Sigetosi Tanisaki

X-Ray Study on the Thermal Hysteresis of the Modulation Wavevector in (Rb1-xKx)2ZnCl4

The modulation wavevector \(k_{z}{=}(\frac{1}{3}{-}\delta)c^{*}\) exhibits remarkable thermal hysteresis over a wide temperature range including the incommensurate-commensurate transition point in the mixed crystal system (Rb 1- x K x ) 2 ZnCl 4 with x =0, 0.005, 0.02, 0.28, 0.92, 0.997 and 1. This hysteresis seems to correspond to the thermal hysteresis observed in the dielectric constant and to support the proposal by Hamano et al . that the pinning of the modulation wave by defects causes the hysteresis. In the Rb rich case ( x ≦0.02), the parameter δ has a tendency to be fixed to the value realized at the normal-incommensurate transition point. In the K rich case ( x =0.997), it decreases monotonically with decreasing temperature, with a plateau in a middle range of the incommensurate phase. In the middle range of x , δ is pinned to a constant value down to low temperature.

X-ray Study on the Ferroelectric Phase Transition of NaNO2

The disordered structure above the Curie temperature (about 160°C) of NaNO 2 was studied by the X-ray diffuse scattering. The observed diffuse scattering, the satellites or the diffuse maxima and the plate-like diffuse scattering normal to the b * axis around them, can be interpreted on the basis of a certain disordered arrangement of two kinds of structure units. The atomic positions in each structure unit were determined from the relative intensity of the Bragg reflections and that of the satellites. The proposed model for the arrangement is the uniform mixing along the a axis of two kinds of small plate-like domains with antiparallel polarizations, “microdomains”, whose widths are of the order of several lattice constants. In the real crystal there is a certain degree of disturbance in the uniform mixing along the a axis, and each microdomain has a certain degree of disorder. The disturbance and the disorder increase with increasing temperature, but the ordered alignment along the b axis remains in som...

X-Ray Studies on Successive Structural Transitions in RbLiSO4

Space groups and super-lattice structures of five phases of RbLiSO 4 are investigated by X-ray diffraction method. Phase I ( T >204^° C ) is paraelectric ( Pmcn - D 2 h 16 ). The incommensurate phase II (202< T <204^° C ) is characterized by a wave number k z =(2+δ)/5 with δ( T )=0.013(204^° C ) ∼0.137(202^° C ). Phase III takes c =2 c 0 structure which belongs to P 2 1 / c 11- C 2 h 5 , where c 0 is the cell dimension in phase I. The ferroelectric phase IV (166< T <185^° C ) belongs to P 11 n - C s 2 and takes c =5 c 0 super-structure. The I-II transition is 2nd order and the critical exponent is β=0.36 ±0.05, while other three transitions are 1st order ones.

X-Ray Diffraction Study on the Incommensurate-Commensurate Phase Transitions in {N(CH3)4}2CoCl4

The space groups of six phases of {N(CD 3 ) 4 } 2 CoCl 4 were determined by X-ray diffraction as P m c n ( Z =4), incommensurate, P 2 1 c n ( Z =20), P 112 1 / n ( Z =12), P 12 1 / c 1( Z =4) and P 2 1 2 1 2 1 ( Z =12), respectively in the order of decreasing temperature. The deuteration causes the increase of the modulation wavenumber. This increase of the wave number agrees qualitatively with that expected from the distribution of modulation wavenumber in the reduced p - T phase diagram.

X-Ray Diffraction Study of the Structures of Normal and Ferroelectric [N (CH3)4]2ZnCl4

The crystal structure of [N (CH 3 ) 4 ] 2 ZnCl 4 was studied at three temperatures, 30, 60and 90°C, in the paraelectric phase and at 5.5°C in the ferroelectric phase by using theX-ray reflection data collected by an automatic four-circle diffractometer. The split-atom method and the significance test showed that the constituent ions are in disorder in the normal phase. In the ferroelectric phase, positional and thermal parameters and occupation probability of ions were calculated by the least-squares method. The density modulation of ZnCl 4 and that of one kind of N(CH 3 ) 4 ion were closely incorporated.

X-Ray Study on the Thermal Hysteresis of the Incommensurate Wave Number in Rb2ZnCl4

The incommensurate wave number \(k_{z}=(\frac{1}{3}-\delta)c^{*}\) in Rb 2 ZnCl 4 has been investigated by X-ray scattering. Over a wide temperature range including the incommensurate-commensurate transition point the parameter δ and the width of the satellite reflection show thermal hystereses which seem to correspond to the thermal hysteresis of the dielectric constant observed by Hamano et al.

Incommensurate-Commensurate Phase Transition of {N(CH3)4}2MnCl4

By means of X-ray diffraction study, an incommensurate phase of {N(CH 3 ) 4 } 2 MnCl 4 was found in the temperature range of 19.1 ∼18.5^° C . Incommensurate satellite reflections were observed at ( h 0 l ±ζ) with ζ≃0.483. In the temperature range of 18.5 ∼-6.5^° C , the value of ζ is \(\frac{1}{2}\) (commensurate). This phase is ferroelastic and monoclinic with the unique axis parallel to the a -axis. Below -6.5^° C , another monoclinic phase ( P 112 1 / n ) with \(\zeta =\frac{1}{3}\) appears.

X-Ray Diffraction Study on the Crystal Structure and the Incommensurate-Commensurate Phase Transition in {N(CH3)4}2CuBr4

The superstructures were found in the low temperature phase of {N(CH 3 ) 4 } 2 CuBr 4 . With decreasing temperature, the cell dimension changes as b 0 , ∼2 b 0 , 2 b 0 and b 0 , respectively. The space group of each phase is Pmcn, incommensurate, Pbc2 1 and P12 1 /cl. As the temperature decreases the modulation wave number q 0 ≈0.62 b * at -1°C decreases continuously to ∼0.57 b * at -31°C and jumps to the commensurate value of 1/2 b * in the ferroelectric phase.

X-Ray Study of the Phase Transitions in {N (CH_3)_4}_2ZnBr_4 and {N (CH_3)_4}_2CoBr_4

The space group of each low temperature phase in {N(CH 3 ) 4 } 2 ZnBr B and {N(CH 3 ) 4 } 2 CoBr 4 was determined by X-ray diffraction as P 12 1 / c 1. The characteristic diffuse streaks along the b * - and c * -directions were observed in the high temperature phase in both materials.The space group of each low temperature phase in {N(CH 3 ) 4 } 2 ZnBr B and {N(CH 3 ) 4 } 2 CoBr 4 was determined by X-ray diffraction as P 12 1 / c 1. The characteristic diffuse streaks along the b * - and c * -directions were observed in the high temperature phase in both materials.

Field Induced Ferroelectric Phase in RbLiSO4

A new polar phase (phase VI) is induced in RbLiSO 4 by the external electric field. The cell dimension of phase VI is tripled in comparison with phase I ( c =3 c 0 ) and the space group is P 2 1 c n - C 2 v 9 , the same as that of the ferroelectric phase of K 2 SeO 4 . In the temperature range of phase II and III, the ferroelectric phase IV is realized at first with increasing the field, and finally a new phase appears. The phase diagram is determined by X-ray scattering.