Ayano Chiba

Vibrational, single-particle-like, and diffusive dynamics in liquid Se, Te, and Te50Se50

Neutron scattering measurements were carried out for liquid Se, liquid Te, and liquid Te50Se50. The results are discussed in the context of semiconductor-to-metal transition. As to liquid Te50Se50, the vibrational modes show a remarkable change when the semiconductor-to-metal transition occurs by temperature increase. A soft stretching mode was observed for the metallic phase and it shows the structural change; the appearance of long bonds in the metallic phase. The vibrational modes for liquid Se and liquid Te were obtained and agreed well with former works. The crossover from collective to single-particle-like regime was also observed for all liquid samples and the transition ranges (Q=4.5–8 A−1) are presented. The diffusive features in the low-Q region and single-particle-like properties in the high-Q region are discussed and are shown to be consistent with the results for the vibrational modes.

Pressure and temperature dependence of the structure of liquid arsenic and antimony

The structure of liquid As and liquid Sb have been measured under high pressure and high temperature by energy-dispersive X-ray diffraction method. The coordination number of liquid As was found to be 3.5 at 1.0GPa, and it showed an increase to around 6 at 10GPa. The change suggests a crossover from a A7-like to a simple-cubic-like structure occurs at this pressure range. Further compression up to 17.1GPa has been made and the structural change in all pressure region is discussed. The pressure dependence of the structure factor, pair-distribution function, and the coordination number for liquid Sb are reported.

Structural Changes of Short- and Intermediate-Range Order in Liquid Arsenic under Pressure

The pressure dependence of the structural properties of liquid arsenic (l-As) is studied in detail by ab initio molecular dynamics simulations and X-ray diffraction experiments. In this study, we have clarified that network structures consisting mainly of As4 units exist at lower pressures and that the correlation between the As4 units is the origin of an intermediate-range order, which is seen as a prepeak of the static structure factor. When the pressure increases, the intermediate-range order disappears and structural change occurs gradually. At approximately 7 GPa, the pair distribution function and the bond angle distribution show some similarities to those of the simple cubic structure as seen in the high-pressure phase of crystalline As.

The structure of liquid Sb under pressure

Synchrotron x-ray diffraction measurements of liquid Sb under pressure have revealed that the structure in the low pressure range is not simple-cubic-like nor slightly Peierls-type distorted one, although the coordination number and the pair correlation function show some similarities to that of simple-cubic-like liquid. Structural changes were observed at around 3GPa and 6.5-9GPa. The changes of the peak positions and the coordination number are reported.

Quasielastic Neutron Scattering of Liquid Te50Se50 in the Semiconductor-to-Metal Transition Range

We have carried out neutron scattering experiments for liquid Te 50 Se 50 at 700 K (semiconducting regime), 900 K (transition regime) and 1100 K (metallic regime). The quasielastic scattering peak is comparatively narrow at the peak positions of S ( Q ). A remarkable increase in the peak width is observed near the first minimum of S ( Q ), when the semiconductor-to-metal transition occurs. Since the first peak of S ( Q ) in this system proves to mainly reflect the inter-chain correlation and the second peak to the intra-chain correlation, the observed broadening may suggest frequent bond-switching between the chains in the metallic regime.

Double Bragg scattering observed in small-angle X-ray scattering from highly oriented pyrolytic graphite

In small-angle X-ray scattering from highly oriented pyrolytic graphite, radial streak patterns are observed. The streaks change their direction with sample rotation and appear and disappear in pairs. This streak pattern can be explained by double Bragg scattering. The directions of the streaks calculated as functions of the rotation angle are in good agreement with experiment. In addition, asymmetry in the intensity of the streaks and a small deviation from the radial pattern are observed, and they can be explained by the finite sample-size effect. The necessity of taking double Bragg scattering into account in small-angle X-ray scattering studies of crystalline materials is emphasized.

Pressure-induced structural change in liquid GeTe and Liquid GeSe

X-ray diffraction has been measured up to 19 GPa for liquid GeTe and up to 15 GPa for liquid GeSe using synchrotron radiations. Static structure factor S(Q) and pair distribution function g(r) was obtained. Both S(Q) of liquid GeTe and liquid GeSe change their shape with increasing pressure, indicating anisotropic contraction of the local structure. A sharp change in S(Q) and g(r) was observed in liquid GeTe between 1.8 GPa and 4.0 GPa. Pressure dependences of S(Q) and g(r) of liquid GeSe change around 6 GPa. In liquid GeSe a small pre-peak was observed around 1.2 A-1 up to 1.7 GPa which suggests the presence of the medium range order of covalent bond network. With increasing pressure the Peierls-type distortion becomes small and the bond angle decreases; the local structure becomes a rhombohedral structure. These pressure dependences of the structure of liquid GeTe and liquid GeSe are different from those of the crystals. These results are compared with those of other liquids and discussed in relation to the bonds between atoms in the liquids.

Neutron scattering studies of liquid tellurium-selenium mixtures

Abstract Neutron scattering experiments for liquid tellurium–selenium mixtures have been carried out in the semiconductor-to-metal transition range. The dynamic structure of liquid Te 50 Se 50 at three temperatures (semiconducting, transition and metallic regime) was measured. We also measured the dynamic structure factor of liquid Te (metallic regime) and liquid Te 40 Se 60 (semiconducting regime) at 500 °C. The vibrational density of states obtained by inelastic scattering studies confirmed that the gap between the bending and stretching modes is filled when the semiconductor-to-metal transition occurs.

The structure of liquid SnTe under pressure.

We investigated the structure of liquid SnTe at high pressures up to 8.2 GPa by energy-dispersive x-ray diffraction. On melting at low pressures, the crystalline B1 structure changed into not B1-like but distorted-B1-like local structure. We also found that the structure changes at around 1.6-3.3 GPa. At high pressures, the bond angle and coordination number approached those for B2-based structure, but still showed clear deviations from B2-like local structure.

Pressure-Induced Structural Change in Liquid Ge0.15Te0.85 alloy

X-ray diffraction has been measured up to 2.4 GPa for liquid Ge0.15Te0.85 alloy using synchrotron radiations. Static structure factor S(Q) and pair distribution function g(r) were obtained. At 0.5 GPa, S(Q) shows different temperature dependence below and above 700 K. With increasing temperature above 700 K the increasing rate of the height ratio between the first and second peaks in S(Q) becomes moderated. This finding suggests a temperature-induced structural transformation occurs at 700K. At 1.8 GPa and 2.4 GPa such anomalous behaviour was not observed in the temperature region from the melting temperature to 850 K. The boundary between the low-temperature and the high-temperature phases has a negative slope. The Clausius-Clapeyron equation suggests the high-temperature phase has larger entropy than the low-temperature one.