Phase Transition in CsH3(SeO3)2

Dielectric, pyroelectric, thermal, optical and crystallographic properties of CsH 3 (SeO 3 ) 2 were studied. Various pyroelectric tests indicate essentially no net spontaneous polarization below the Curie point, -128°C. Correspondingly, no ferroelectric hysteresis loop appears; D vs E curves are linear except for an impressive form having the critical field, E c , in a narrow temperature range just below the Curie point, T c , under an applied ac field perpendicular to (100). The critical field increases with decreasing temperature and is given with sufficient approximation by an expression; E c =2.3×10 4 ( T c - T ) 1/2 V/cm. The dielectric constant, ε ⊥(100) , shows a peak at the Curie point, which shifts, to lower temperature on application of a biasing dc field. These dielectric results may well be interpreted as a field-induced transition phenomenon from an antiferroelectric state to a paraelectric one. The measurement of specific heat as a function of temperature yields δ S =0.81 cal/mole·deg . The upper phase has symmetry P 1 and the cell dimensions are a u =9.32 6 Å, b u =6.52 6 Å, c u =5.83 5 Å. α u =91.5°, β u =105.1°, γ u =91.1°. The lower phase also has symmetry P 1 , but superstructure along the c axis, i.e. , \(cₗ{}2cᵤ\).