Yuki Nakamoto

Superconductivity of Ca Exceeding 25 K at Megabar Pressures

The pressure dependence of the superconducting transition temperature T c in calcium was measured up to 161 GPa. T c increased significantly with pressure and reached 25 K at 161 GPa, which is the highest observed T c for all elements. Compared with the result obtained in a recent structural experiment, T c increases within the simple cubic structure phase and becomes rather stable but still increases in the Ca-IV and Ca-V phases.

Structural transition of post-spinel phases CaMn2O4, CaFe2O4, and CaTi2O4 under high pressures up to 80 GPa

Abstract Three structures of CaMn2O4, CaFe2O4, and CaTi2O4 have been proposed as post-spinel phases. Because these structures are very similar, several ambiguities and inconsistencies appear in high-pressure studies, leading to many problems that are yet to be solved. Systematic powder diffraction studies related to these three phases were conducted under high pressure using synchrotron radiation. All three samples have further high-pressure polymorphs. CaMn2O4 transforms to the CaTi2O4-type structure at about 30 GPa. The MnO6 octahedron in the lower-pressure structure is distorted by the Jahn-Teller effect. A new phase was observed at pressures above 50 GPa during compression of CaFe2O4. Rietveld profile fitting analysis of diffraction data at 63.3 GPa demonstrated that the high-pressure structure, with space group Pnam, is produced via a martensitic transformation by displacing atoms in every third layer perpendicular to the c axis. CaTi2O4 also has a new high-pressure polymorph above 39 GPa with space group Bbmm. The most probable post-spinel candidate in the mantle is the CaTi2O4-type structure. The CaMn2O4-type structure is only formed at high pressure from spinel phases with atoms susceptible to Jahn-Teller distortion.

New high-pressure phase of calcium

An angular dispersive X-ray diffraction experiment on calcium metal (Ca) has been performed at high pressures up to 139 GPa. Ca forms a face-centered cubic (fcc) lattice at ambient conditions, transforms to a body-centered cubic (bcc) lattice at 20 GPa, and further transforms to a simple cubic structure at 32 GPa. We found the simple cubic phase in a wide pressure range, from 32 to about 109 GPa, and discovered a new high-pressure phase above 113 GPa.

Generation of Multi-Megabar Pressure Using Nano-Polycrystalline Diamond Anvils

A nano-polycrystalline diamond was synthesized from graphite by direct conversion under high pressure. The nano-polycrystalline diamond consists of nanosized diamond grains oriented in random directions and has higher toughness and more isotropic mechanical properties than the single-crystal diamond. We generated the pressure using a pair of anvils composed of nano-polycrystalline diamond particles. The highest generated pressure achieved was 210 GPa. The generated maximum pressure was almost the same as that achieved by the single-crystal diamond anvils.

Note: high-pressure generation using nano-polycrystalline diamonds as anvil materials.

Nano-polycrystalline diamonds (NPDs) consist of nanosized diamond grains oriented in random directions. They have high toughness and isotropic mechanical properties. A NPD has neither the cleavage feature nor the anisotropy of hardness peculiar to single-crystal diamonds. Therefore, it is thought to be useful as a diamond anvil. We previously reported the usefulness of a NPD as an anvil for high-pressure development. In this study, some additional high-pressure generating tests using diamond anvils of various shapes prepared from NPDs were conducted to investigate the advantage of using NPDs for anvil applications. The results revealed that the achievable pressure value of a NPD anvil with a culet size of more than 300 μm is about 1.5 to 2 times higher than that of single-crystal diamond anvils, indicating that NPD anvils have considerable potential for large-volume diamond anvils with large culet sizes.

High-pressure phase transitions of Fe3–xTixO4 solid solution up to 60 GPa correlated with electronic spin transition

