Hiroyuki Mashiyama

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 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.

A Model of Successive Transitions in {N(CH3)4}2ZnCl4

On the basis of a model free energy, mechanisms of the successive transitions from a normal phase to an incommensurate one, a commensurate one and finally to some low temperature phases are investigated. The free energy consists of terms of six physical quantities, and is invariant in the normal phase Pmcn . It is shown that the incommensurate phase transforms to the commensurate ferroelectric phase with fivefold cell-length. Further the temperature is decreased, then a triple cell-length phase is realized. The present model is appropriate for {N(CH 3 ) 4 } 2 ZnCl 4 and {N(CH 3 ) 4 } 2 CoCl 4 , for example, and differs from those proposed for K 2 SeO 4 and the isomorphous materials.

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.

A Structural Study of the Successive Transitions in LiRbSO4

The crystal structures of LiRbSO 4 are analyzed in the disordered phase (250, 220 and 210°C), the antiferroelectric phase (190°C), the ferroelectric phase (170°C) and the paraelectric phase (160 and -100°C) from X-ray diffraction data. In the disordered phase, it is concluded that the SO 4 tetrahedron is in disorder by using of the split atom method as well as the difference Fourier maps. Following the incommensurate-commensurate transition, the tetrahedra occupy one of two configurations alternately. The modulation period 2 c 0 at 190°C is elongated to 5 c 0 in the ferroelectric phase, where c 0 is the basic cell dimension in the disordered phase. The structural transformation is characterized by the change of the occupation pattern of the SO 4 tetrahedron correlated to the displacement of its center of mass and positions of Li and Rb atoms.

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.

Incommensurate-Commensurate Transition in K2ZnBr4 and K2CoBr4

The structural phase transitions of K 2 ZnBr 4 and K 2 CoBr 4 are investigated by means of X-ray scattering. The transition from the room-temperature α phase to the high-temperature β phase takes place at about 200°C. It is found that the β phase is an incommensurate one with the modulation wave vector q =(1/3-δ) c * , which is followed by a normal phase of Pmcn above 288°C (Zn-salt) or 282°C (Co-salt). If the β phase is supercooled to below room temperature, then the crystal is a commensurate phase with q =1/3 c * . With increasing temperature, the commensurate-incommensurate phase transition is observed at 5°C in Zn-salt. The transition sequence of normal, incommensurate and commensurate phases is quite common in A 2 X Y 4 -type incommensurate materials, though the α phase is very different from others.