M. Takumi

X-Ray, Raman and Photoluminescence Study of Vacancy Ordered β-Ga2Se3 under High Pressure

The effect of pressure on the structure and optical properties of vacancy-ordered β-Ga2Se3 has been investigated by X-ray diffraction, Raman and photoluminescence spectroscopies using diamond anvil cell. From the shift of the photoluminescence peak with pressure, the pressure dependence of the band gap energy (dEg/dP) is derived to be 46 meV/GPa. From Rietveld analysis, the pressure dependencies of the lattice parameters and the atomic positions are obtained. The relation between these crystal data and the pressure dependence of the band gap energy is discussed. From the X-ray measurements, a structural phase transition at 14 GPa is observed, as is observed in α-Ga2Se3. In the Raman scattering measurement, pressure-tuned resonant phenomenon is observed near 2.5 GPa, where the band gap energy of β-Ga2Se3 coincides with the photon energy of the argon ion laser (λ = 514.5 nm) used for Raman measurement as excitation light.

X-ray structural analysis of the high-pressure phase III of tellurium

A structure of a high-pressure phase of tellurium (Te-III) has been re-examined by an x-ray diffraction method. This phase appears at a pressure between 7 and 27 GPa, where a second-order phase transition to Te-IV occurs. The newly obtained crystal lattice of Te-III at 8 GPa is monoclinic with a = 8.4682(14) A, b = 4.7424(8) A, c = 3.9595(7) A, β = 88.112(11)°. The space group is C2/m, with six atoms in the unit cell, two in positions 2a at (0, 0, 0) and four in positions 4i at (x, 0, z) with x = 0.324(11), z = 0.675(3).

Structural Phase Transitions of Ga2Se3 and GaSe under High Pressure

High pressure X-ray powder diffraction studies have been made on α-Ga 2 Se 3 up to 27 GPa and e-GaSe up to 64 GPa by using synchrotron radiation. A structural phase transition is observed for α-Ga 2 Se 3 at 14 GPa, as is observed in β-Ga 2 Se 3 , The diffraction patterns of the high pressure phases of α- and β-Ga 2 Se 3 are almost the same, showing that the structures of the high pressure phases of both α- and β-Ga 2 Se 3 are the same. On the other hand, the structural phase transition for e-GaSe is observed near 25 GPa. The diffraction pattern of the high pressure phase of GaSe is similar to that of Ga 2 Se 3 . This shows that the high pressure phase of GaSe is similar to that of Ga 2 Se 3 , although the atomic concentrations in the alloys are different between Ga 2 Se 3 and GaSe. Most of the diffraction profile of the high pressure phase is explained by a NaCI type structural model, suggesting that the high pressure phase has a structure similar to NaCI.

Pressure-Induced Metallization and Structural Transition of Orthorhombic Se

Powder X-ray diffraction experiments and electrical resistance measurements were performed at pressures up to 36 GPa for orthorhombic Se consisting of cyclic Se 7 molecules. A successive phase transformation was observed to occur in three stages under pressure. The forth phase (final phase) was assigned to a base-centered orthorhombic lattice, which was the same as the Se-IV phase, the high pressure phase of trigonal Se. Pressure-induced metallization was observed at pressure around 25 GPa, which corresponded to the second stage of transition. The crystal lattices of the second phase and the third phase have not been clarified yet.

Optical Properties of Ga2Se3 under High Pressure

The optical absorption spectrum of β-Ga2Se3 has been measured at pressures up to 7 GPa using a diamond anvil cell. The exciton absorption is observed in the optical absorption spectrum at ambient pressure. With increasing pressure, the exciton line shifts to the higher energy region. From the shift of the exciton line with pressure, the pressure dependence of the band gap energy (dEg/dP) is estimated to be (45 ± 4) meV/GPa at ambient pressure. This pressure coefficient agrees well with that obtained previously by photoluminescence measurements. The pressure coefficient of the band gap and the intensity of the exciton line decrease gradually with pressure, and become almost zero at 7 GPa, what suggests that a pressure induced direct- to indirect-gap semiconductor transition occurs.

The Design and Performance of Beamline BL15 at SAGA Light Source

A new X‐ray beamline has been designed and constructed at SAGA Light Source. The beamline, named BL15, covers an energy range from 2.1 to 14.2 keV and is intended for the characterization of various materials developed for industrial purposes. The beamline has an experimental station for the performance of several experiments: e.g. X‐ray absorption fine structure (XAFS) measurement; high resolution diffractometry; topographic imaging; energy‐dispersive four‐circle diffractometry. The photon flux passing through a receiving slit at XAFS measurement position is more than 1 × 1011 photons/sec with focusing mirror in the energy range of less than 8keV. The XANES spectra measurements of Cu and Ti K‐edge show that the beamline has a good energy resolution.

Optical absorption spectrum of trigonal selenium under high pressure

Optical absorption spectra of trigonal selenium have been measured at pressure up to 5 GPa using a diamond anvil cell with two kinds of pressure-transmitting media, that is, a mixture of methanol, ethanol and water, and a potassium bromide, to confirm no effects of them. With increasing pressure, the absorption edge shifts to a lower energy and the pressure coefficients of the absorption edge at atmospheric pressure obtained by using both pressure-transmitting media agree with each other. At high pressure, as more clearly observed in the spectra obtained by using a potassium bromide, the edge shifts to a lower energy become moderate near 3 GPa, where an abrupt decrease in intramolecular covalent bond length was observed. This result must be related to the transition of the electronic band structure in trigonal selenium at approximately 3 GPa.

Pressure effects of a genuine organic ferromagnet p-NPNN

Pressure-induced ferro-antiferromagnetic transition in genuine organic compounds was observed for the first time in the β-phase p-NPNN crystal. This phenomenon is understood by the charge-transfer mechanism, in which some of the ferromagnetic exchange integral comes to compete with the antiferromagnetic one being enhanced via molecular orbitals under the hydrostatic pressure.