Iwao Shibuya

A Neutron Diffraction Study of the Ferroelectric Transition of CsH2PO4

Hydrogen atoms in ferroelectric phase of CsH 2 PO 4 have been located from low temperature neutron diffraction experiment. Chains of hydrogen bonds extending to both the crystallographic b - and c -axes are confirmed. The hydrogen atom in a shorter O–H··O bond becomes ordered at low temperature phase with bond length O-H=1.02 A auad H··O=1.46 A. Structural change below T c shows that the hydrogen ordering accompanies a slight rotation of PO 4 tetrahedron about the one P–O bond.

Crystal structure refinement of ferroelectric rochelle salt

The crystal structure of Rochelle salt has been refined at 0°C, where the crystal is ferroelectric. Intensity data were collected on a fully automatic four-circle neutron diffractometer. An external dc field of 90 V was applied along the ferroelectric a-axis of the specimen during the course of measurement. The R-factor of 0.102 were obtained.

A neutron diffraction study on the deuteration effect in CsH2PO4 crystal

The deuteration rate x of partially deuterated crystals, Cs(H 1- x D x ) 2 PO 4 , was determined through the neutron diffraction experiment. It was found that two non-equivalent positions of hydrogen atoms take slightly different deuteration rates. The deuteration rate x 1 of the shorter hydrogen bond which is closely related to the ferroelectric phase transition is smaller than the rate x 2 of the longer one. The observed dependence of the phase transition temperature T c on x 1 is found to be linear in contrast to the concaved dependence predicted in the one-dimensional proton-pseudospin coupled model. The neutron diffraction experiment also revealed that the distance of the shorter hydrogen bond increased linearly with x 1 , whereas that of the longer one remained constant.

A neutron diffraction study on deuterated rochelle salt in the paraelectric phase

Abstract The crystal structure of Rochelle salt was refined at two temperatures of paraelectric phase by using three-dimensional neutron diffraction data from deuterated single crystal. The high temperature (40°C) structure exhibits enhanced anisotropic thermal motion about particular deuterium atoms while the low temperature (-195°C) structure shows quite normal state with lessened anisotropic thermal behaviors. These anisotropies are further refined by split-atom model and it was shown that split-atom model gives better agreement. Based on these structures, phase transition scheme was examined. Previous model of the phase transition does not contradict to the present result but needs some modifications.

Neutron diffraction analysis on a possible disordered structure of paraelectric rochelle salt

Abstract Neutron diffraction refinement of paraelectric structure of deuterated Rochelle salt at 40 °C and −195 °C was performed. Existence of large anisotropic thermal motions suggests a possibility of disordered structure with small atomic shifts. Split atom method was applied and a disordered structure model for the paraelectric phase was proposed. Phase transition based on this model was discussed.

Neutron Diffraction Study of Ferroelectric Transition in Li2Ge7O15

The crystal structure of Li2Ge7O15 in its ferroelectric state has been determined and refined as well with an R-value of 0.063 for 1202 neutron diffraction data. The crystal belongs to the orthorhmbic system with space group C2V5-Pbc21 and unit cell dimensions a=7.36, b=16.66 and c=9.69 A (at 77 K). Atomic displacements at the phase transition (283.5 K) are evaluated in comparison with the paraelectric structure determined by Vollenkle et al., in 1970. Consequently, tiltings of GeO4 tetrahedra closely related to the soft phonon mode observed by Raman scattering and far-infrared measurements have clearly been observed.

Structure of M(HCOO)2·2(NH2)2CO: M = Mg, Co, Zn

The present status of the structure determination is summarized for the monoclinic C2/c phase of divalent metal formate di-urea. The final result on Mg and Zn salts shows that they consist of a metal-HCOO quasi-quadratic lattice separated by urea molecules.

Neutron diffraction study on CDP-DCDP mixed crystals

Abstract Deuteration rate (x) dependent behavior of transition temperatures (Tc ) and hydrogen bond distances in Cs(H1-x D x )2PO4 mixed crystals were examined through neutron diffraction analysis and dielectric constant measurements. The neutron analysis proved that a hydrogen atom, H(1), involved in a shorter hydrogen bond has slight difficulty in substitution than the other, H(2), in a longer hydrogen bond in the region of intermediate x. The experimental ΔTc vs. x relation was compared with a theoretical result which was derived taking one-dimensional character of CDP into account. The ferroelectric structure, disorder in the paraelectric phase, change of the hydrogen bond by deuteration and its relation to geometrical isotope effect are discussed.

Low-Temperature Structure of K2Ba(NO2)4

The crystal structure of the low-temperature phase (γ-phase) of the dipolar frustrated crystal K 2 Ba(NO 2 ) 4 was determined at -120°C by X-ray measurement of superlattice reflections. A superlattice model proposed for the γ-phase has been refined. The space group is reasonably assigned as monoclinic C 2/ m - C 2h 3 , and the configurations of all the NO 2 groups are found to be ordered. The symmetry agrees with the theoretical prediction made by Ishibashi et al ., implying that the γ-phase is ferroelastic.

Structural Investigations of High-Tc Superconducting Untwinned DyBa2Cu3O7-y Single Crystal by X-Ray Diffraction

The crystal structure of high-Tc superconducting orthorhombic DyBa2Cu3O7-y has been determined by X-ray diffraction using an untwinned single crystal. Fourier and difference Fourier analyses revealed exact occupancy factors of O(1) and O(2) in this crystal to be 0.84 and 0.08, respectively. The crystal has one of the highest orthorhombicity ever found with (a-b)/a=0.016, and hence, distinct structural differences of the a and b directions.