Charge density study under high pressure

The experimental and analytical method of the high-pressure powder experiment at BL10XU, SPring-8, is described. There is no doubt that BL10XU must be one of the most appropriate beam lines for high pressure X-ray diffraction experiment taking advantage of third generation synchrotron source. As an example of the advanced charge density study under high pressure, the structural change of Cs₂Au₂Br₆ by applying pressure is studied by Rietveld/MEM analysis. It reveals that the structural change of Cs₂Au₂Br₆ by applying pressure occurs basically at electron level, such as valence state change and chemical bonding, which may be called the electronic phase transition.     

Structure and high pressure behaviour of organic crystals based on aromatically substituted 1,3,4-oxadiazoles

The crystalline structure of a new compound containing the 1,3,4-oxadiazole moiety, 4-(5-methyl-1,3,4-oxadiazole-2yl-)-N,N′-dimethyl-phenylamine (MODPA) was determined. It shows a monoclinic structure with space group P2₁/c and lattice parameters: a=1.02997(6), b=0.64840(4), c=1.58117(10) nm and β=99.4820(10)°. To study the intermolecular interactions in oxadiazole containing organic crystals, X-ray studies on MODPA and 2,5-diphenyl-1,3,4-oxadiazole (DPO) were performed up to 5 GPa at room temperature. The Murnaghan equation of state is used to describe the compression behaviour of both substances. From these results, the bulk modulus and its pressure derivative were determined. The values obtained are: K₀=6.3 GPa and K′₀=6.8 for MODPA and K₀=7.3 GPa and K′₀=6.7 for DPO. Additionally, measurements under increasing temperature at ambient pressure were carried out to evaluate the thermal expansion coefficient: α=1.8×10⁻⁴ K⁻¹ for MODPA and α=1.9×10⁻⁴ K⁻¹ for DPO.     

High pressure equation of state studies using methanol-ethanol-water and argon as pressure media

We have measured the equation of state of the intermetallic compound AuIn₂ up to 20 GPa and Cd_0.8Hg_0.2 up to 50 GPa using methanol-ethanol-water solution or argon as pressure media. In the experiments performed with argon as pressure medium, we minimized non-hydrostatic conditions by thermally annealing the sample. We present data revealing compressibility anomalies in AuIn₂ at 2.7 GPa and in Cd_0.8Hg_0.2 near 8, 18 and 34 GPa with methanol-ethanol-water and argon. At pressures above 5 GPa the P-V data for AuIn₂ and Cd_0.8Hg_0.2 from experiments preformed with argon as a pressure medium start deviating from those using methanol-ethanol-water, and the equation of state based on experiments in argon is stiffer compared with that in methanol-ethanol-water. This behavior is consistent with the relative merits of the two pressure transmitting media as documented in the literature. We also provide a brief summary of the results of electronic structure calculations that associate these anomalies with electronic topological transitions.     

First-principles calculations of structural stability and mechanical properties of tungsten carbide under high pressure

The structural stability and mechanical properties of WC in WC-, MoC- and NaCl-type structures under high pressure are investigated systematically by first-principles calculations. The calculated equilibrium lattice constants at zero pressure agree well with available experimental and theoretical results. The formation enthalpy indicates that the most stable WC is in WC-type, then MoC-type finally NaCl-type. By the elastic stability criteria, it is predicted that the three structures are all mechanically stable. The elastic constants C_ij, bulk modulus B, shear modulus G, Young?s modulus E and Poisson?s ratio ν of the three structures are studied in the pressure range from 0 to 100 GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is assessed. Moreover, the elastic anisotropy of the three structures up to 100 GPa is also discussed in detail.     

