , S.Dacko , O.Ya.Myshchyshyn being of a discontinuous type (first order). It was established that the changes of physical parameters at the IV ↔ V phase transition exceed those occurring at the III ↔ IV and II ↔ III phase transitions [1].…”