Yasuo Nishihata

Local structure anomaly around Ge dopants in Mn3Cu0.7Ge0.3N with negative thermal expansion

Local structure analysis of Cu and Ge atoms in the negative thermal expansion material Mn3Cu0.7Ge0.3N was conducted using x-ray absorption fine structure measurements. The temperature dependence of the interatomic distance was found to reflect the macroscopic negative thermal expansion both for Cu–Mn and Ge–Mn shells, although the magnitude of the relative change was much larger for Ge–Mn than Cu–Mn. An enhanced anomaly of the Debye–Waller factor was observed for the Ge–Mn shell in the temperature region of the negative expansion, indicating the presence of static local disorder around Ge impurities. These local structure anomalies strongly suggest that the local and inhomogeneous strain around Ge is essential in broadening the discontinuous volume contraction.

On the Ferroelectric Phase Transition in Li2Ge7O15

X-ray scattering experiments of Li 2 Ge 7 O 15 were carried out above and below the ferroelectric phase transition temperature T c =283.5 K. The space groups are confirmed to be D 14 2 h - P b c n and C 5 2 v - P b c 2 1 above and below T c , respectively. The coupling between the ferroelectric displacements of atoms along the c -direction and the displacements of atoms along the a -direction is important to cause the phase transition.

NO dissociation on Cu(111) and Cu2O(111) surfaces: a density functional theory based study

NO dissociation on Cu(111) and Cu(2)O(111) surfaces is investigated using spin-polarized density functional theory. This is to verify the possibility of using Cu-based catalyst for NO dissociation which is the rate limiting step for the NO(x) reduction process. The dissociation of molecularly adsorbed NO on the surface is activated for both cases. However, from the reaction path of the NO-Cu(2)O(111) system, the calculated transition state lies below the reference energy which indicates the possibility of dissociation. For the NO-Cu(111) system, the reaction path shows that NO desorption is more likely to occur. The geometric and electronic structure of the Cu(2)O(111) surface indicates that the surface Cu atoms stabilize themselves with reference to the O atom in the subsurface. The interaction results in modification of the electronic structure of the surface Cu atoms of Cu(2)O(111) which greatly affects the adsorption and dissociation of NO. This phenomenon further explains the obtained differences in the dissociation pathways of NO on the surfaces.

X-Ray Study on Dipole-Glass Phase in Random Mixture of Ferroelectric and Antiferroelectric: Rb1-x(NH4)xH2PO4

X-ray study on a random mixture of a ferroelectric and an antiferroelectric, Rb 1- x (NH 4 ) x H 2 PO 4 , has been carried out. The glassy phase (dipole-glass phase) is found to be tetragonal, although the deviation of a lattice parameter from the Debye approximation is seen at low temperatures. The intensity of the X-ray diffuse scattering increases with decreasing temperature in the glassy phase. The results suggest a freezing of the short range order or the local polarization in the glassy phase.

Roles of Cation and Anion Molecules in Ferridistortive Phase Transition in [A(CH_3)_4]_2XBr_4 Type Crystals

In order to clarify the roles of cation and anion molecules in the ferridistortive phase transition, four kinds of single crystals of [A(CH 3 ) 4 + ] 2 XBr 4 2- type, where cat-ion sites are occupied either by tetramethylphosphonium ion P(CH 3 ) 4 + or by tetramethylammonium ion N(CH 3 ) 4 + , and anion site either by tetrabromocobaltate ion CoBr 4 2- or by tetrabromomanganate ion MnBr 4 2- , have been grown. The temperature dependence of the spontaneous shear strain x 5 s ( T )=(π/ 180° )(β( T )-90° ), where β( T ) is the monoclinic angle, has been measured from 420 K down to 20 K. The substitution for cations is found to cause a major change in the temperature dependence of x 5 s ( T ), while the substitution for anion only a minor one. The results are explained on the basis of a ferridistortive model whose spontaneous strain consists of two kinds of sublattice strains. It is concluded that cations A(CH 3 ) 4 + play a primary role in the ferridistortive phase transition in [A(CH 3 ) 4 + ] 2 XBr 4 2- typ...

Anomalous metallic state in the vicinity of metal to valence-bond solid insulator transition in LiVS2.

We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2 known for the formation of a valence-bond solid (VBS) state out of an S=1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first order transition from a paramagnetic metal to a VBS insulator at Tc approximately 305 K upon cooling. The presence of a VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of a VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudogap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K.

Dynamic structural change in Pd-perovskite automotive catalyst studied by time-resolved dispersive x-ray absorption fine structure

Dynamic structural change in Pd-perovskite automotive catalyst, LaFe0.9Pd0.1O3, which has a high catalytic activity during aging, was studied by in situ time-resolved dispersive x-ray absorption fine structure spectroscopy at 200–500 °C. An Al2O3-based conventional catalyst was also studied. In a reductive atmosphere, both catalysts showed similar temperature dependences of structural transformation from an oxide to a metal. However, different temperature dependence was observed in an oxidative atmosphere. A faster response in the structural change was observed in the Pd-perovskite catalyst than in the Pd/Al2O3 catalyst. It was revealed that Pd-perovskite shows a considerably fast structural change to the oxidized state via the movement of Pd atoms into the perovskite crystal, in comparison with Pd/Al2O3 showing two-step structural change for making PdO.

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.

Dynamic structural change of Pd particles on LaFeO3 under redox atmosphere and CO/NO catalytic reaction studied by dispersive XAFS

The local structure of Pd metal fine particles on LaFeO3 which has a high catalytic activity was observed by dispersive XAFS optics from the viewpoint of dynamical structure change of Pd during oxide-metal change and CO/NO catalytic reaction. The oxide-metal change of Pd nanoparticles on LaFeO3 and Al2O3 was investigated by 20–50 Hz rate. It was recognized that, under the reductive atmosphere, Pd atoms show similar speed of movement from oxide to metal state both on the two supports. However, under the oxidative atmosphere, Pd atoms on LaFeO3 show faster movement from metal to oxide state than those on Al2O3. CO/NO catalytic reaction on Pd metal nanoparticles was also observed by 0.2 Hz rate. Slow observation mode made the four EXAFS parameters: coordination number, interatomic distance, Debye-Waller factor and edge shift, precisely determined during catalytic reaction. There are two particular differences between Pd particles on LaFeO3 and Al2O3. Large enhancement of interatomic distance of Pd particle was only observed on Al2O3. Stable surface oxide layer of Pd particle is created on LaFeO3.

Structural changes of quartz-type crystalline and vitreous GeO2 under pressure

Using a large volume high pressure apparatus, quartz-type crystalline GeO2 and Li2O-4GeO2 glass have been compressed up to 14 GPa at room temperature and their local structural changes have been investigated by an in-situ XAFS method. In quartz-type crystalline GeO2, the change of the coordination number from 4 to 6 begins above 8 GPa and finishes below 12 GPa. On decompression, reversal transition begins below 8 GPa and there is a large hysteresis. Almost no sixfold coordination of Ge is preserved after releasing pressure. Change of coordination number in vitreous Li2O-4GeO2 begins above 6 GPa and is completed below 10 GPa. Reversal transition begins below 10 GPa and the hysteresis is smaller than that of quartz-type GeO2. Change of coordination number in vitreous Li2O-4GeO2 is also reversal.