Takanao Asahi

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 .

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.

A structural study of [N(CH3)4]2HgCl4 in connection with its phase transition

The crystal structure of [N(CH 3 ) 4 ] 2 CdBr 4 was studied at 21°C in the room temperature phase and at -3, -8, -35, -80 and -115°C in the low temperature phase by single crystal diffractometry. In the room temperature phase, CdBr 4 ion and two kinds of N(CH 3 ) 4 ions were determined to be in disorder from Fourier synthesis and the significance test. In the low temperature phase, the occupation probabilities of ions as well as positional and thermal parameters were determined. The structure is characterized by two kinds of chains composed of CdBr 4 and N(CH 3 ) 4 ; one is along [001] and the other is along [100]. The deviation Δβ of monoclinic angle β from 90° shows the characteristic temperature dependence as reported in [N(CH 3 ) 4 ] 2 ZnBr 4 and [N(CH 3 ) 4 ] 2 CoBr 4 ; Δβ increases rapidly just below the transition temperature then decreases almost linearly as temperature decreases and changes its sign at about -72°C.

Disordered Configuration of Methylammonium of CH3NH3PbBr3 Determined by Single Crystal Neutron Diffractometry

In order to understand the mechanism of the structural phase transition in CH 3 NH 3 PbBr 3 , neutron diffraction data from the single crystal is analyzed by the least-squares method and the maximum entropy method. The disordered configuration of a methylammonium ion in a cage surrounded by eight PbBr 6 octahedrons is identified unambiguously. With comparing the low-temperature structure, the transition is characterized by the condensation of the rotational motion of the PbBr 6 octahedron around the Pb-Br axis, which accompanies the partial ordering of the methylammonium ion to reduce the site symmetry.

X-ray study of high pressure phases in NH4LiSO4

Abstract High-pressure X-ray scattering study of NH4LiSO4 has been made up to 16 kbar at room temperature in a gasketted diamond anvil cell. It undergoes phase transitions at about 6.5 kbar from the ferroelectric phase to an intermediate pressure phase, then to a super structure phase (a=2a0, b=2b0, c=c0) at about 12.4 kbar. Symmetries of the intermediate and super structure phases have been determined as P2111(Z=4) and C111 (Z=48). Crystal structure was analyzed at 5.7, 6.9 and 9.5 kbar.

Crystal Structure of the High Pressure Phase VI of Thiourea

Crystal structure of the high pressure phase VI of thiourea, SC(NH 2 ) 2 , has been analyzed at 0.97 GPa, 295 K from X-ray diffraction data; orthorhombic, P b n m , a =5.503 A, b =7.138 A, c =24.788 A, Z =12, R =0.055 for 351 independent reflections. Displacements of molecules at V-VI phase transition bring about reconstruction of hydrogen bonds network.

Structural phase transitions of betaine phosphite studied by X-ray scattering

Abstract DSC and X-ray scattering studies of betaine phosphite, (CH3)3NCH2COO·H3PO3, single crystals have been made. The space group of Phase I (T > 355 K) and that of Phase III (T < 216K) are determined as P121/ml and P121 1, respectively, where the cell length of the c axis in Phases II and III are two times larger than that in Phase I. The crystal structure is determined at 373 K. in Phase I and at 173 K in Phase III. The phosphite molecules are disordered in Phase I.

X-Ray Study of Phase Transition in Betaine Phosphite

The phase transition of betaine phosphite crystal associated with the ordering of PO 3 radicals at phase I to II transition has been studied by X-ray scattering: The observations are concerned on (1) the temperature dependence of lattice constants, (2) the temperature dependence of the order parameter of PO 3 which followed the relation η∝( T c - T ) β , β=0.33(1), and (3) the anisotropic distribution of critical diffuse scattering in the reciprocal space above T c ; elongated along the c -direction, shortened along the b * -direction. The critical X-ray diffuse scattering has been analyzed on an Ising model to obtain effective interaction between PO 3 ions. The results are summarized as follows. (1) The correlation lengths along three principal directions of anisotropic scattering distribution was estimated. (2) The temperature dependence of the correlation length follows nearly the mean-field behavior, ξ∝( T - T c ) -ν , ν=0.5.

Phase transitions in mixed crystals LiRb x Cs1−x SO4

Abstract Phase transitions in mixed crystals LiRb x Cs1−x SO4 with x = 0.986, 0.977, 0.940, 0.926 and 0.913 were studied by X-ray diffraction. As the concentration of LiCsSO4 increases, the temperature range of incommensurate and ferroelectric phases shifts to the low-temperature side and these phases faded away at about x = 0.95. Temperature range of antiferroelectric phase increased on substitution of Cs for Rb. The form of double-well potential of SO4 in LiRbSO4 (220°C) and LiRb x Cs1−x SO4 (x = 0.913, 230°C) was estimated by using reflection data. Potential became shallow on substitution; the heights of the potential barrier in pure and mixed crystals are 1.27 × 10−13 and 0.606 × 10−13 erg respectively.

Single-Crystal Neutron Structural Analyses of Potassium Dihydrogen Phosphate and Potassium Dideuterium Phosphate

Single-crystal neutron structural analyses have been performed on both potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (DKDP) in order to discuss the isotope effect from structural viewpoints. The values of spontaneous polarization are calculated from the refined structural parameters by the point-charge method, and the calculated values are almost in good agreement with the experimental values of each compound. The temperature dependences of the anisotropic atomic displacement parameters U 33 s and positional shifts of potassium, phosphorus, oxygen, and hydrogen/deuterium atoms along the polar c -axis is compared between KDP and DKDP. It is concluded that the paraelectric-ferroelectric transitions are perfectly of the improper order–disorder type in both KDP and DKDP; the ordering of the hydrogen atom induces the spontaneous displacements of both potassium and phosphorus atoms. The interatomic distances and angles in the paraelectric and ferroelectric phases of KDP and DKDP are ...