Theoretical study of the structural phase transition and elastic properties of HfN under high pressures

The effect of hydrostatic pressure on the structures of HfN at 0 K was investigated by using the projector augmented wave (PAW) within the Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA). The transition pressure between NaCl (B1) and CsCl (B2) structures is predicted to be 277.3 GPa. This value is consistent with that reported by Kroll, while in contrast to the results obtained by Ojha et al. and Meenaatci et al. Moreover, the elastic properties of B1-HfN and B2-HfN under high pressures are successfully obtained. It is found that the elastic constants, bulk modulus B, shear modulus G, compressional and shear wave velocities increase monotonically with increasing pressure. The Debye temperature Θ calculated from the elastic constants of HfN is in good agreement with the experimental values. The anisotropies of B1-HfN and B2-HfN at zero pressure have also been discussed.     

Theoretical investigation on the oligothienoacenes under the influence of external electric field

Theoretical investigation on a series of oligothienoacenes has been carried out at the B3LYP/6-31G* level by considering the influence of the external electric field. With the electric field increasing, the carbon-carbon single bonds become shorter and the carbon-carbon double bonds become longer, resulting in a better conjugation. Due to the different electron density, the charge mobility of the sulfur is more obvious than that of the carbon. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap decreases with the EF intensity increasing. The applied EF also changes the spatial distribution of the molecular orbits: LUMO and several higher orbitals shift to the high potential side, whereas HOMO and several lower ones shift to the low potential side. All these features behave more pronounced with increasing conjugated chain length.     

Dynamical stability and high pressure phases of platinum carbide

Using the density-functional linear response method, we study the dynamical properties of ground state zinc-blende and high pressure NaCl phases of platinum carbide (PtC). The calculated phonon dispersion curve does not show any soft modes for all wave vectors, indicating the dynamic stability of the ground state zinc-blende phase. The high pressure rock-salt phase exhibits imaginary frequencies, practically along all directions of the Brillouin zone, which means that PtC cannot exist in the NaCl phase at least up to a high pressure of 100 GPa.     

High pressure study of structural and electronic properties of magnesium telluride

The full-potential linear muffin-tin orbital method (FP-LMTO) within the local density approximation (LDA) is used to calculate the electronic band structures and the total energies of MgTe in its stable (NiAs-B8) and high pressure phases. The latter provide us with the ground state properties such us lattice parameter, bulk modulus and its pressure derivatives. The transition pressure at which this compound undergoes the structural phase transition from the NiAs to CsCl phase is calculated. The energy band gaps and their volume and pressure dependence in the stable NiAs-B8 phase are investigated. The ground state properties, the transition pressure are found to agree with the experimental and other theoretical results. The elastic constants at equilibrium in both NiAs and CsCl structure are also determined.     

Phase transition of Ta2NiO6 with the trirutile-type structure under high pressure and high temperature

High-pressure phase transition of Ta₂NiO₆ with the trirutile-type structure was investigated from the viewpoint of crystal chemistry. A new quenchable high-pressure phase was found in the pressure range higher than 7 GPa and 900°C. The high-pressure phase has an orthorhombic cell (a=4.797(1) Å, b=5.153(2) Å and c=14.85(1) Å and space group; Abm2), and it is more dense by 9.6% than the trirutile-structured phase. Infrared spectra of the trirutile-type phase and the high-pressure phase show that Ni²⁺ ions in the high-pressure phase are still in octahedral sites. The crystal structure of the high-pressure phase is considered as a cation-ordering trifluorite-type structure, which can be stabilized by a crystal field effect of Ni²⁺ ions.     

Raman spectra for hydrogen hydrate under high pressure: Intermolecular interactions in filled ice Ic structure

Raman studies of a high-pressure structure of hydrogen hydrate, a filled ice Ic structure, were performed using a diamond anvil cell in the pressure range 3.2-44.1 GPa. The Raman spectra of a vibron revealed that extraction of hydrogen molecules from the filled ice Ic structure occurred above 20 GPa. In addition, the Raman spectra of a roton revealed that a rotation of hydrogen molecules in the filled ice Ic structure was suppressed at around 20 GPa and then the rotation recovered, and the rotation of hydrogen molecules was suppressed again above 35.5 GPa. These results indicate that intermolecular interactions increased between guest hydrogen molecules and host water molecules at around 20 and 35.5 GPa. These intermolecular interactions were considered to be induced to stabilize the filled ice Ic structure. Above 40 GPa, symmetrization of hydrogen bond was considered to contribute to the stability of hydrogen hydrate.     

