Takeshi Usuki

Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous states

The three-dimensional atomic configuration of amorphous Ge2Sb2Te5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge2Sb2Te5 can be regarded as “even-numbered ring structure,” because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge–Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge2Sb2Te5 is the key for the fast crystallization speed of the material.

Inelasticity Effect on Neutron Scattering Intensities of the Null-H2O

Time-of-flight (TOF) neutron scattering measurements have been carried out for liquid null-H2O, in which the average coherent scattering length of hydrogen atoms is zero. In order to determine the inelasticity effect depending on both the scattering angle and the neutron flight path ratio, γ [ = l_{s}/(l_{0} + l_{s}), l 0 and l s denote the moderator-sample and sample-detector distances, respectively], neutron scattering measurements have been performed using three neutron spectrometers, HIT-II, RAT, and SWAN, installed at KENS, Tsukuba, Japan. The self-scattering intensity for the null-H2O was derived by subtracting the known O–O partial structure factor from the observed scattering cross-section. It has been revealed that the magnitude of the inelasticity distortion involved in the self-scattering term is still significant even at a smaller scattering angle than that expected from the first-order inelasticity correction formulas proposed in the literature. The inelasticity distortion in the self-scatter...

Atomic and electronic structures of an extremely fragile liquid

The structure of high-temperature liquids is an important topic for understanding the fragility of liquids. Here we report the structure of a high-temperature non-glass-forming oxide liquid, ZrO2, at an atomistic and electronic level. The Bhatia–Thornton number–number structure factor of ZrO2 does not show a first sharp diffraction peak. The atomic structure comprises ZrO5, ZrO6 and ZrO7 polyhedra with a significant contribution of edge sharing of oxygen in addition to corner sharing. The variety of large oxygen coordination and polyhedral connections with short Zr–O bond lifetimes, induced by the relatively large ionic radius of zirconium, disturbs the evolution of intermediate-range ordering, which leads to a reduced electronic band gap and increased delocalization in the ionic Zr–O bonding. The details of the chemical bonding explain the extremely low viscosity of the liquid and the absence of a first sharp diffraction peak, and indicate that liquid ZrO2 is an extremely fragile liquid.

Network topology for the formation of solvated electrons in binary CaO–Al2O3 composition glasses

Glass formation in the CaO–Al2O3 system represents an important phenomenon because it does not contain typical network-forming cations. We have produced structural models of CaO–Al2O3 glasses using combined density functional theory–reverse Monte Carlo simulations and obtained structures that reproduce experiments (X-ray and neutron diffraction, extended X-ray absorption fine structure) and result in cohesive energies close to the crystalline ground states. The O–Ca and O–Al coordination numbers are similar in the eutectic 64 mol % CaO (64CaO) glass [comparable to 12CaO·7Al2O3 (C12A7)], and the glass structure comprises a topologically disordered cage network with large-sized rings. This topologically disordered network is the signature of the high glass-forming ability of 64CaO glass and high viscosity in the melt. Analysis of the electronic structure reveals that the atomic charges for Al are comparable to those for Ca, and the bond strength of Al–O is stronger than that of Ca–O, indicating that oxygen is more weakly bound by cations in CaO-rich glass. The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass [Kim SW, Shimoyama T, Hosono H (2011) Science 333(6038):71–74] synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.

EXAFS study on amorphous and nanocrystalline M–W (M=Fe,Ni) alloys produced by electrodeposition

Abstract Ductile amorphous and nanocrystalline M–W (M=Fe,Ni) alloys were produced by electrodeposition. Structural studies of these alloys were made by using EXAFS and SAXS methods. The crystal structure of the electrodeposited Ni–W alloy was similar to that of the Ni4W crystal phase. The nearest Ni–Ni distance was determined to be 2.49 A; the Ni–W distance was determined to be 2.50 A. The atomic distances in the nanocrystalline Ni–W alloys increases with the tungsten concentration. The average crystal grain size of the Ni–W alloy was estimated at about 2.5 nm from a Guinier plot of the SAXS spectra. The electrodeposited amorphous Fe–21.6 at.% W alloy has only short range order, and no medium range order.

Structural and physical properties of Ag-As-Te glasses

Abstract Differential scanning calorimetric (DSC), dc electrical conductivity, X-ray diffraction and EXAFS measurements for (Ag 2 Te) x (AsTe) 1− x glasses with x =0 to 0.3 have been carried out to investigate physical properties and the co-ordination environment of constituting atoms in ternary Ag–As–Te glasses. Results of the DSC measurement suggest that the glass structure is more thermally unstable with increasing Ag content. The incorporation of Ag 2 Te into AsTe glass is responsible for a pronounced increase in the electrical conductivity and corresponding decrease in the electrical activation energy. Least-squares fit analyses for the observed X-ray structure functions have been carried out under the assumption that the first co-ordination shell is composed of As–Te, Ag–Te and As–As correlations. The results indicate that the interatomic distances of three atomic correlations are all composition-independent. The co-ordination number of As atoms is determined to be about three and remains practically unchanged. Ag atoms are roughly fourfold co-ordinated with Te atoms, as suggested by the formal valence shell (FVS) model proposed for the chalcogenide glasses. The co-ordination number of Te atoms increases from 1.93 to 3.60 with increasing Ag content. The thermal stability and electrical properties of the present glasses may be associated with the local structure and bonding nature of Ag–Te bonds.

