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Monoclinic ZrO2 baddeleyite exhibits anomalous softenings of the bulk modulus and atom vibrations with compression. The pressure evolution of the structure is investigated using neutron powder diffraction combined with ab initio calculations. The results show that the anomalous pressure response of the bulk modulus is related not to the change in the bonding characters but to the deformation of an oxygen sublattice, especially one of the layers made of oxygen atoms in the crystallographic a* plane. The layer consists of two parallelograms; one is rotated with little distortion and the other is distorted with increasing pressure. The deformation of this layer lengthens one of the Zr—O distances, resulting in the softening of some atom vibrational modes.

Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure induced Dirac system as predicted by band structure calculations, has been studied by $^{31}$P-nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states $D(E)$. The temperature and pressure dependences of nuclear spin lattice relaxation rate $1/T_1$ in the semiconducting phase are well reproduced using the derived $D(E)$, and the resultant pressure dependence of semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of $1/T_1$ data with the complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of $D(E)$ and chemical potential, as well as to decompose observed $1/T_1$ into intrinsic and extrinsic contributions. An abrupt increase in $1/T_1$ at 1.63GPa indicates that the semiconducting gap closes, resulting in an enhancement of conductivity., 11 pages, 4 figures

Monoclinic ZrO

The structural stability of transition metal vanadium (V) under high pressure is investigated by using an x-ray diffraction technique combined with a laser annealing method. Although the body-centered cubic (bcc) phase of V showed rhombohedral distortion above 30–69 GPa, as reported in previous studies, with increasing pressure at 298 K, the distortion was eliminated at 58, 118, 119, and 189 GPa by laser annealing up to 2200 K; the bcc phase was observed at room temperature after annealing. These results suggested that the bcc phase is stable up to 189 GPa at room temperature, and the rhombohedral phase with a rhombohedral angle α > 109.47° of the primitive cell for the bcc lattice reported in existing studies is a metastable phase induced by non-hydrostatic pressure. Furthermore, after annealing at 242 GPa, a new high-pressure phase appeared and coexisted with the bcc phase. From different experimental runs up to 300 GPa at 298 K, the transition to the new high-pressure phase was also confirmed at approximately 200 GPa, and the high-pressure phase was explained to be a new rhombohedral phase with α

In this study, the lattice compression properties of monoclinic HfO2 (m-HfO2) at room temperature are investigated by performing high-pressure powder x-ray diffraction experiments. The lattice constants are precisely determined from the Rietveld analysis. The linear compressibility of each crystal axis a, b, and c is estimated as 10.0 × 10−3, 3.8 × 10−3, and 12.7 × 10−3/GPa, respectively, indicating a strong anisotropy. The equation of state for m-HfO2 is obtained using the third-order Birch–Murnaghan equation based on the volume and pressure data. The obtained unit cell volume (V0), bulk modulus (B0), and its pressure derivative (B0′) at ambient pressure are 138.3(1) A, 195(2) GPa, and –5.4(5), respectively. The negative value of B0′ implies the bulk modulus softening with an increase in pressure. The fact that the elastic softening has also been observed in m-ZrO2 suggests that the abnormal behavior is a characteristic of the monoclinic structure (P21/c). The transition to the orthorhombic phase is observed at approximately 6.4 GPa. The transition is sluggish, and two phases coexist at a pressure of up to 10 GPa.