Ionic to electronic dominant conductivity in Al2(WO4)3 at high pressure and high temperature

Electrical conduction and crystal structure of Al₂(WO₄)₃ at 400 °C have been studied as a function of pressure up to 5.5 GPa using impedance methods and synchrotron radiation X-ray diffraction, respectively. AC impedance spectroscopy and DC polarization measurements reveal an ionic to electronic dominant transition in electrical conductivity at a pressure as low as 0.9 GPa. Conductivity increases with pressure and reaches a maximum at 4.0 GPa, where the conductivity value is 5 orders of magnitude greater than the 1 atm value. Upon decompression, the conductivity retains the maximum value until the sample is cooled at 0.5 GPa. The high pressure-temperature X-ray diffraction results show that the lattice parameters decrease as pressure increases and the crystal structure undergoes an orthorhombic to tetragonal-like transformation at a pressure ∼3.0 GPa. The change of conduction mechanism from ionic to electronic may be explained by means of pressure-induced valence change of W⁶⁺→W⁵⁺, which results in electron transfer between W⁵⁺-W⁶⁺ sites at high pressure.     

First-principles study of the optical properties of BeO in its ambient and high-pressure phases

Optical properties such as the dynamic dielectric function, reflectance, and energy-loss function of beryllium oxide (BeO) in its ambient and high-pressure phases are reported for a wide energy range of 0-50 eV. The calculations of optical properties employ first-principles methods based on all-electron density functional theory together with sum over states and finite-field methods. Our results show subtle differences in the calculated optical properties of the wurtzite, zincblende, rocksalt and CsCl phases of BeO, which may be attributed to the higher symmetry and packing density of these phases. For the wurtzite phase, the calculated band gap of 10.4 eV corresponds well with the experimental value of 10.6 eV and the calculated (average) index of refraction of 1.70 shows excellent agreement with the experimental value of 1.72.     

Solvated fullerenes,a new class of carbon materials suitable for high-pressure studies: A review

As the list of known carbon compounds grows longer, solvated fullerenes have become a more important class of carbon materials. Their general properties and methods of synthesis have both attracted considerable attention. The study of the behavior of these compounds under high-pressure conditions has revealed several new phenomena that have never been observed in pure fullerene. This article is a review of all recent progress in this field.     

High pressure X-ray diffraction study of ReS2

The high-pressure behavior of rhenium disulfide (ReS₂) has been investigated to 51.0 GPa by in situ synchrotron X-ray diffraction in a diamond anvil cell at room temperature. The results demonstrate that the ReS₂ triclinic phase is stable up to 11.3 GPa, at which pressure the ReS₂ transforms to a new high-pressure phase, which is tentatively identified with a hexagonal lattice in space group P6?m2. The high-pressure phase is stable up to the highest pressure in this study (51.0 GPa) and not quenchable upon decompression to ambient pressure. The compressibility of the triclinic phase exhibits anisotropy, meaning that it is more compressive along interlayer directions than intralayer directions, which demonstrates the properties of the weak interlayer van der Waals interactions and the strong intralayer covalent bonds. The largest change in the unit cell angles with increasing pressures is the increase of β, which indicates a rotation of the sulfur atoms around the rhenium atoms during the compression. Fitting the experimental data of the triclinic phase to the third-order Birch-Murnaghan EOS yields a bulk modulus of K_OT=23±4 GPa with its pressure derivative K_OT′= 29±8, and the second-order yields K_OT=49±3 GPa.     

