Ayako Ohmura

Pressure-induced phase transition of Bi2Te3to a bcc structure

The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been studied by synchrotron x-ray diffraction measurements at room temperature using a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the pressure from 29.8 GPa, the diffraction pattern changes with pressure hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc structure can explain the phase IV peaks. We assumed that the structural model of phase IV is analogous to a substitutional binary alloy; the Bi and Te atoms are distributed in the bcc-lattice sites with space group Im-3m. The results of Rietveld analysis based on this model agree well with both the experimental data and calculated results. Therefore, the structure of phase IV in Bi2Te3 can be explained by a solid solution with a bcc lattice in the Bi-Te (60 atomic% tellurium) binary system.

Development of a single-crystal X-ray diffraction system for hydrostatic-pressure and low-temperature structural measurement and its application to the phase study of quasicrystals

We constructed a single-crystal synchrotron X-ray diffraction system to precisely study the structure under hydrostatic pressure conditions at low temperatures and applied it to a study on the phase transition phenomena of a Cd–Yb periodic approximant and a Cd–Yb quasicrystal. Four phases were newly observed for the 1/1 approximant crystal in a P–T range up to 5.2 GPa and down to 10 K. The innermost part of the atomic clusters of Cd4 tetrahedra exhibited various orientational ordering sensitively depending on pressure and temperature. High pressure diffraction measurements using a highly parallel synchrotron X-ray beam and a hydrostatically compressed single crystal enabled us to detect the weak diffractions due to the subtle structural changes.

Photochromism in yttrium hydride

Transparent orange yttrium hydride turns to black when illuminated by visible laser light at pressures of several gigapascals at room temperature. The marked reduction in optical transmittance extends over the infrared region, suggesting that illumination creates persistent free carriers. The opaque black sample returns to the transparent orange hydride during room-temperature annealing for a few hours. Photochromism is pronounced for the coexistent state of the metallic fcc-YH2 and the insulating hexagonal-YH3 state but is depressed for the single phase of hexagonal-YH3. The results indicate that light illumination can modify the optical and possibly electronic properties during a certain period of times.

Raman and optical absorption studies of rare-earth hydrides under high pressure

Raman and visible absorption study were performed for YH3 and ScH3 at high pressures, in order to clarify the structural and electronic phase transitions. The Raman results of YH3 revealed the presence of an intermediate phase at 9-24 GPa between the low-pressure hexagonal and high-pressure fcc phases. The characteristic behaviour was commonly observed for vibrational properties of YH3 and ScH3, suggesting a common mechanism of the structural transformation. The results of the absorption measurements for YH3 demonstrate that the optical band gap starts to close in response to the phase transition.

Pressure-induced topological phase transition in the polar semiconductor BiTeBr

We performed X-ray diffraction and electrical resistivity measurement up to pressures of 5 GPa and the first-principles calculations utilizing experimental structural parameters to investigate the pressure-induced topological phase transition in BiTeBr having a noncentrosymmetric layered structure (space group P3m1). The P3m1 structure remains stable up to pressures of 5 GPa; the ratio of lattice constants, c/a, has a minimum at pressures of 2.5 - 3 GPa. In the same range, the temperature dependence of resistivity changes from metallic to semiconducting at 3 GPa and has a plateau region between 50 and 150 K in the semiconducting state. Meanwhile, the pressure variation of band structure shows that the bulk band-gap energy closes at 2.9 GPa and re-opens at higher pressures. Furthermore, according to the Wilson loop analysis, the topological nature of electronic states in noncentrosymmetric BiTeBr at 0 and 5 GPa are explicitly revealed to be trivial and non-trivial, respectively. These results strongly suggest that pressure-induced topological phase transition in BiTeBr occurs at the pressures of 2.9 GPa.

Collapse of CuO Double Chains and Suppression of Superconductivity in High-Pressure Phase of YBa2Cu4O8

The crystal structure and electrical resistivity of YBa2Cu4O8 (Y124) were studied under high pressure up to 18 GPa using diamond-anvil cells, respectively, in order to clarify its conduction mechanism. Y124 causes the first-order phase-transition into the orthorhombic Immm at pressure around 11 GPa. The high-pressure phase (HPP) also shows the superconductivity, while the manner of temperature dependence of electrical resistance and the pressure dependence of transition temperature, Tc, drastically change above 11 GPa. The CuO2 plane persists in HPP but the CuO double chains collapse with the phase transition and transform into three-dimensional Cu–O network, resulting in the renewal of conduction system.

Powder x-ray diffraction study of Ne up to 240 GPa

Powder x-ray diffraction experiments have been done on Ne up to 237 GPa at room temperature. Ne remains in the fcc phase up to the highest pressure. In order to observe the weak 200 reflection at ultrahigh pressures, we have used the tube-cross slit at the exit side of x-rays, which effectively reduced the scattered x-rays by the diamond anvil. The lattice parameters obtained from the 111 and 200 reflections significantly differ from each other, implying the existence of sizable uniaxial stress in Ne at ultrahigh pressures. On the basis of the calculated elastic constants of Ne at high pressures, we conclude that the 111 reflection is least affected by the uniaxial stress. The equation of state constructed from the 111 reflection agrees well with previous data.