Abstract The structural phase transition of the titanomagnetite (Fe3-xTixO4) solid solution under pressures up to 60 GPa has been clarified by single-crystal and powder diffraction studies using synchrotron radiation and a diamond-anvil cell. Present Rietveld structure refinements of the solid solution prove that the prefered cation distribution is based on the crystal field preference rather than the magnetic spin ordering in the solid solution. The Ti-rich phases in 0.734 ≤ x ≤1.0 undergo a phase transformation from the cubic spinel of F̅d̅3̅m to the tetragonal spinel structure of I41/amd with c/a < 1.0. The transition is driven by a Jahn-Teller effect of IVFe2+ (3d6) on the tetrahedral site. The c/a < 1 ratio is induced by lifting of the degeneracy of the e orbitals by raising the dx2-y2 orbital below the energy of the dz2 orbital. The distortion characterized by c/a < 1 is more pronounced with increasing Ti content in the Fe3-xTixO4 solid solutions and with increasing pressure. An X-ray emission experiment of Fe2TiO4 at high pressures confirms the spin transition of FeKβ from high spin to intermediate spin (IS) state. The high spin (HS)-to-low spin (LS) transition starts at 14 GPa and the IS state gradually increases with compression. The VIFe2+ in the octahedral site is more prone for the HS-to-LS transition, compared with Fe2+ in the fourfold- or eightfold-coordinated site. The transition to the orthorhombic post-spinel structure with space group Cmcm has been confirmed in the whole compositional range of Fe3-xTixO4. The transition pressure decreases from 25 GPa (x = 0.0) to 15 GPa (x = 1.0) with increasing Ti content. There are two cation sites in the orthorhombic phase: M1 and M2 sites of eightfold and sixfold coordination, respectively. Fe2+ and Ti4+ are disordered on the M2 site. This structural change is accelerated at higher pressures due to the spin transition of Fe2+ in the octahedral site. This is because the ionic radius of VIFe2+ becomes 20% shortened by the spin transition. At 53 GPa, the structure transforms to another high-pressure polymorph with Pmma symmetry with the ordered structure of Ti and Fe atoms in the octahedral site. This structure change results from the order-disorder transition.

Emergence of double-dome superconductivity in ammoniated metal-doped FeSe

The pressure dependence of the superconducting transition temperature (Tc) and unit cell metrics of tetragonal (NH3)yCs0.4FeSe were investigated in high pressures up to 41 GPa. The Tc decreases with increasing pressure up to 13 GPa, which can be clearly correlated with the pressure dependence of c (or FeSe layer spacing). The Tc vs. c plot is compared with those of various (NH3)yMxFeSe (M: metal atoms) materials exhibiting different Tc and c, showing that the Tc is universally related to c. This behaviour means that a decrease in two-dimensionality lowers the Tc. No superconductivity was observed down to 4.3 K in (NH3)yCs0.4FeSe at 11 and 13 GPa. Surprisingly, superconductivity re-appeared rapidly above 13 GPa, with the Tc reaching 49 K at 21 GPa. The appearance of a new superconducting phase is not accompanied by a structural transition, as evidenced by pressure-dependent XRD. Furthermore, Tc slowly decreased with increasing pressure above 21 GPa, and at 41 GPa superconductivity disappeared entirely at temperatures above 4.9 K. The observation of a double-dome superconducting phase may provide a hint for pursuing the superconducting coupling-mechanism of ammoniated/non-ammoniated metal-doped FeSe.

Crystal Structure of the High-Pressure γ Phase of Mercury: A Novel Monoclinic Distortion of the Close-Packed Structure

The crystal structure of the high-pressure γ phase of solid mercury, stable between 12 and 37 GPa, has been determined by powder x-ray diffraction experiments. The structure is monoclinic C 2/ m with six atoms in the unit cell. A mercury atom is coordinated by 10 to 11 atoms, which are distributed at distances up to 10% longer than the first nearest one. The structure is intimately related to α (simple rhombohedral), β (body-centered-tetragonal) and δ (hexagonal-close-packed) phases of mercury, where the stacking of pseudohexagonal layers is systematically modified. This novel monoclinic distortion of the close-packed structure serves as a new class of dense structures for metallic elements.

Superconductivity and crystal structure of lithium under high pressure

The superconductivity and structural properties of lithium under high pressure are investigated by simultaneous measurements of X-ray diffraction and electrical resistance at low temperature below 25 K. The structural transitions fcc - hR1 - cI16 near 40 GPa and the possible structural transition to higher pressure phases were observed above 70 GPa. The superconducting transition temperature Tc increases with applied pressure and has maximum in fcc phase. At higher pressures, Tc decreases in hR1 phase and increases again in cI16 phase. We did not observe Tc above 9 K for pressures above 70 GPa. Superconductivity possibly disappears in high pressure phases above 70 GPa.

Plastic deformation and optical behavior of high-purity synthetic diamond crystal subjected to high stress load at room temperature

The optical behavior around the culet of a diamond anvil made of high-purity and defect-free synthetic diamond crystal, which was plastically deformed at room temperature, was investigated by cathodoluminescence spectroscopy. It was found that the free exciton peaks weaken while the A-band and 2BD bands appear at the culet center where plastic deformation occurred. It was demonstrated that the free exciton peaks near the edge of the culet shift to the long wavelength side, indicating that the band structure of the peripheral areas of the culet changes because of residual strain caused by the plastic deformation in the culet center.