Electrical property and phase transition of CdSe under high pressure

In situ electrical resistivity measurement of CdSe was performed under high pressure and moderate temperature using a diamond anvil cell equipped with a microcircuit. With the pressure increasing, a sharp drop in resistivity of over two orders of magnitude was observed at about 2.6 GPa, it was caused by the transition to the rock-salt CdSe. After that, the resistivity decreased linearly with pressure. However, in different pressure range, the decreasing degree was obviously different. This attributed to the different electron structures. By fitting to the curve of pressure dependence of resistivity in different pressure range, the relationship of the band gap to pressure was given and the metallization pressure was speculated to be in the range of 70-100 GPa. The temperature dependence of resistivity showed that in the experimental temperature and pressure range the resistivity had a positive temperature coefficient.     

Analysis of structural properties of refractory compounds at high temperature and pressure using a potential model

In this paper we focused on the structural and elastic properties of four transition metal mononitrides (TMNs) (M=Ti, Nb, Hf and Zr) by using realistic three body interaction potential (RTBIP) model, including the role of temperature. These TMN compounds have been found to undergo NaCl (B1) to CsCl (B2) phase transition, at a pressure quite high as compared to other binary systems. We successfully obtained the phase transition pressures and volume changes at different temperatures. In addition, elastic constants of TMNs at different temperatures are discussed. The present theoretical results have been compared with the available experimental data and predictions of LDA theory.     

Structural,electronic and optical properties of LiBeP in its normal and high pressure phases

An investigation on the structural stabilities, electronic and optical properties of LiBeP under high pressure was conducted using the all-electron density functional theory within the local density approximation. Our results show that the sequence of the pressure induced phase transition of LiBeP is the Cu₂Sb-type structure (P4/nmm), the MgSrSi-type structure (Pnma) and the LiGaGe-type structure (P6₃mc). The first transition (P4/nmm to Pnma) takes place at 2.95 GPa and the second (Pnma to P6₃mc) at 6.65 GPa. In the three phases, the bandgap is indirect and the valence band maximum is at the zone center. With increasing pressure LiBeP in the LiGaGe structure becomes a direct gap semiconductor at 19.75 GPa. The assignments of the structures in the optical spectra and the band structure transitions are discussed. The mean value of the optical dielectric constant for the Cu₂Sb phase is smaller than that for the MgSrSi and the LiGaGe ones. This compound has a positive uniaxial anisotropy in the LiGaGe structure. The absorption coefficient along the z   direction, _αzz${\alpha }_{\mathit{zz}}$, for the MgSrSi structure is higher than that in the other two structures in the visible regime.     

Structural stability of tin clathrates under high pressure

High pressure Raman scattering experiments have been performed for Rb₈Sn₄₄□₂ in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization.     

Volume shrinkage dependence of ferromagnetic moment in lanthanide ferromagnets gadolinium, terbium, dysprosium, and holmium

The Gd-Ho series of lanthanide ferromagnets, which includes gadolinium (Gd), terbium (Tb), dysprosium (Dy), and holmium (Ho), undergoes similar structural transitions, e.g., the hcp→Sm-type→dhcp→fcc transitions, under pressure. Through high-field DC magnetic measurements and structural analyses, we found that the ferromagnetic moments disappeared at a specified critical pressure, which resulted in volume shrinkage of 16.7±1.7% for each ferromagnet. The results of the present study suggest that the disappearance of the ferromagnetic moments of Gd-Ho under pressure could be understood within the framework of a band picture related to volume shrinkage.     

Theoretical study of the transition metal ring functionalized tips of open-ended (5,5) boron nitride nanotube (BNNT)

The nanotube with open edges is an excellent candidate for designing efficient tip for atomistic scanning probes or field emission display (FED) devices. In the present work, we have studied the functionalization of an open-ended boron nitride nanotube (BNNT) with a series of transition metal rings and the effects on the properties of open-ended BNNT through density functional theory (DFT) calculations. The results show that the TM-BNNT complexes are energetically favorable. Moreover, it is found that the functionalization (a) significantly decreases the band gap of BNNT to different degrees, which might effectively modify the electronic properties of the open-ended BNNT; and (b) efficiently lowers the work function, which might improve the field emission properties. Our results might be helpful not only to design specific BNNT-based tips but also to further discuss the chemical vapor deposition (CVD) growth of BNNT on nanoparticles.