Toru Ozaki

Plastic Deformation by Twinning in CsHSO4 Single Crystal

Plastic deformation by mechanical twinning with (001) twin plane has been observed in the b -plate of CsHSO 4 single crystals at room temperature. A parallelogram-shaped hysteresis loop is found between the external shear stress and the deformation angle of the crystal plate. The maximum deformation angle is found to be 24° which is very close to 2(β-90°), where β=101.51° is the monoclinic angle of the crystal lattice. A microscopic model based on the crystal structure for the twin structure is proposed.

Measurement of Initial Dielectric Constant of KH2PO4 by Thermal Noise Method

The dielectric constant of KH 2 PO 4 along the ferroelectric axis is measured at 1 kHz between 90 K and 130 K by a thermal noise method by which one can obtain the dielectric constant without applying Chi external field. The results well agree with the values obtained by the usual bridge method, using the external field of 1 to 20 V/cm, when extrapolation has been made toward zero external field. In the ferroelectric phase, the initial dielectric constants of the multi-domain crystal thus determined are of the order of 10 2 times as large in magnitude as those of the single domain crystal. It is concluded that the most part of the initial dielectric constants of multi-domain KH 2 PO 4 crystal are due to the domain wall motion. A simple model is proposed to give a relation between the domain wall motion and the initial dielectric constant.

Dielectric Dispersion in Ferroelectric RbHSO4

Complex dielectric constants of RbHSO 4 along the ferroelectric c -axis were measured over the frequency range from 10 kHz to 9.50 GHz as functions of temperature. Monodispersive dielectric dispersion was observed around the Curie temperature, T c =-8.56°C, with relaxation time τ p expressed as τ p =7.7×10 -11 /( T - T c ) s for T > T c and τ p =2.9×10 -11 /( T c - T ) s for T < T c . Electro-mechanical clamping effects were found to contribute to τ p in the ferroelectric phase.

Curie-Weiss Behavior of Ferroelectric RbHSO4

Measurements of the dielectric constant of RbHSO 4 along the ferroelectric axis have been made for a well annealed specimen. The result indicates that, contrary to the previously reported results of the spontaneous polarization and the deuteron magnetic resonance in RbDSO 4 , the dielectric constant e obeys a Curie-Weiss law of e - e 0 = C /( T - T 0 ) in 0.06°C< T - T 0 <20°C with e 0 =4.7, C =144 K and T 0 =-7.9°C in agreement with the Landau type mean field theory.

Dielectric Critical Phenomena in RbHSO4

Measurements of the spontaneous polarization are made for RbHSO 4 . The results in the temperature range 0.1^° C < T 0 - T <100^° C are well explained by using the elastic Gibbs function of G 1 - G 10 =(1/2)χ P 2 +(1/4)ξ P 4 +(1/6)ζ P 6 and χ=(4π/ C )( T - T 0 ) with T 0 =-8° C , C =144°C, ξ=7.2×10 14 Jm 5 C -4 and ζ=4.2×10 19 Jm 9 C -6 . The value of δ defined by P ∝ E 1/δ at T 0 is obtained to be 3, the mean field value, in the range E ≥∼200 V/cm.

Growth of a Two-Dimensional Nucleus on a Cleaved (010) Surface of (NH2CH2COOH)3 H2SO4.

On a cleaved (010) surface of ferroelectric (NH 2 CH 2 COOH) 3 ·H 2 SO 4 (TGS), time evolution of round islands with height corresponding to one chemical unit of TGS has been investigated using an atomic force microscope (AFM) in air at room temperature. The time evolution of the round islands at the measured humidity was characterized by a critical radius r c of the island as follows. In case of an island with a radius r r c , the radius increased or was almost unchanged. The critical radius r c increased with an increase of the relative humidity. The relative humidity is associated with adsorbed water. These results are discussed with respect to a basic theory of growth of a two-dimensional nucleus in supersaturated aqueous solution.

Fractal Aspects of the Ferroelectric Domain Structures in KH2PO4 Crystal

Detailed structures of the ferroelectric domains in KH 2 PO 4 have been observed between parallel polarizers. Three types of domain patterns which show self-similarity are found in the pyramidal growth sectors of c plate crystals. The patterns are characterized with fractal aspects of the “pentad” Cantor sets having the Hausdorff dimension D ≈0.756.

Orientations of the Optical Indicatrices of KH2PO4 and NH4H2PO4 in Their Low Temperature Phases

For KH 2 PO 4 (KDP) and NH 4 H 2 PO 4 (ADP), the orientation of the optical indicatrix for NaD light (589 nm) has been determined in the low temperature orthorhombic phase, through the optical and X-ray measurements. For both KDP and ADP, the principal refractive indices, n a , n b and n c have been found to satisfy n a > n b > n c for Fdd2 cell in KDP, and for P2 1 2 1 2 1 cell in ADP. Both crystals are optically negative and the c axes are the acute bisectrices with the optical axial angle 2Ω=61° in KDP at 90 K and 2Ω=20° in ADP at 122 K. The {110} twin of ADP observed in the low temperature phase has been found to form the rotation twin with the two fold axis normal to the {110} twin boundary.

Fractal Aspects of Lamellar Ferroelectric Domain Structures Formed under the Influence of Depolarization Fields in CsH2PO4 and (NH2CH2COOH)3·H2SO4

Ferroelectric 180° domain structures of CsH 2 PO 4 (CDP) and (NH 2 CH 2 COOH) 3 ·H 2 SO 4 (TGS) have been observed in unelectroded b plates using the secondary emission mode of a scanning electron microscope (SEM). It is found that a lamellar domain structure is formed in CDP. The lamellar structure shows the fine periodicity along the normal to the walls nearly parallel either to the (100) plane or to the (801) plane. The periodic pattern corresponds to the first prefractal domain pattern that is characterized by the first prefractal of the pentad Cantor set. It is also revealed that the lamellar domain structure of TGS is observed to be the first prefractal domain pattern. The SEM observations establish that the prefractal domain patterns first dis- covered in KDP are common to the lamellar 180° domain structures which are formed in proper ferroelectrics under the influence of depolarization fields. CDP, TGS, ferroelectric domain, SEM, fractal, Cantor set, depolarization field, surface energy density

Birefringence Induced by Lattice Imperfections in the Paraelectric Phase of KH2PO4

Birefringence for light travelling along the c axis was observed above the Curie temperature T c in the strained regions characteristic of the KH 2 PO 4 crystal. The birefringence and the area of the birefringent regions increase as the temperature approaches T c . In the strained regions, the crystal is optically biaxial and negative, and the optical indicatrix has the same orientation as that in the ferroelectric phase. It is found that the birefringence is induced by the strain of x 6 type around the lattice imperfections through a photoelastic effect.