Haruki Kawamura

High-pressure Raman spectroscopy of diamond anvils to 250GPa: Method for pressure determination in the multimegabar pressure range

The first-order Raman spectra of diamond anvils used in a gasketed high-pressure cell have been measured at pressure up to 250GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the culet face, is represented by a function of pressure in the sample chamber up to 250GPa. The dependence is almost independent on loading conditions. The application of the pressure dependence for pressure determination up to the multimegabars pressure region is proposed.

Pressure calibration of diamond anvil Raman gauge to 310GPa

In order to develop an optical method for pressure determination in the multimegabar region, the first-order Raman spectra of diamond anvils were investigated at pressures up to 310GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the anvil culet due to the normal stress, was calibrated against the sample pressure derived from the equation of state of Pt. The obtained pressure dependence of the edge frequency demonstrates the reliability of this diamond anvil Raman gauge. Up to the maximum pressure of this study, the relation between Raman frequency and normal stress at the diamond anvil culet is formally similar to the equation of state of a hydrostatically compressed isotropic elastic body having a bulk modulus of K0=547(11)GPa and a pressure derivative of the bulk modulus K0′=3.75(20).

Equation of state of bismuth to 222 GPa and comparison of gold and platinum pressure scales to 145 GPa

Unit cell volumes of Bi, Pt, and Au have been measured simultaneously to megabar pressures by x-ray powder diffraction using a diamond anvil cell and a synchrotron radiation source. The body-centered cubic (bcc) phase of Bi was found to be stable up to 222 GPa. The equation of state (EoS) of bcc-Bi was determined using the Pt pressure scale. A fit of the Vinet EoS to the volume compression data gave B0=35.22(19) GPa, B0′=6.303(18), and 1 atm atomic volume V0=31.60(4) A3. Because of the high compressibility, the use of bcc-Bi as a pressure marker is expected to give improved precision in pressure measurement. The Pt and Au pressure scales were compared up to 145 GPa. The Au pressure scale gave lower pressure than the Pt pressure scale. The deviation between the two scales became noticeable at ∼30 GPa and diverged with increasing pressure, reaching ∼20 GPa at 145 GPa. A fit of the Vinet EoS to Au compression data on the Pt pressure scale gave: B0=166.34(77) GPa, and B0′=6.244(33).

Anomalous superconductivity in black phosphorus under high pressures

Abstract Pressure induced superconductivity in single crystals of black phosphorus has been studied. Maximum onset Tc was near 13 K. The anomalous superconductivity may be explained in terms of excitonic mechanism.

High-Pressure X-Ray Diffraction Study on Electronics-d Transition in Zirconium

High-pressure X-ray diffraction study on the electronic s - d transition in IVa transition metal Zr has been carried out up to 68 GPa. The successive structural transitions of hcp-hexagonal-bcc were observed at 6.7 GPa and 33 GPa, respectively. An isostructural transition was also observed in the bcc phase around 56 GPa. The values of the isothermal bulk modulus, K 0 , and the pressure derivative of K 0 , K 0 ′ , for the hexagonal phase were calculated to be 121 GPa and 1.69, respectively. Abnormally small K 0 ′ suggests softening in the pressure-volume curve. The atomic volume of the bcc-Zr at the transition pressure of 33 GPa agreed with that of the bcc-Nb at zero pressure. These results suggest that the s - d transition of Zr metal progresses at each pressure and is complete with an isostructural transition at 56 GPa.

Raman study of black phosphorus up to 13 GPa

Raman scattering experiments of a single crystal of black phosphorus have been carried out at room temperature in a wide pressure range from 0.55 to 13 GPa in order to study a band overlapped metallization at 1.5 GPa and the structural phase transitions. Five Raman-active modes in the orthorhombic (A11) phase were detected as a function of pressure. The softening of the B1g and B3g1 modes was observed around 2 GPa. Above 4.7 GPa, these modes showed a sudden decrease in the intensity and two new broad bands appeared at 388 and 308 cm−1, corresponding to a transition to the rhombohedral (A7) phase. The broad bands which were assigned to the A1g and Eg modes of the A7 phase, exhibited remarkable softening with pressure. The A7 phase coexisted with the A11 phase up to 7 GPa. The transition to the simple cubic phase was also observed at 10.5 GPa with vanishment of the A1g mode.

Pressure calibration of diamond anvil Raman gauge to 410 GPa

The first-order Raman band of diamond anvils has been investigated at pressure up to 410 GPa in order to develop an optical pressure determination method. The high-frequency edge of the band was calibrated by the pressure scale of the equation of state of Pt. A unique relationship between the edge-frequency and the sample pressure was confirmed up to the highest pressure, while the edge frequency reaches to 1907 cm−1 at 410 GPa. Usefulness of the diamond anvil Raman gauge as a laboratory-based pressure determination method in the multimegabar pressure range has been discussed.

Pressure-induced superconductivity and phase transition in selenium and tellurium

Abstract The pressure dependences of the electrical resistance and the superconducting transition temperature, Tc, in Se and Te have been measured up to 57 GPa and 42 GPa, respectively. A semiconductor- metal transition of Se and Te, which occured at 23 GPa and 4 GPa, respectively, corresponded to the structural transition to monoclinic phase. The pressure dependence of Tc of Se showed an anomaly at about 36 GPa, corresponding to a structural transition from monoclinic to orthorhombic phase. The Tc of monoclinic-Se was 5.6 K at 26 GPa and independent of pressure. The Tc of Te exhibited a significant enhancement at 35 GPa, which resulted from the structural transition from β-Po type to bcc phase, and reached a maximum value of 7.4 K.

Structural Phase Transition of Rutile-Type MgH2 at High Pressures

The pressure-induced structural phase transition of MgH 2 was examined up to 57 GPa at room temperature. α-MgH 2 with the rutile-type structure at ambient conditions transformed to γ-MgH 2 with the α-PbO 2 structure. A complete conversion to the γ-phase, however, did not take place and these two phases coexisted up to 9 GPa. Above 9 GPa, MgH 2 transformed to the orthorhombic phase (HP1-phase), and further to another orthorhombic phase (HP2-phase) at around 17 GPa. Space groups of the HP1-phase and the HP2-phase were concluded to be P b c 2 1 and P n m a (cotunnite-type), respectively. Cotunnite-type MgH 2 was stable at pressures up to 57 GPa. Thus the proposed structural sequence of MgH 2 under pressure is rutile-type → α-PbO 2 → P b c 2 1 → cotunnite-type, with an increase in the coordination number of magnesium ions from 6 to 9.

Superconductivity and phase transition of zirconium under high pressure up to 50 GPa

The superconducting transition temperature, T c , of group-IV transition metal zirconium has been studied as a function of pressure up to 50 GPa in relation to the pressure-induced structural phase transition. Corresponding to a phase transition from hexagonal to bcc structure, a discontinuous increase of the T c is observed at 30 GPa. For the bcc phase, a high T c of 11 K has been developed. The result suggests that an electronic configuration similar to that of the bcc Nb is induced in the bcc phase as a result of an increase in valence d -electron numbers by the s - d electronic transformation. The observation of the high T c is decisive experimental evidence for a pressure-induced transformation from group-IV transition metal to group-V transition metal.