The high-pressure properties of the supercritical fluid H2 have been investigated in the Raman scattering and synchrotron radiation x-ray diffraction experiments at room temperature. The pressure dependence of four vibrational modes, i.e., Q1(0), Q1(1), Q1(2), and Q1(3), and four rotational modes, i.e., S0(0), S0(1), S0(2), and S0(3), were precisely obtained, and three rotational constants under pressure, i.e., B0, D0, and H0, were estimated from theoretical formulas. A peculiar change in the pressure dependence of the Raman spectra was observed at 1–2 GPa. Through x-ray experiments, halo patterns were collected within a wide pressure range of 0.1–5 GPa, and the molar volume at each pressure was estimated from the d-value of the halo peak. The obtained pressure–volume relation suggested that the fluid H2 showed a change in compressibility at around 1 GPa and became incompressible above this pressure because the repulsive term of the intermolecular potential became dominant. The dependence followed the relational expression of P ∼ Vm−3.11 above 1 GPa, whereas fluid O2 and N2 of the same homonuclear diatomic molecule followed the relational expression of P ∼ Vm−4.32 above 0.2 GPa. It was found that the fluid H2 behaves differently from fluid O2 and N2 and is more easily compressed than those. The behavior of Vm was significantly correlated with the pressure dependence of the Raman spectra, and the peculiar change of the Raman spectra has been attributed to the enhancement of the intermolecular interaction due to the transfer to the solid-like pressure–volume relation.

In this study, the pressure-induced phase transition of the 3d transition metal titanium (Ti) has been investigated at pressures up to 290 GPa at room temperature using an x-ray diffraction technique with a micro-beam. From the analysis of powder x-ray diffraction patterns, the high-pressure δ phase (orthorhombic, Cmcm) was found to transform to the β phase (body-centered cubic: bcc, Im3m) at 243 GPa. Although the volume reduction at this phase transition was less than 1%, the coexistence of the δ phase suggests that the transition is of the first order. The isothermal bulk modulus at zero pressure, B0, and the pressure derivative of B0, B0′, were estimated to be 125(2) GPa and 3.46(6) GPa, respectively, by least squares fitting to the Vinet equation of state based on the P–V data of the δ and β phases. This transition to the bcc phase verified predictions from theoretical studies and demonstrated the systematics of the pressure-induced structural phase transition in transition metals.

Hydrodynamic flow of phonons, previously detected only in a handful of solids, is observed in black phosphorus., The travel of heat in insulators is commonly pictured as a flow of phonons scattered along their individual trajectory. In rare circumstances, momentum-conserving collision events dominate, and thermal transport becomes hydrodynamic. One of these cases, dubbed the Poiseuille flow of phonons, can occur in a temperature window just below the peak temperature of thermal conductivity. We report on a study of heat flow in bulk black phosphorus between 0.1 and 80 K. We find a thermal conductivity showing a faster than cubic temperature dependence between 5 and 12 K. Consequently, the effective phonon mean free path shows a nonmonotonic temperature dependence at the onset of the ballistic regime, with a size-dependent Knudsen minimum. These are hallmarks of Poiseuille flow previously observed in a handful of solids. Comparing the phonon dispersion in black phosphorus and silicon, we show that the phase space for normal scattering events in black phosphorus is much larger. Our results imply that the most important requirement for the emergence of Poiseuille flow is the facility of momentum exchange between acoustic phonon branches. Proximity to a structural transition can be beneficial for the emergence of this behavior in clean systems, even when they do not exceed silicon in purity.

The double carrier transport has been observed in thin film black phosphorus (BP) field effect transistor (FET) devices in highly electron doped region. BP thin films with typical thickness of 15 nm were encapsulated by hexagonal boron nitride (h-BN) thin films to avoid degradation by air exposure. Their Hall mobility has reached 5300 cm2/Vs and 5400 cm2/Vs at 4.2 K in the hole and electron doped regions, respectively. The gate voltage dependence of conductivity exhibits an anomalous shoulder structure in electron doped region. In addition, at gate voltages above the shoulder, the magnetoresistance changes to positive, and there appears an additional slow Shubnikov-de Haas oscillation. These results strongly suggest the appearance of the second carriers, which originate from the second subband with localized band edge., Comment: 16 pages, 4 figures

