Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS_2

The insulator-metal transition triggered by pressure in charge transfer insulator NiS2 is investigated by combining high-pressure electrical transport,synchrotron x-ray diffraction and Raman spectroscopy measurements up to40-50 GPa.Upon compression,we show that the metallization firstly appears in the low temperature region at～3.2 GPa and then extends to room temperature at～8.0 GPa.During the insulator-metal transition,the bond length of S-S dimer extracted from the synchrotron x-ray diffraction increases with pressure,which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa.Considering the decreasing bonding-antibonding splitting due to the expansion of S-S dimer,the charge gap between the S-ppπ* band and the upper Hubbard band of Ni-3 d eg state is remarkabl.y decreased.These results consistently indicate that the elongated S-S dimer plays a predominant role in the insulator-metal transition under high pressure,even though the p-d hybridization is enhanced simultaneously,in accordance with a scenario of charge-gap-controlled type.     

Anomalous Pressure Behavior of N-Cluster Emissions in GaAs0.973Sb0.022N0.005

Photoluminescence of GaAs0.973Sb0.022N0.005 is investigated at different temperatures and pressures. Both the alloy band edge and the N-related emissions, which show different temperature and pressure dependences, are observed. The pressure coefficients obtained in the pressure range 0-1.4GPa for the band edge and N-related emissions are 67 and 45meV/GPa, respectively. The N-related emissions shift to a higher energy in the lower pressure range and then begin to redshift at about 8.5GPa. This redshift is possibly caused by the increase of the X-valley component in the N-related states with increasing pressure.     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

High-Pressure Annealing Effect on Glass Transformation Temperature of Zr41Ti14Cu12.5Ni10Be22.5 Bulk Metallic Glass

Zr41 Ti14 Cu12.5Ni10Be22.5 bulk metallic glasses （BMG） are annealed at a temperature of 603 K under ambient and high pressures in the range of 3-6 GPa. The effect of high pressure annealing on the nanocrystallization process of compressed specimens is investigated by x-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Experimental results show that the grain size of the crystalline phase decreases with the increasing pressure. For the Zr41Ti14Cu12.5Nil0Be22.5 BMG annealing at 603K in the pressure range of 0- 6 GPa, the activation energy 159.68 kJ/mol and the activation volume △V＊ =0.94 cm^3/mol are determined. The mechanism for the effects of the high pressure on the nanocrystallization process of the BMG is discussed.     

Static strength of gold compressed up to 127 GPa

Gold powder is compressed non-hydrostatically up to 127 GPa in a diamond anvil cell(DAC),and its angle dispersive X-ray diffraction patterns are recorded.The compressive strength of gold is investigated in a framework of the lattice strain theory by the line shift analysis.The result shows that the compressive strength of gold increases continuously with the pressure up to 106 GPa and reaches 2.8 GPa at the highest experimental pressure(127 GPa) achieved in our study.This result is in good agreement with our previous experimental result in a relevant pressure range.The compressive strength of gold may be the major source of the error in the equation-of-state measurement in various pressure environments.     

Anomalous behavior and phase transformation of α-GaOOH nanocrystals under static compression

The structural compression mechanism and compressibility of gallium oxyhydroxide, α-GaOOH, are investigated by in situ synchrotron radiation x-ray diffraction at pressures up to 31.0 GPa by using the diamond anvil cell technique. Theα-GaOOH sustains its orthorhombic structure when the pressure is lower than 23.8 GPa. The compression is anisotropic under hydrostatic conditions, with the a-axis being most compressible. The compression proceeds mainly by shrinkage of the void channels formed by the coordination GaO_3(OH)_3 octahedra of the crystal structure. Anomaly is found in the compression behavior to occur at 14.6GPa, which is concomitant with the equatorial distortion of the GaO_3(OH)_3 octahedra. A kink occurs at 14.6 GPa in the plot of finite strain f versus normalized stress F, indicating the change in the bulk compression behavior. The fittings of a second order Birch–Murnaghan equation of state to the P–V data in different pressure ranges result in the bulk moduli B_0= 199(1) GPa for P 14.6 GPa and B_0= 167(2) GPa for P 14.6 GPa. As the pressure is increased to about 25.8 GPa, a first-order phase transformation takes place, which is evidenced by the abrupt decrease in the unit cell volume and b and c lattice parameters.     

