Kiyoshi Deguchi

Effects of Deuteration on the Dielectric Properties of Ferroelectric CsH2PO4. I. Static Dielectric Properties

The dielectric constant along the ferroelectric b -axis and the spontaneous polarization have been measured for CsH 2(1- x ) D 2 x PO 4 ( x =0∼0.98) over wide temperature ranges. The temperature dependences of the dielectric constant in the paraelectric phase are well described on the basis of the quasi-one-dimensional Ising model except in the vicinity of the Curie temperatures, 0K\({\leq}T{-}T_{\text{c}}{\lesssim}20\) K, for all the specimens with different amounts of x . The saturated value of the spontaneous polarization increases with increasing x . The observed results are discussed in terms of the quasi-one-dimensional model.

Effect of Heat-Treatment of Dielectric Properties of Ca2Sr(C2H5CO2)6

The dielectric constant and the spontaneous polarization of Ca 2 Sr(C 2 H 5 CO 2 ) 6 have been measured for unannealed crystals and for crystals annealed at various temperatures between 150°C and 390°C. Almost all the unannealed crystals contain the internal bias field of about a few kilovolt per centimeter. For annealing temperatures higher than 330°C, the internal bias field vanishes completely and the Curie temperature T c shifts remarkably toward lower temperatures. Deviations from the Curie-Weiss law are observed for dipolar dielectric constant e - e ∞ near T c . It is found that Δ T = T o - T c , where T o is the extrapolated Curie-Weiss temperature, increases with decreasing T c .

Dielectric Critical Slowing-Down in Ferroelectric (CH3NHCH2COOH)3·CaCl2

Complex dielectric constants of (CH 3 NHCH 2 COOH) 3 ·CaCl 2 and (CH 3 NDCH 2 COOD) 3 ·CaCl 2 have been measured. between 10 4 Hz and 10 9 Hz in the Curie temperature region. No appreciable isotope effect was found in the complex dielectric constants as well as in the Curie temperature. Dielectric dispersion of the Debye type which showed a critical slowing-down was observed above about 100 MHz. Discussion was made of the relation between the observed critical slowing-down and the softening of the optical phonon reported previously.

Static and Dynamic Critical Properties of Dielectric Constant in Ferroelectric CsH2PO4 and CsD2PO4

Monodispersive dielectric dispersion has been observed in CsH 2 PO 4 and CsD 2 PO 4 in the frequency range between 1 MHz and 1 GHz. Critical anomalies found in the static dielectric constant and the relaxation time, e H s and τ H for CsH 2 PO 4 and e D s and τ D for CsD 2 PO 4 , respectively, show pronounced deviations from the Curie-Weiss behavior. Simple relations of e D s / e H s =1.3 and τ D /τ H =5.2 for the values at the same T - T c , where T c is the transition temperature of each material, are found in wide temperature ranges above T c . It is hard to explain these results by the tunneling model.

Pinning Effect on Dynamical Dielectric Properties of Rb2ZnCl4 in the Incommensurate Phase

Dielectric dispersion has been studied for several crystals of Rb 2 ZnCl 4 prepared under different conditions in the frequency range from 10 3 Hz to 10 9 Hz around the incommensurate-commensurate phase transition temperature T c . Although the dielectric dispersion of the relaxational type is observed for all the crystals, the apparent critical slowing down is found only for the less defective crystals in which the relaxation is monodispersive in the temperature range T > T c +5 K, but becomes polydispersive near T c in the incommensurate phase. In the defective crystals, a wide distribution of the relaxation times was observed in the whole temperature range of the incommensurate phase.

Dielectric critical slowing-down in hydrogen-bonded ferroelectrics: PbHPO4 and PbDPO4

Complex dielectric constants of PbHPO 4 and PbDPO 4 have been studied at frequencies from 10 3 Hz to 10 9 Hz in the Curie temperature region. The Curie-Weiss behavior of the low-frequency dielectric constant is confirmed. The dielectric dispersion of the Debye type which shows a critical slowing-down has been observed in both materials. The phase transition is considered to be of an order-disorder type. The effects of deuteration in the static and dynamic dielectric properties are discussed.

X-Ray and Dielectric Studies of Commensurate-Incommensurate Phase Transition in Rb2ZnCl4

The commensurate (C)-incommensurate (IC) phase transition in Rb 2 ZnCl 4 has been investigated through studies of the X-ray diffraction and the P - E hysteresis loop. Simultaneous observation of the P - E hysteresis loop and the misfit parameter reveals that the double hysteresis loop observed in the IC phase corresponds to the field induced C-IC transition. The “triple”-like hysteresis loop observed in the vicinity of the C-IC phase transition temperature corresponds to the coexistence of the IC and the C state near E =0 followed by the field induced C-IC transition at a finite value of E . The effect of pinning on the electric field dependence of the misfit parameter and the polarization is considered on the basis of the Landau type theory. The tendency of the calculated results agrees well with that of the experimental results.

Dielectric Dispersion and Twin Structure in Hexagonal Barium Titanate

The complex dielectric constant of hexagonal BaTiO 3 , (h-BT) along the ferroelectric axis has been measured in the frequency range from 10 3 Hz to 10 9 Hz. It is found that the specimen cooled in a strong d.c. electric field exhibits resonant-type dielectric dispersions around 1 MHz and 300 MHz both in the phase II (222 K> T > T c =62 K) and the phase III ( T c > T ). The dispersion around 1 MHz does not appear for the specimen cooled without the d.c. electric field. The effect of the field-cooling on these dispersions is discussed in connection with the twin structure expected from a group theoretical consideration. The dielectric anomaly around T c diminishes in the damped dielectric constant, indicating that the ferroelectric phase transition in h-BT is closely related to an elastic instability.

Re-Examination of the Dielectric Critical Slowing Down in Ferroelectric Triglycine Sulfate

Dielectric dispersion of triglycine sulfate in the vicinity of the Curie temperature T c was observed in the frequency range from 1 kHz to 1 GHz. The relaxation time τ of the dispersion is well expressed by the power law τ∝( T - T c ) - Δ in the region 0.07 K\( \lesssim T-T_{\text{c}} \lesssim \)10 K with Δ =1.07 that is quite different from the non-classical value, Δ ≃2/3, reported recently. The magnitude of the activation energy of dipole reversal is found to be about 3 k T c , which is fairly larger than that in the earlier study, indicating that the phase transition is of the order-disorder type.

A neutron diffraction study on the deuteration effect in CsH2PO4 crystal

The deuteration rate x of partially deuterated crystals, Cs(H 1- x D x ) 2 PO 4 , was determined through the neutron diffraction experiment. It was found that two non-equivalent positions of hydrogen atoms take slightly different deuteration rates. The deuteration rate x 1 of the shorter hydrogen bond which is closely related to the ferroelectric phase transition is smaller than the rate x 2 of the longer one. The observed dependence of the phase transition temperature T c on x 1 is found to be linear in contrast to the concaved dependence predicted in the one-dimensional proton-pseudospin coupled model. The neutron diffraction experiment also revealed that the distance of the shorter hydrogen bond increased linearly with x 1 , whereas that of the longer one remained constant.