Transient electron-hole pairs generated in semiconductors can exhibit unconventional excitonic condensation. Anisotropy in the carrier mass is considered as the key to elongate the life time of the pairs, and hence to stabilize the condensation. Here we employ time- and angle-resolved photoemission spectroscopy to explore the dynamics of photo-generated carriers in black phosphorus. The electronic structure above the Fermi level has been successfully observed, and a massive-and-anisotropic Dirac-type dispersions are confirmed; more importantly, we directly observe that the photo-carriers generated across the direct band gap have the life time exceeding 400 ps. Our finding confirms that black phosphorus is a suitable platform for excitonic condensations, and also open an avenue for future applications in broadband mid-infrared BP-based optoelectronic devices., Comment: 13 pages, 5 figures

The structural stability of indium was investigated under high pressures up to 247 GPa using synchrotron X-ray diffraction techniques. A reentrant structural transition from a face-centered tetragonal (fct-I with c/a > 1) to a face-centered orthorhombic (fco) and further to an fct-I structure was revealed. The first transition (from fct-I to fco) and the second transition (from fco to fct-I) were observed at approximately 50 GPa and 150 GPa, respectively. The second transition was of the second order despite the first transition being of the first order. The theoretically predicted fct-II (c/a

We report on the electronic transport properties of black phosphorus and analyze them using a two-carrier model in a wide range of pressure up to 2.5 GPa. In a semiconducting state at 0.29 GPa, the remarkable nonlinear behavior in the Hall resistance is reasonably reproduced by assuming the coexistence of two kinds of holes with different densities and mobilities. On the other hand, two-carrier analyses of the magnetotransport properties above 1.01 GPa suggest the coexistence of high-mobility electron and hole carriers that have almost the same densities, i.e., nearly compensated semimetallic nature of black phosphorus. In the semimetallic state, analyses of both the two-carrier model and quantum oscillations indicate a systematic increase in the carrier densities as pressure increases. An observed sign inversion of Hall resistivity at low magnetic fields suggests the existence of high-mobility electrons $(\ensuremath{\sim}{10}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, which is roughly ten times larger than that of holes, in the semimetallic black phosphorus. We conclude that the extremely large positive magnetoresistance that has been observed in the semimetallic state cannot be reproduced by a conventional two-carrier model.

The pressure-temperature $(P\text{\ensuremath{-}}T)$ phase diagram of ${\mathrm{N}}_{2}\text{\ensuremath{-}}{\mathrm{O}}_{2}$ mixture with a composition of ${\mathrm{N}}_{2}$-48 mol % ${\mathrm{O}}_{2}$ has been investigated using x-ray diffraction and the phase stability of a hexagonal phase (space group: $P6$/mmm), with the kagome lattice examined under high-pressure and low-temperature conditions. While the phase appears as a low-temperature phase of the cubic phase $(Pm3n)$ with the structure of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{O}}_{2}$ or $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ and is stable in a wide range of pressures and temperatures, it transforms to lower symmetry monoclinic or orthorhombic phases at lower temperature, accompanied with a distortion of the kagome lattice. Based on Rietveld refinements, the monoclinic and orthorhombic phases are found to be in the $P{2}_{1}/a$ and Cmmm space groups, respectively. In magnetization measurements, a magnetic transition is observed with a relatively large drop of magnetization, corresponding to the cubic-to-hexagonal phase transition. This suggests that the hexagonal phase has a certain magnetic ordered state that arises from the molecular magnetic moment of ${\mathrm{O}}_{2}$.

We investigated molecular dissociation induced by 10-keV X-ray irradiation in dense ice at pressures up to 40 GPa at 300 K. The dissociation yield estimated from the oxygen K-edge X-ray Raman spectra, showed that the molecular dissociation was enhanced up to 14 GPa and gradually suppressed on further compression to 40 GPa. The molecular dissociation was detected for a rather narrow pressure span of 2–40 GPa by the X-ray spectroscopy. The pressure variation of the dissociation yield was similar to that observed in the electric conductivity of ice VII and likely interpreted in terms of proton mobility.