Phase Transition Behavior of LiCr 0.35 Mn0.65O2 under High Pressure by Electrical Conductivity Measurement

The electrical conductivity of powdered LiCr 0.35 Mn0.65O2 is measured under high pressure up to 26.22 GPa in the temperature range 300-413 K by using a diamond anvil cell. It is found that both conductivity and activation enthalpy change discontinuously at 5.36 GPa and 21.66 GPa. In the pressure range 1.10-5.36 GPa, pressure increases the activation enthalpy and reduces the carrier scattering, which finally leads to the conductivity increase. In the pressure ranges 6.32-21.66 GPa and 22.60-26.22 GPa, the activation enthalpy decreases with pressure increasing, which has a positive contribution to electrical conductivity increase. Two pressure-induced structural phase transitions are found by in-situ x-ray diffraction under high pressure, which results in the discontinuous changes of conductivity and activation enthalpy.     

Raman Investigation of BaWO4-II Phase under Hydrostatic Pressures up to 14.8GPa

The BaW04-17 phase is synthesized at 5.0 GPa and 610~C with a cubic-anvil apparatus and identified by XRD. Raman scattering measurement is carried out to investigate the phase behaviour of a pure BaW04-Ⅱ phase （space group P21/n, Z = 8） under hydrostatic pressures up to 14.8 GPa at ambient temperature. In each spectrum recorded for this phase, 27 Raman modes are observed, and all bands shift toward higher wavenumber with a pressure dependence ranging from 3.8 to 0.2 cm- 1/GPa. No pressure-driven phase transition occurs in the entire pressure range in this study. Our results indicate that the previously reported high pressure phase of Ba WO4 at pressure above about 10 GPa and room temperature （Errandonea et al. Phys. Rev. B 73（2006）224103） is not the BaW04-Ⅱ phase.     

High-Pressure Phase Transition in Cyclo-octane

Structural behaviour of cyclo-octane under high pressure is studied by using a synchrotron x-ray source in a diamond anvil cell （DAC） up to 40.2 GPa at room temperature. The cyclo-octane firstly solidifies to the triclinic phase at 0.87 GPa. With the increasing pressure, the phase of cyclo-octane changes to the tetragonal phase at about 6.0 GPa and then transforms to amorphous phase above 18.2 GPa, which is kept till to 40.2 GPa. All the phase transitions of cyclo-octane are irreversible.     

Influence of Pressure on the Annealing Process of β-Ca_2SiO_4(C_2S) in Portland Cement

Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-speciality grout. Dicalcium silicate(Ca_2SiO_4) is the primary constituent of a number of different types of cement. The β-Ca_2SiO_4 phase is metastable at room temperature and will transform into γ-Ca_2SiO_4 at 663 K. In this work, Portland cement is annealed at a temperature of 950 K under pressures in the range of 0–5.5 GPa. The high pressure experiments are carried out in an apparatus with six anvil tops. The effect of high pressure on the obtaining nano-size β-Ca_2SiO_4(C_2 S) process is investigated by x-ray diffraction and transmission electron microscopy. Experimental results show that the grain size of the C_2 S decreases with the increase of pressure. The volume fraction of the C_2 S phase increases with the pressure as the pressure is below3 GPa, and then decreases(P 3 GPa). The nano-effect is very important to the stabilization of β-Ca_2SiO_4. The mechanism for the effects of the high pressure on the annealing process of the Portland cement is also discussed.     

Effects of Temperature and Pressure on a Novel 1,3,4-Oxadiazole Derivative

The electrical resistivity variation of 1,4-bis[(4-methylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-1) microcrystal is studied under variable pressure and temperature conditions by a quasi four-probe method in a diamond anvil cell. The sample resistivity is calculated with a finite element analysis method. The temperature and pressure dependencies of resistivity of OXD-1 microcrystal are measured up to 150℃ and 15 GPa. The resistivity decrease with temperature increasing indicates that OXD-1 exhibits an organic-semiconductor transport property in the experimental pressure region. With pressure increasing, the resistivity of OXD-1 increases firstly and reaches the maximum at about 6.2 GPa, and then begins to decrease as the pressure increases continuously. In situ x-ray diffraction data under pressure provide obvious prove that the anomaly of resistivity variation at 6.2 GPa is caused by the pressure-induced amorphism of OXD-1.     

