Yukinobu Kawakita

Comparison of partial structures of melts of superionic AgI and CuI and non-superionic AgCl

Neutron and high-energy x-ray diffraction analyses of molten AgI have been performed and the partial structures are discussed in detail with the aid of the structural modelling procedure of the reverse Monte Carlo (RMC) technique by comparison with those of molten CuI and AgCl. It is well known that AgI and CuI have a superionic solid phase below the melting point, in which the cations favour a tetrahedral configuration, while solid AgCl has a rock-salt structure with an octahedral environment around both Ag and Cl atoms. Even in the molten states, there is a significant difference between superionic and non-superionic melts. The cation is located on the triangular plain formed by three iodine ions in molten AgCl and CuI, while molten AgCl favours a 90° Cl-Ag-Cl bond angle, which is understood to maintain a similar local environment to that in the solid state. The atomic configurations of the RMC model suggest that the cation distributions in superionic melts of CuI and AgI exhibit large fluctuations, while Ag ions in the non-superionic melts of AgCl are distributed much more uniformly.

Polar rotor scattering as atomic-level origin of low mobility and thermal conductivity of perovskite CH 3 NH 3 PbI 3

Perovskite CH3NH3PbI3 exhibits outstanding photovoltaic performances, but the understanding of the atomic motions remains inadequate even though they take a fundamental role in transport properties. Here, we present a complete atomic dynamic picture consisting of molecular jumping rotational modes and phonons, which is established by carrying out high-resolution time-of-flight quasi-elastic and inelastic neutron scattering measurements in a wide energy window ranging from 0.0036 to 54 meV on a large single crystal sample, respectively. The ultrafast orientational disorder of molecular dipoles, activated at ∼165 K, acts as an additional scattering source for optical phonons as well as for charge carriers. It is revealed that acoustic phonons dominate the thermal transport, rather than optical phonons due to sub-picosecond lifetimes. These microscopic insights provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized.

Sub-picosecond dynamics in liquid Si

We are the first group to succeed in measuring the dynamic structure factor S(Q,ω) of liquid Si close to melting using high-resolution inelastic x-ray scattering. The spectra clearly demonstrate the existence of propagating short wavelength modes in the melt with a Q–ω relation similar to those in other liquid metal systems. A specific variation of the quasi-elastic line shape with increasing Q is observed close to the structure factor maximum. This observation is related to the onset of atomic correlations on the sub-picosecond timescale in the vicinity of a metal-to-insulator transition. Such observations have been made previously only in computer simulations of metallic systems with increasing covalent character. Our data provide the first experimental evidence for these ultrashort density correlations.

Structure of Liquid Ge at High Temperatures

Neutron diffraction measurements have been performed for liquid germanium over a wide temperature range from 1000°C to 1550°C. The structure factor of liquid germanium just above the melting point has a pronounced shoulder in the high momentum transfer region of the first peak, as also by our data confirmed. With increasing temperature, the position of the first peak moves slightly to a high value of Q and the shoulder becomes inconspicuous.

Structural disorder in lithium lanthanum titanate: the basis of superionic conduction

High-energy x-ray and neutron diffraction measurements on polycrystalline La(2/3-x)Li(3x)TiO(3) (0.075 < x < 0.165) were performed. The total scattering structure factors were analysed by the reverse Monte Carlo (RMC) modelling technique, resulting in three-dimensional particle configurations. These configurations were then used for revealing the distributions of La and Li ions and to understand the relationship between these distributions and ionic conduction. An alternating arrangement of La-rich and La-poor layers along the c-axis was found in the x = 0.075 composition. Intriguingly, this arrangement has gradually disappeared in samples with higher Li concentration. Furthermore, RMC models exhibit disordered distributions of Li ions, situated mainly on the La-rich layer, and there is a significant probability of Li ions occupying the interstitial sites (T site) between the O-3 triangle plane of the TiO(6) octahedron and an La ion or its vacancy site. It was also found on the basis of the RMC models that the bond valence sum (BVS) for Li ions behaves differently on La-rich and La-poor layers at low Li concentration compositions, but they are similar at high Li concentration compositions. This is consistent with the behaviour of the alternating arrangement of La-rich and La-poor layers. It is also suggested that the Li ions around the bottleneck at (1/2, 0, 0) (bottom layer) can jump to an adjacent bottleneck at (0, 1/2, 0) through the T site and not only Li ions in the La-poor layers but also Li ions in the La-rich layers contribute to the bottleneck-bottleneck Li conduction.