The structural phase transitions of tellurium (Te) are investigated at pressures of up to 330 GPa at 298 K using an X-ray powder diffraction technique. In the experiments, it was found that the high-pressure bcc phase (Te-V) transitioned to the fcc phase (Te-VI) at 99 GPa, although a double hcp phase (dhcp) coexisted with the fcc phase. As the pressure was increased and decreased, the dhcp phase vanished at 255 and 100 GPa, respectively. These results suggest that the dhcp phase is metastable at 298 K and the structure of the highest-pressure phase of Te is fcc. The present results provide important information regarding the high-pressure behavior of group-16 elements.

Recently, we have succeeded to extend the attainable static pressure to 410 GPa using a diamond anvil cell (DAC). In this article, our recent advances in the ultra-high pressure generation technique of the DAC are reviewed, and the key factors for the high-pressure generation beyond 300 GPa were discussed. In addition, in such extremely high-pressures range, an optical pressure determination method and a direct observation method of the stress-state by the diamond anvil Raman spectroscopy have been presented.

The first-order Raman band of diamond anvils has been investigated at pressure up to 380 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. The universality of the relationship between the sample pressure and the edge-frequency was confirmed up to 370 GPa and the usefulness of the diamond anvil Raman gauge was demonstrated. Using the diamond anvil Raman spectroscopy, the stress-state of the anvil culet was directly observed in the multimegabar pressure range. Obtained pressure dependence of the shear stress suggested further extension of feasible pressure beyond 400 GPa.

Up to now a Ni2In structure is a final step in the structural sequence of ionic AX2 compounds under high pressure. Powder X-ray diffraction experiments on BaH2 were performed at room temperature and high pressures up to 69 GPa. Successive phase transformations were observed to occur in two stages. The first was from the cotunnite to the Ni2In structure at 2.5 GPa. The second transition commenced at pressures around 50 GPa and was completed at 65 GPa. At the transition the arrangement of a cation sublattice changes from an hcp to a simple hexagonal lattice. This is the first observation of the post Ni2In phase.

Magnetoresistance in single crystals of black phosphorus is studied at ambient and hydrostatic pressures. In the semiconducting states at pressures below 0.71 GPa, the magnetoresistance shows periodic oscillations, which can be ascribed to the magneto-phonon resonance that is characteristic of high mobility semiconductors. In the metallic state above 1.64 GPa, the both transverse and longitudinal magnetoresistance show titanic increase with exhibiting superposed Shubnikov-de Haas oscillations. The observed small Fermi surfaces, high mobilities and light effective masses of carriers in semimetallic black phosphorus are comparable to those in the representative elemental semimetals of bismuth and graphite., 11 pages, 5 figures, to be published in J. Phys. Soc. Jpn

Polycrystalline bismuth sample was compressed in a diamond anvil cell and X-ray diffraction patterns from the body centered cubic phase of bismuth (bcc-Bi) recorded in the pressure range 12-222GPa. The analysis of diffraction data indicates that the factor lt;igt;Slt;/igt;=(lt;igt;Slt;/igt;lt;subgt;11lt;/subgt;-lt;igt;Slt;/igt;lt;subgt;12lt;/subgt;-lt;igt;Slt;/igt;lt;subgt;44lt;/subgt;/2) is positive, where lt;igt;Slt;/igt;lt;subgt;ijlt;/subgt; are the single-crystal elastic compliances. The data suggest lt;igt;Slt;/igt;/lt;igt;Slt;/igt;lt;subgt;11lt;/subgt;amp;ap;amp;equiv;0.5 in the entire pressure range if stress continuity across the grain boundaries is assumed, and lt;igt;Slt;/igt;/lt;igt;Slt;/igt;lt;subgt;11lt;/subgt;amp;ap;amp;equiv;0.9 if a condition halfway between the stress and strain continuities is assumed. These results are compared with the first principle calculations of the elastic moduli carried out recently. The upper bound of the uniaxial stress component (the difference between the axial and radial stress components) increases linearly from very low value at 12 to amp;ap;3GPa at 222GPa.