Superconductivity in Topological Semimetal θ-TaN at High Pressure

Recently,θ-TaN was proposed to be a topological semimetal with a new type of triply degenerate nodal points.Here,we report studies of pressure dependence of transport,Raman spectroscopy and synchrotron x-ray diffraction on θ-TaN up to 61 GPa.We find that θ-TaN becomes superconductive above 24.6 GPa with T_c at 3.1 K.The θ-TaN is of n-type carrier nature with carrier density about 1.1 × 10~(20)/cm~3 at 1.2 GPa and 20 K, while the carrier density increases with the pressure and saturates at about 40 GPa in the measured range.However,there is no crystal structure transition with pressure up to 39 GPa,suggesting the topological nature of the pressure induced superconductivity.     

Detailed structural,mechanical,and electronic study of five structures for CaF2 under high pressure

Detailed density functional theory(DFT)calculations of the structural,mechanical,thermodynamic,and electronicproperties of crystalline CaF2 with five different structures in the pressure range of 0 GPa–150 GPa are performed byboth GGA(generalized gradient approximation)-PBE(Perdew–Burke–Ernzerhof)and LDA(local density approximation)-CAPZ(Cambridge Serial Total Energy Package).It is found that the enthalpy differences imply that the fluorite phase→PbCl2-type phase→Ni2In-type phase transition in CaF2 occurs at PGGA1=8.0 GPa,PGGA2=111.4 GPa by usingthe XC of GGA,and PLDA1=4.5 GPa,PLDA2=101.7 GPa by LDA,respectively,which is consistent with previousexperiments and theoretical conclusions.Moreover,the enthalpy differences between PbCl2-type and Ni2In-type phases inone molecular formula become very small at the pressure of about 100 GPa,indicating the possibility of coexistence of twophase at high pressures.This may be the reason why the transition pressure of the second phase transition in other reportsis so huge(68 GPa–278 GPa).The volume changed in the second phase transition are also consistent with the enthalpydifference result.Besides,the pressure dependence of mechanical and thermodynamic properties of CaF2 is studied.Itis found that the high-pressure phase of Ni2In-type structure has better stiffness in CaF2 crystal,and the hardness of thematerial has hardly changed in the second phase transition.Finally,the electronic structure of CaF2 is also analyzed withthe change of pressure.By analyzing the band gap and density of states,the large band gap indicates the CaF2 crystal isalways an insulator at 0 GPa–150 GPa.     

Hydrostaticity of Pressure Media in Diamond Anvil Cells

Hydrostaticity under high pressure of several materials from solid, fluid to gas, which are widely used as pressure media in modern high-pressure experiments, is investigated in diamond anvil ceils. Judging from the R-line widths and R1 - R2 peak separation of Ruby fluorescence, the inert argon gas is hydrostatic up to about 30 GPa. The behavior of silicon oil is found to be similar to argon at pressures less than 10 GPa, while the widening of R-lines and increase of R1 - R2 peak separation at higher pressure loads indicate a significant degradation of hydrostaticity. Therefore silicon oil is considered as a good pressure medium at pressures less than 10 GPa but poor at higher pressures.     

Thermoelasticity of CaO from first principles

The thermoelastic properties of CaO over a wide range of pressure and temperature are studied using density functional theory in the generalized gradient approximation. The transition pressure taken from the enthalpy calculations is 66.7GPa for CaO, which accords with the experimental result very well. The athermal elastic moduli of the two phases of CaO are calculated as a function of pressure up to 200GPa. The calculated results are in excellent agreement with existing experimental data at ambient pressure and compared favourably with other pseudopotential predictions over the pressure regime studied. It is also found that the degree of the anisotropy rapidly decreases with pressure increasing in the B1 phase, whereas it strongly increases as the pressure increases in the B2 phase. The thermodynamic properties of the B1 phase of CaO are predicted using the quasi-harmonic Debye model; the heat capacity and entropy are consistent with other previous results at zero pressure.     

Radial X-Ray Diffraction Study of Static Strength of Tantalum to 80 GPa

We study the strength and texture of tantalum(Ta) under uniaxial compression up to 80 GPa using an angledispersive radial x-ray diffraction technique together with the lattice strain theory in a diamond anvil cell at ambient temperature. The ratio of differential stress to shear modulus(t/G) is found to remain constant above～60 GPa, indicating that the Ta starts to experience macro yield with plastic deformation at this pressure.Combined with independent constraints on the high-pressure shear modulus, we find that the Ta sample could support a differential stress of～4.67 GPa when it starts to yield with plastic deformation at～60 GPa under uniaxial compression. The differential stress in Ta ranges from 0.216 GPa to 4.67 GPa with pressure increasing from 1 GPa to 60 GPa and can be expressed as t-0.199(33)十 0.075(1)P, where P is the pressure in GPa. A maximum differential stress as high as～5.37 GPa can be supported by Ta at the high pressure of～80 GPa. In addition, we investigate the texture of Ta under nonhydrostatic compression to 80 GPa using the software package material analysis using diffraction. It is proven that the plastic deformation due to stress under high pressures is responsible for the development of texture.     

