Katsuhiko Hasebe

Structural Study on Cubic-Tetragonal Transition of CH3NH3PbI3.

The cubic–tetragonal phase transition of CH 3 NH 3 PbI 3 was investigated by single crystal X-ray diffractometry. The crystal structure was refined at five temperatures in the teragonal phase. The PbI 6 octahedron rorates around the c -axis alternatively to construct the SrTiO 3 -type tetragonal structure. A methylammonium ion is partially orderd; 24 disordered states in the cubic phase are reduced to 8. With decreasing temperature, the rotation angle of the octahedron increases monotonically, which indicates it is an order parameter of the cubic-tetragonal transition.

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 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.

Studies of the Crystal Structure of Ammonium Sulfate in Connection with Its Ferroelectric Phase Transition

The crystal structure of ammonium sulfate is studied at -140, -90, -64 and -53.5°C in the ferroelectric phase, and at -48.5 and -40°C in the paraelectric phase, by the use of the data collected by a four-circle diffractometer. The difference Fourier synthesis at -53.5°C shows a clear indication of the disordering of the ammonium ion I but does not show the disordering of the sulfate ion. The configuration of the sulfate ion below T c can be derived from that above T c by a condensation of one of the librational B 1 u symmetry modes. The order parameters of two kinds of ammonium ions are determined by the use of the reflection data below T c . From the temperature dependence of the displacement of the sulfate ion and order parameters of two kinds of ammonium ions below T c , a pseudospin-phonon coupled model is proposed.

X-Ray Study of LiCsSO4 in Connection with Its Ferroelastic Phase Transition

The crystal structure of LiCsSO 4 was studied at 25, 0, -20 and -40°C in the normal phase and -71, -78 and -100°C in the ferroelastic phase by using X-ray reflection data collected by an automatic four circle diffractometer. The librational motion of SO 4 is highly anisotropic in the high temperature phase. Temperature variation of the atomic position of S or O atoms was small in the low temperature phase. The mutual proportionality between the temperature dependence of observed structure factors of reflections 201, 203, 205, 311 and 131 and that of the deviation angle Δ γ of the monoclinic angle γ from 90° was found, which suggests that the phase transition is characterized by the order-disorder nature of the SO 4 ion coupled with the shear strain x 6 .

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.

Refinement of the crystal structure of [N(CH3)4]2ZnBr4 in connection with its phase transition

The crystal structure of [N(CH 3 ) 4 ] 2 ZnBr 4 was studied at 25°C in the normal phase and 13, 11, 6, 1.5, -10, -40 and -100°C in the ferroelastic phase by single crystal diffractometry. The split-atom method and the significance test showed that the constituent ions are in disorder in the normal phase. In the ferroelastic phase, positional and thermal parameters and occupation probabilities of ions were determined. The structure in the ferroelastic phase is characterized by two kinds of chains composed of ZnBr 4 and N(CH 3 ) 4 -1; one kind of which is along the c -direction, while the other kind is along the a -direction.

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.