The ultrahigh-pressure technique in multimegabar pressures range has been developed using a diamond anvil cell (DAC) and the feasible range of static pressure is extended beyond 300 GPa. By synchrotron radiation x-ray diffraction and Raman scattering experiments on the base of this technique, high-pressure behaviors of elemental materials, that is, O2, S, Se, Te, P, Mg, Al, Ti and Sc have been investigated, and unique and/or unexpected high-pressure phenomena, such as structure phase transitions, metallization and molecular dissociation are revealed at ultra-high pressures. These results will help scientists to enrich their understanding (physical view) of materials under the extremely high-density condition.

Single-crystal X-ray experiments have been performed for calcite, calcite II, and calcite III at 300 K and pressures up to 4.1 GPa. The volume decrease from calcite to calcite II is estimated to be 0.54 cm3/mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm3/mol in the calcite to calcite II transition at 1.4 GPa reported previously based on piston-cylinder volume-compression experiments. The crystal data of calcite III have been determined using single-crystal X-ray analyses at 300 K and pressures between 2.3 and 4.1 GPa. The calcite III crystal is found to be monoclinic with the space group either C2, Cm, or C2/m from the systematic absence of reflections, and with the refined cell parameters of a = 8.707(7), b = 9.315(6), c = 6.154(4) A, and β = 115.18(2)° at 2.3 GPa. These crystal data of calcite III at 2.3 GPa yield a density of 2.94 g/cm3 for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm3 at pressures from 1.9 to 2.9 GPa, using powder X-ray analyses based on orthorhombic structural models for calcite III, but is generally consistent with the reported data by the piston-cylinder experiments. Thus the present study reveals the density of calcite III is substantially less than that of aragonite (3.02 g/cm3 at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.

An experimental system for X-ray magnetic diffraction (XMD) under extreme conditions was constructed on the beamline BL39XU at SPring-8. This system aims at studying magnetic properties of ferromagnets through the measurements of magnetic form factors under the conditions of low temperature (5 K), high magnetic field (6 T) and high pressure (10 GPa). This system consists of a superconducting magnet (SCM), a diamond anvil cell (DAC), a two-axis manipulator for the DAC, a five-axis goniometer for the SCM, and an X-ray polarizer with a phase plate. Details of this system are presented. Experimental results on uranium telluride are shown as a performance test with this instrumentation.

High-pressure Raman scattering experiments of O2 and N2 were carried out at 296 K and at pressures up to 10 and 6 GPa, respectively, and the pressure dependence of vibrational spectra of O2 was studied on the basis of molecular interactions in comparison with N2. An appreciable contribution of the attractive interaction to the vibrational frequency was observed for fluid O2. The linewidth of the spectra for fluid O2 showed a sharp increase with increasing pressure, and the fluid–solid transition was observed at 5.9 GPa with a sudden drop of the linewidth.

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.

UV absorption spectra of molecular oxygen were measured at pressure up to 1.5 GPa and temperature between 17 and 297 K. In the supercritical phase, an absorption band was observed in a UV region between 4.5 and 7 eV and showed an appreciable enhancement in the intensity with pressure. The absorption cross-section, σ eff , of the UV band at 1.5 GPa, estimated from the molar volume obtained by present X-ray diffraction experiments, reached three orders of magnitude of previous one measured at 1.0 MPa. The observed UV absorption of high-density oxygen was attributed to the two-molecule process by an antiferromagnetic O 2 pair.