Pressure Driven Structural Evolutions of 0.935(Na_(0.5)Bi_(0.5))TiO_3-0.065BaTiO_3 Lead-Free Ferroelectric Single Crystal through Raman Spectroscopy

Pressure evolution of local structure and vibrational dynamics of the perovskite-type relaxor ferroelectric single crystal of 0.935(Na0.5Bi0.5)TiO3-0.065BaTiO3(NBT-6.5BT)is systematically investigated via in situ Raman spectroscopy.The pressure dependence of phonon modes up to 30GPa reveals two characteristic pressures:one is at around 4.6GPa which corresponds to the rhombohedral-to-tetragonal phase transition,showing that the pressure strongly suppresses the coupling between the off-centered A-and B-site cations;the other structural transition involving the oxygen octahedral tilt and vibration occurs at pressure～13–15GPa with certain degree of order-disorder transition,evidenced by the abnormal changes of intensity and FWHM in Raman spectrum.     

Superconducting properties of barium in three phases under high pressure from first principles

Electron-phonon coupling (EPC) in the three high-pressure phases of Ba is investigated using a pseudopotential plane-wave method based on density functional perturbation theory. The calculated values of superconducting critical temperature T c of Ba-I and Ba-II under pressure are consistent well with the trends observed experimentally. Moreover, Ba-V is found to be superconducting with a maximum T c exceeding 7.8 K at 45 GPa. With the increase of pressure, the values of T c increase in Ba I and Ba-II but the value of T c decreases in Ba-V. For Ba-I at pressures below 2 GPa, the increases of logarithmic average frequency ω log and electron-phonon coupling parameters λ both contribute to the enhancement of T c . For all the three phases at pressures above 2 GPa, T c is found to be primarily determined by λ . Further investigation reveals that for all the three phases, the change in λ with pressure can be explained mainly by change in the phonon frequency. Thus for Ba-II and Ba-V, although they exhibit completely different superconducting behaviors, their superconductivities have the same origin; the pressure dependence of T c is determined finally by the pressure dependence of phonon frequency.     

Influence of Polar Pressure Transmission Medium on the Pressure Coefficient of Excitonic Interband Transitions in Monolayer WSe_2

The influence of the pressure transmission medium(PTM)on the excitonic interband transitions in monolayer tungsten diselenide(WSe_2)is investigated using photoluminescence(PL)spectra under hydrostatic pressure up to 5GPa.Three kinds of PTMs,condensed argon(Ar),1:1 n-pentane and isopentane mixture(PM),and4:1 methanol and ethanol mixture(MEM,a PTM with polarity),are used.It is found that when either Ar or PM is used as the PTM,the PL peak of exciton related to the direct K-K interband transition shows a pressure-induced blue-shift at a rate of 32±4 or 32±1 meV/GPa,while it turns to be 50±9meV/GPa when MEM is used as the PTM.The indirect A-K interband transition presents almost no shift with increasing pressure up to approximatel.y 5 GPa when Ar and PM are used as the PTM,while it shows a red-shift at the rate of-17±7meV/GPa by using MEM as the PTM.These results reveal that the optical interband transitions of monolayer WSe_2 are very sensitive to the polarity of the PTM.The anomalous pressure coefficient obtained using the polar PTM of MEM is ascribed to the existence of hydrogen-like bonds between hydroxyl in MEM and Se atoms under hydrostatic pressure.     

In-Situ Measurement of Electrical Character of PbTe at High Pressure and High Temperature

There is a widespread interest in lead telluride (PbTe) as a good thermoelectric material. We report the temperature dependence of thermopower S(T) and resistance R(T) for PbTe at the different pressures of from 1.8GPa to 5 GPa obtained by using the cubic anvil high pressure apparatus. With increasing pressure, R(T) and S(T) decrease. The effect of pressure on R(T) is larger than that on S(T). The power factor that is determined by thermopower and resistivity increases with increasing pressure. This method is an efficient tool for synthesizing good thermoelectric materials at high pressure and high temperature.