Electronic and atomic structures of liquid tellurium containing alkali elements

The measurements of electrical conductivity σ, density, EXAFS and neutron scattering were carried out for liquid K–Te and Rb–Te mixtures. The conductivity σ decreases rapidly with alkali concentration and a metal-semiconductor transition occurs at about 10 at.% alkali. It is found that the compositional variation of σ is nearly independent of the alkali species. The Te–Te bond length deduced from EXAFS and neutron scattering measurements is 2.8 A and changes little with alkali concentrations. The average distances from K and Rb atom to Te atoms are 3.6 A and 3.8 A, respectively. Two kinds of relaxation processes are observed in quasielastic neutron scattering for K 20 Te 80 . Upon the addition of alkali the interaction between the neighbouring Te chains, which is responsible for the metallic conduction, weaken considerably.

The Performance of TOF near Backscattering Spectrometer DNA in MLF, J-PARC

The time-of-flight (TOF) type near-backscattering spectrometer (n-BSS), DNA, with Si crystal analyzers was built and started operation in 2012 at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). DNA is the first n-BSS with pulse shaping chopper installed at a spallation pulsed neutron source. It offers currently the highest energy-resolution of about 2.4 micro eV by operating a pulse shaping double-disk chopper at 225 Hz whose phase is optimized to the narrowest slit of 10 mm width. Energy resolution can be flexibly compromised with intensity during experiment by using two type slits with different widths and changing the copper frequency. An example of measurement with high energy-resolution under the condition that the pulse shaping chopper was operated is shown, where the limited measurable energy range was widely expanded by multi incident energy band technique. The experimental data demonstrate extremely high signal-to-noise ratio (~10 5 ) of this spectrometer.

Neutron diffraction data and molecular dynamics simulations of the molten mixture Ag(Br0.7I0.3).

The structure factors of the ionic liquid mixture Ag(Br(0.7)I(0.3)) at three temperatures, 723, 923, and 1023 K, as well as of the pure molten AgI at 923 K and the pure molten AgBr at 773 and 923 K, were studied experimentally and by means of molecular dynamics simulations. The experiments were carried out using the high intensity total scattering time-of-flight spectrometer, HIT-II, at the KENS spallation neutron source in Japan. The experimental data are very reliable, with the possible exception of the small momentum transfer region, whose accessibility is limited by neutron energy and detector positions. The simulations made use of the semiempirical rigid ion potentials of the Vashishta-Rahman [Phys. Rev. Lett. 40, 1337 (1978)] type using a new set of parameters appropriate for the mixture. Within the known constraints of the pairwise rigid ion potentials, the simulated structure factors are in fair agreement with experiment. The results for the pair distribution functions suggest that the molten mixture retains the superionic character found in previous calculations of both the AgI and AgBr melts. This suggestion is confirmed by the results for the self-diffusion coefficients. Values obtained for the ionic conductivities are also presented.

Coherent dynamic scattering law of divalent liquid Mg

The dynamic structure factor S ( Q ,ω) of liquid ( l -) Mg was measured at 700°C using high-resolution inelastic X-ray scattering. Based on the high-quality experimental data presented in this paper, we discuss the particle dynamics in dense l -Mg using the generalized hydrodynamic theory for phonon excitations. The particle motion in this liquid is influenced by collective longitudinal modes, similar to other simple liquid metals such as l -alkali metals. The Q dependence of quasielastic line width can be interpreted based on an existing theory modified from the de Gennes formalism.

Structure and dynamical properties of liquid In2Te3

Abstract Liquid In–Te has a compound-forming behavior around the In 2 Te 3 composition. The large temperature dependences of many physical properties at this composition are strongly related to structural changes in this liquid. Energy dispersive X-ray diffraction measurements were carried out to study the change of the local structure in liquid In 2 Te 3 with temperature over a wide temperature range up to 1200 °C. The observed total structure factor, S ( Q ), shows a distinct second peak around 3.05 A −1 at lower temperatures and, with increasing temperature, this peak decreases, becoming a small shoulder at the highest temperature. The neutron diffraction spectra for liquid In 2 Te 3 was also measured at 750 °C and a similar profile for S ( Q ) to the X-ray results was obtained. The formation of triangular-pyramidal units just above the melting point and the appearance of chain like molecules in the high temperature liquid are proposed, along with a bond fluctuation model.