Hironobu Kasano

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.

Structure study of paraelectric-ferroelectric phase transition in monoclinic K2ZnBr4

The phase transition from the paraelectric phase (α phase) to the ferroelectric one of monoclinic K 2 ZnBr 4 at about 159 K is investigated by means of X-ray diffraction. The space groups of the para- and ferroelectric phases are confirmed to be P 2 1 / m and P 2 1 , respectively. The crystal structures of both phases are also determined at 296 and 123 K, respectively. In the paraelectric phase, the displacive and the disordered models are examined: The significance test shows that ZnBr 4 tetrahedron is in disorder. In the ferroelectric phase, ZnBr 4 tetrahedron is ordered and two independent potassium ions are translated in a same direction along the b -axis.

Incommensurate Phase Transition in 4,4'-Dichlorodiphenyl Sulfone

The structural phase transition in a 4,4 ′ -dichlorodiphenyl sulfone single crystal was investigated by means of X-ray diffraction. It has been found that the crystal shows the normal-incommensurate phase transition at 150 K with the modulation wavevector q = a * ±(1/5+δ) b * , where a * and b * represent the reciprocal lattice vectors in the normal phase and δ represents the small misfit parameter in the incommensurate phase. No lock-in phase transition was confirmed within the temperature range 20-150 K.

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.

Refined Crystal Structure of LiNH4SO4 Including Hydrogen Atoms in Phases II and III

Crystal structure of LiNH 4 SO 4 was refined in phase II (room temperature) and phase III (190 K) by the use of X-ray diffraction intensity with including hydrogen atoms. The least-squares calculations converged with residual parameter R =0.0362 and 0.0378 for phases II and III, respectively. At room temperature, thermal parameters of hydrogen atoms are large and residual electron density is notable around ammonium ions. This may indicate a partial disorderness of the ammonium ions. In the monoclinic phase III, the calculation was performed with considering the effect of twin domains. In the refined superstructure, ammonium ions with hydrogen atoms are perfectly ordered. Hydrogen atoms link nitrogen and oxygen of sulphate ions.

Monoclinic-orthorhombic transition in K2ZnBr4 and Rb2ZnI4

Abstract The first-order transition from the monoclinic α-phase to the orthorhombic β-phase in K2ZnBr4 and Rb2ZnI4 is investigated by X-ray diffraction. The structure of β-phase is confirmed to be isostructural with disordered β-K2SO4. Although structures of α − and β-phases are different so much with each other, the transformation proceeds without cracking or deforming the crystal.

Synchrotron X-Ray Diffraction Study of Third-Order Fibonacci Lattices

Third-order Fibonacci lattices have been grown by means of molecular beam epitaxy. Many satellite peaks are observed by using synchrotron X-rays. A broad peak observed by a conventional diffraction method is found, at high resolution, to be a gathering of many sharp peaks. The satellite peaks are assigned by a power law, implying that the third-order Fibonacci lattices have a self-similar feature.

Phase transitions in the mixed crystals LiRb1-x(NH4)xSO4

Abstract The successive phase transitions of mixed crystal systems of LiRbSO4-LiNH4SO4 were studied by X-ray diffraction, dielectric measurement, thermal analysis and SHG detection as functions of temperature and ammonium concentration x. Although the ammonium ion is almost of the same size as the rubidium ion, a small amount of ammonium reduces the transition temperatures and the incommensurate and the fivefold commensurate phases of LiRbSO4 fade out for x>0.1. With further replacing rubidium by ammonium, the antiferroelectric phase of LiRbSO4 does not appear and the phase sequence is similar to LiNH4SO4 for x>0.25. The phase diagram and the modulated structure are discussed in reference to an Ising model with long range interactions.

Reinvestigation of Low-Temperature Phase Transitions in Rb2ZnBr4

Low-temperature phase transitions in Rb 2 ZnBr 4 single crystal were reinvestigated by means of neutron and X-ray diffractions. Below 116 K some superlattice reflections appeared at the same position (h/2, k/2, 0) as in other members of Rb 2 ZnCl 4 group crystals, where h and k denote the Miller indices in the normal phase (space group P m c n ). The temperature dependence of the integrated intensity of the superlattice reflection changed below 78 K. However, the anomaly around 50 K reported previously was not observed. Below 116 K an incommensurate phase as reported for K 2 ZnCl 4 was not detected. The space group of phase between 78 and 116 K was directly confirmed to be C 1 c 1 by the verification of extinction rules of X-ray diffraction.

A Structural Study of Phase Transitions in [N(CH 3) 4] 2ZnCl 4 at Low Temperature

The crystal structures of the monoclinic low-temperature phase (phase V) at 161 K and the threefold-modulated orthorhombic phase (phase VI) at 149 K of [N(CH 3 ) 4 ] 2 ZnCl 4 are determined. The crystal structure of the threefold-modulated monoclinic phase (phase IV) at 223 K is also reexamined in order to investigate the structural relation to other phases. The modulation pattern of the fivefold-modulated orthorhombic phase (phase III) reported previously is interpreted to be constructed with that of phase IV. The structural changes on the phase transitions from the disordered phase (phase I) to phase V are described mainly by the rotations about the b - and c -axes and the translations along the a -axis of ZnCl 4 and one kind of N(CH 3 ) 4 . On the phase transition from phase V to phase VI, the magnitudes of the displacements of each ions along the b - and c -axes become larger than those on other phase transitions.