Hydrogen-bonded structure in aqueous sulfuric acid solutions

Abstract X-ray and time-of-flight (TOF) neutron diffraction and Raman spectroscopic measurements have been carried out at 25°C on concentrated aqueous sulfuric acid solutions, (H 2 SO 4 ) x (H 2 O) 1−x , x = 0, 0.25, 0.5, 0.75 and 0.86, in order to investigate the hydrogen-bonded intermolecular structure in the solutions. The intramolecular parameters for regular tetrahedral SO 4 unit were determined from the least squares fit to observed X-ray and neutron interference terms in the range of Q ≥ 8 A −1 . The nearest neighbor intermolecular hydrogen-bonded distances, r O…O = 2.7 A and r O…D = 1.7 A, respectively determined from X-ray and neutron intermolecular distribution functions, exhibited both ∼0.2 A shorter values than those reported for pure liquid water. The isotropic Raman spectra of O-H stretching region for solutions of higher H 2 SO 4 content (x ≥ 0.5) indicated an extremely broadened band centered at 3030 cm −1 , which corresponds to ∼300 cm −1 lower frequency shift compared with that for pure liquid H 2 O. These results sugges the existence of strong hydrogen-bonded intermolecular interaction in these solutions.

Structure of HCOOK hydrated melts

Abstract X-ray diffraction and ATR-IR spectroscopic measurements have been carried out for HCOOK hydrated melts, (HCOOK) x (H 2 O) 1− x with x =0.3, 0.35 and 0.40, in order to obtain structural information on both the ion–water and ion–ion interactions in extremely concentrated aqueous solutions. The observed X-ray intermolecular interference terms were analyzed through the least-squares fitting procedure. The present value of the nearest neighbor K + ⋯H 2 O distance, r K + ⋯H 2 O =2.77(1) A, for the 40 mol% HCOOK solution is in good agreement with that observed in various aqueous solutions. On the other hand, the coordination number, n K + ⋯H 2 O , was revealed to be 2.6(1), which is much smaller than the value, n K + ⋯H 2 O ∼6, reported for more dilute aqueous solutions. Structural parameters for the contact ion pair, HCOO − ⋯K + , were satisfactorily determined to be r O(HCOO − )⋯K + =2.80(1) A, n O(HCOO − )⋯K + =1.3(1) and ∠C–O⋯K + =100(1)°, respectively. Hydration numbers, such as n K + ⋯H 2 O and n HCOO − ⋯H 2 O were found to be strongly concentration dependent from the ATR-IR spectra for uncoupled O–D stretching vibrational bands of HDO molecules in the melts.

Local arrangement in GeSeI glasses

Abstract X-ray diffraction and Raman scattering measurements on amorphous (GeSe3.5)100 - xIx have been carried out over a wide composition range. The position of the well-resolved first peak of the pair distribution function gradually increases with increasing I content due to the substitution of the GeI bond for the GeSe bond. The coordination number for the Ge atoms obtained by a least-squares fit between the observed and theoretical intensity functions is nearly four at any composition, indicating that the glasses contain some types of tetrahedral unit with the Ge atoms located at the central position, as well as Se cluster molecules. The nature of the GeI bonding in the tetrahedral unit is ideally covalent, similar to that for GeSe, which enables the formation of GeSe (4-n) 2 I n (n = 1–3) mixed-anion tetrahedral units. A series of Raman bands for 160 ≤ v ≤ 201 cm−1, which can be assigned to the A1 vibrational mode of these units, indicates their presence.

The coordination structure of Li+ in highly concentrated methanolic LiBr and LiI solutions

Time-of-flight neutron diffraction measurements have been carried out on 25 mol% LiBr and 33 mol% LiI solutions in CD 3 OD, using the HIT-II spectrometer installed at KENS. The 6 Li/ 7 Li isotopic substitution technique was applied to lithium ions in both solutions in order to determine the coordination structure around the lithium ion in highly concentrated non-aqueous solutions. The partial distribution function around Li + , G Li (r), has been derived from the Fourier transform of the first-order difference function, Δ Li (Q), obtained from a numerical difference in observed scattering cross-sections between two solutions with different 6 Li/ 7 Li composition. The Li + ... methanol molecule configuration is characterized by the intermolecular distance and coordination number, r LIO = 1.97 ± 0.05 A and n LiO = 3.0 ± 0.5 for the LiBr solution and r LiO = 1.93 ± 0.05 A and n LiO = 1.8 ± 0.5 for the LiI solution, respectively. These values indicate that Li + has a well-defined first coordination shell. The possibility of the formation of a contact ion pair, Li + ... X - (X = Br or I), in such a highly concentrated solution is discussed using the present data.