Samples of silver and gold have been compressed together in a diamond anvil cell up to ∼150 GPa and the x-ray diffraction patterns recorded using a synchrotron radiation source. The parameters αt for silver and gold, where α is a factor that determines the relative weights of iso-stress and iso-strain conditions and t is the difference between the axial and radial stress components, are estimated from the analysis of the diffraction data. The αt-values are small and increase with pressure reaching 2 GPa and 0.5 GPa at 150 GPa for silver and gold, respectively. The effects of varying α between 1 and 0.5 on the hydrostatic volume compression derived from the measured data and the pressures computed therefrom are discussed. The compression data for silver and gold, corrected for nonhydrostatic compression effects assuming iso-stress condition (α=1), fit the relation (V/V0)Ag=1.5946(V/V0)Au3−3.0024(V/V0)Au2+2.8285(V/V0)Au−0.4207. The pressures computed from (V/V0)Ag and (V/V0)Au using equations of state from ...

Powder X-ray diffraction experiments of Re–hydrogen system were performed at high pressures and room temperature. Superstructure reflection lines, which were produced by the displacements of rhenium atoms from the hcp lattice points, were observed. This means that hydrogen is incorporated in every second layer along the c-axis of an hcp lattice, which results in an anti-CdI 2 type structure. The displacements of rhenium atoms were determined from the relative intensities of these reflection lines.

Powder x-ray diffraction experiments on a low-Z element, Be, were performed at pressures up to 171 GPa using the synchrotron radiation source at SPring-8. All profiles were assigned to the hcp structure of the ambient phase, and the theoretically predicted hcp–bcc phase transition was not observed. The previously proposed hcp–hexagonal phase transition around 14.5 GPa was also not confirmed. The equation of state of hcp-Be was determined by a least mean squares fitting to the Birch–Murnaghan equation from these diffraction data. Calculated values of K0 and K0 ' were 97.2 ± 2.5 GPa and 3.61 ± 0.07, respectively.

Structural phase transitions of titanium (Ti) have been investigated at pressures up to 220 GPa at room temperature using a monochromatic synchrotron x-ray diffraction technique. At 140 GPa, the hexagonal phase (ω-Ti) was transformed into an orthorhombic phase (δ-Ti) with a distorted bcc structure via an intermediate phase (γ-Ti), which has recently been proposed. The δ-Ti and γ-Ti phases had a unique zigzag-chain-like structure, which resulted from an orthorhombic distortion.

The structure and lattice compression of the β-O2 phase have been studied by synchrotron x-ray diffraction experiments at 277 and 300 K. The pressure dependence of the lattice constants, a and c, obtained from a powder indicated anisotropic compressibility. a is almost three times as compressible as c. This large linear compressibility of a may be attributed to the antiferromagnetic interaction in the basal plane. Single-crystal analyses at 6.1 GPa and 300 K suggested that the oxygen atom is disordered with a threefold axis of symmetry.

Kohei Nishiyama1, Takeshi Mito1, Gabriel Pristas1,2, Yukiko Hara1, Takehide Koyama1, Koichi Ueda1, Takao Kohara1, Yuichi Akahama1, Slavomir Gabani2, Marian Reiffers2, Karol Flachbart2, Hideto Fukazawa3, Yoh Kohori3, Nao Takeshita4, and Natalia Shitsevalova5 1Graduate School of Material Science, University of Hyogo, Hyogo 678-1297, Japan 2Institute of Experimental Physics, Slovak Academy of Science, 04001 Kosice, Slovakia 3Graduate School of Science, Chiba University, Chiba 263-8522, Japan 4National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8562, Japan 5Institute for Problem of Material Science, National Academy of Science of Ukraine, 03680 Kiev, Ukraine E-mail: mito@sci.u-hyogo.ac.jp

X-ray diffraction and magnetic-susceptibility measurements have been carried out for single phase KxC70 (x=1, 3, 4, 6 and 9) compounds synthesized by heating stoichiometric amount of K9C70 and C70. The x-ray diffraction profiles show no structural transition down to 10K. The fairly large temperature-independent paramagnetic contribution was observed in x=3 and 4. The electrical resistivity has been measured for K evaporated C70 film with increasing K thickness. Two resistivity minima were observed at x=1 and 4.