Kazuyuki Itoh

X-Ray Structural Study of Ferroelectric Cesium Dihydrogen Phosphate at Room Temperature

The atomic coordinates and the anisotropic thermal parameters of CsH 2 PO 4 crystal in the paraelectric phase have been obtained using three-dimensional X-ray intensity data collected by a four-circle automatic diffractometer at room temperature. The structure is refined to R =0.029 for 780 independent reflections. Two types of hydrogen bonds are clearly distinguished in the electron density maps which display distributions corresponding to order and disorder of protons, respectively.

Disordered Structure of Rb2ZnCl4 in the Normal Phase

The crystal structure of Rb 2 ZnCl 4 was determined at several temperatures in the normal phase by using X-ray reflection data collected by an automatic four-circle diffractometer. The β-K 2 SO 4 type structure with large thermal amplitudes was obtained. The split-atom method reveals that the ZnCl 4 group lies with equal probability in two equilibrium arrangements. In addition to the disordered motion, softening of a rotational motion of the ZnCl 4 group is suggested from the temperature variation of thermal vibrations. The magnitude of the thermal amplitude gives an information about the mechanism of the phase transition in the β-K 2 SO 4 type ferroelectrics.

Order-disorder type phase transition in ferroelectric CsD2PO4 studied by X-ray structure analysis

The mechanism of the ferroelectric phase transition of CsD 2 PO 4 was investigated by examining the crystal structures determined by X-ray structure analysis at seven different temperatures above and below the transition temperature T c . The PO 4 group has two equilibrium arrangements in the paraelectric phase while only one of them remains in the ferroelectric phase. The thermal amplitude of the D atom which forms a one-dimensional chain along the ferroelectric b -axis reduces drastically at T c with decreasing temperature. These results are well described on the basis of the mechanism of the order-disorder type phase transition.

X-Ray Study of High-Temperature Phase Transitions in KH2PO4

High-temperature phase transitions in KH 2 PO 4 near 110°C and 180°Chave been studied by X-ray diffraction. The extinction rule observed at 123°C is the same as that in the room-temperature phase. It is concluded that on heating through 187°C crystal system changes from tetragonal to monoclinic whose space group is P 2 1 or P 2 1 / m . The lattice parameters at 195°C are given by a =7.47A, b =7.33A, c =14.49A, α=β=90° and γ=92.2° .

Refinement of Crystal Structure of Ferroelectric Ca_2Sr (C_2H_5CO_2)_6 in the Paraelectric Phase

The absolute structure of Ca 2 Sr(C 2 H 5 CO 2 ) 6 at room temperature was determined by using X-ray reflection data collected by a four-circle automatic diffractometer with the R -factor of 0.027. The space group of levorotatory crystals is D 4 4 -P4 1 2 1 2 and that of dextrorotatory ones D 4 8 -P4 3 2 1 2. A difference Fourier synthesis revealed that electron densities of the methyl groups exhibited doubly peaked or largely elongated contours. The split-atom calculation has established that four of the methyl groups together with one of the α-carbons in a formula unit are in disordered state occupying respectively two equilibrium atomic positions with the same probability.

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.

Partially Disordered Structure of Ca2Sr(C2H5CO2)6 in the Ferroelectric Phase

The crystal structure of Ca 2 Sr(C 2 H 5 CO 2 ) 6 in the ferroelectric phase was determined at -40°C by X-ray reflection data from a monodomain crystal. The least-squares calculation was converged with the R -factor of 0.044. Among the six independent propionate ions in a formula unit, four ones which are disordered perfectly in the paraelectric phase are in partially disordered states with two equilibrium atomic positions. Correlation between the positions of methyl groups in the disordered propionate ions is discussed on the basis of the van der Waals version.

An Atomic Order Parameter in Disordered Crystals Undergoing Structural Phase Transitions and Its Determination by the Crystal Structure Analysis

An atomic order parameter is defined by the difference of the probabilities with which the atom occupies two local equilibrium positions in the split-atom potential. An expression of this order parameter is given as a function of the temperature and the potential difference between two local equilibrium positions. It is shown that the thermal averages of the atomic position and the square of the atomic thermal amplitude, which can be determined from the crystal structure analysis, are expressed as a function of this atomic order parameter. The characteristic temperature variation of these values observed in the ferroelectric CsD 2 PO 4 is successfully explained by the predicted temperature dependence of the atomic order parameter.

Refinement of Crystal Structure of SbSI in the Ferroelectric Phase

Crystal structure analysis of SbSI in the ferroelectric phase has been made at 1.3°C by using full three-dimensional Bragg reflection data collected with X-ray automatic four-circle diffractometer. The discrepancy factor is 0.030. The atomic shifts from the positions in the paraelectric phase take place along the ferroelectric c -axis by 0.103 A for Sb atoms and 0.019 A for S atoms. A large thermal parameter of 0.16 A in amplitude is seen for Sb atoms along the c -axis. The split atom method has shown that the nature of the phase transition is very close to the displacive type.

Refinement of crystal structure of SbSI

Abstract The crystal structure of SbSI has been refined at 27°C and 48°C, where the crystal is paraelectric. The anisotropies in the temperature factors were taken into account. The three-dimensional x-ray intensity data used were gathered by an automatic four-circle diffractometer, and the final R values of 0.048 were obtained in both cases. A large anisotropic temperature factor B33 of the Sb atom is obtained, and a trial was made to determine whether the large anisotropy could be explained as disorder of the Sb atom. The result shows that no choice could be made between a displacive or an order-disorder type phase transition.