Metallization of fluid hydrogen at 140 GPa (1.4Mbar) by shock compression

Abstract

The minimum electrical conductivity of a metal was produced in dense hydrogen using shock compression. Metallization occurs at 140 GPa (1.4 Mbar), 0.6 g/cm³ (ninefold compression of initial liquid-H₂ density), and 3000 K. The relatively modest temperature generated by a reverberating shock wave produced the metallic state in a warm quantum fluid at a lower pressure than expected previously for the crystallographically ordered solid at low temperatures. Future research directions are discussed. Possible scientific and technological uses of metastable solid metallic hydrogen are speculated upon in the unlikely event that the metallic fluid can be quenched to this state at ambient pressure and temperature.     

High-pressure phase transitions in UP1-xSx

Abstract

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP_1-x -S_x (X=0.1; 0.25; 0.4) up to 53 GPa UP_1-x S_x is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP_1-x S_x with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP^1-x S_x 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur.     

The hydrogen‐bond effect on the high pressure behavior of hydrazinium monochloride

The first high pressure study of solid hydrazinium monochloride has been performed by in situ Raman spectroscopy and synchrotron X‐ray diffraction (XRD) experiments in diamond anvil cell (DAC) up to 39.5 and 24.6 GPa, respectively. The structure of phase I at room temperature is confirmed to be space group C2/c by the Raman spectral analysis and Rietveld refinement of the XRD pattern. A structural transition from phase I to II is observed at 7.3 GPa. Pressure‐induced position variation of hydrogen atoms in NH₃⁺ unit during the phase transition is attributed to the formation of N―H…Cl hydrogen‐bonds, which play a vital role in the stability and subsequent structural changes of this high energetic material under pressure. This inference is proved from the abnormal pressure shifts and obvious Fermi resonance in NH stretching mode of N₂H₅⁺ ion in the Raman experiment. Finally, a further transition from phase II to III accompanied with a slight internal distortion in the N₂H₅⁺ ions occurs above 19.8 GPa, and phase III persists up to 39.5 GPa. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermoelectric properties of p-Bi2 ? xSbxTe3 solid solutions under pressure

This paper reports on a study of the Seebeck coefficient and power factor κ of p-Bi_{2 − x} Sb _x Te₃ solid solutions with different contents of antimony atoms in the bismuth sublattice for x = 0, 1.4, 1.5, and 1.6 under variation of pressure of up to 15 GPa. The magnitude of κ has been found to grow nonmonotonically within the pressure region of 2–4 GPa. The effective mass of the density of states m/m ₀ and the mobility μ₀ have been calculated with due account of degeneracy within the parabolic model of the energy spectrum assuming isotropic charge carrier scattering. It has been shown that application of pressure brings about a decrease of the effective mass m/m ₀ and an increase of carrier mobility. The power factor κ of the p-Bi_0.6Sb_1.4Te₃ composition exhibits at the pressure P ≈ 4 GPa the largest increase of the power factor κ as a result of a weak decrease of the effective mass m/m ₀ and an increase of carrier mobility as compared to the other solid solution compositions. The specific feature of the variation of the power factor κ with a change of the pressure in bismuth telluride near P ≈ 3 GPa, which is accompanied by formation of a knee in the m/m ₀ vs. P dependence, can be assigned to an electronic topological transition.     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

First principles calculations on the effect of pressure on SiH4(H2)2

The effect of pressure on the strength of H₂ covalent bond in the molecular solid SiH₄(H₂)₂ has been investigated using quantum molecular dynamics simulations and charge density analysis. Our calculations show, in agreement with the implications of the experimental results, that substantial elongation of H₂ bond can be achieved at low pressures, with the onset of rapid changes close to 40 GPa. Model calculations show redistribution of charge from bonding to antibonding states to be responsible for the behavior. Our computed Raman spectra confirm the dynamic exchange of hydrogen atoms speculated to be operative in SiH₄–D₂ mixture by experiments. This exchange is shown to be a three step process driven by thermal fluctuations.     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

Pressure-induced phase transitions in gypsum

Abstract

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO₄ · 2H₂O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO₄ · 2H₂O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure.

Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.     

Dissociation and dissociative phase transition in dense hydrogen

A simple physical model is proposed for dissociating dense fluid hydrogen. We propose that free dissociated atoms interact via quantum electron-electron exchange analogously to the interaction in the liquid-metal phase of alkali metals. The density dependence of a hydrogen atom’s binding energy in such a quasi-liquid is calculated. It is shown that the transition from the molecular fluid to liquid hydrogen is a first-order phase transition. The critical parameters of the transition are determined: P _c = 72 GPa, T _c = 10500 K, and ρ _c = 0.5 g/cm³. The possibility of the metastable existence of atomic liquid hydrogen in a dissociated molecular fluid under decreased pressure is established.     

High pressure X-ray diffraction study of copper hydride at room temperature

Abstract

High pressure X-ray studies on CuH up to 23 GPa have been performed at room temperature using a gasketed diamond anvil cell. The experimental data on the molar volume of CuH as a function of pressure have been fitted to Murnaghan's equation of state giving a bulk modulus: B₀ = 72.5±2 GPa and B₀ = 2.7 ± 0.3. By comparison with the equation of state for pure copper the effective additive volume of hydrogen has been evaluated as a function of pressure. It decreases from 3.2 cm³/mol H, at ambient pressure reaching a flattening value of 1.7cm³hol H at about 60 GPa. This suggests a continuous transition of CuH from ionic or covalent character at normal pressure to metallic hydride behavior at high pressure     

High‐pressure Brillouin study of the elastic properties of rare‐gas solid xenon at pressures up to 45 GPa

The pressure dependences of three adiabatic elastic constants, adiabatic bulk modulus, refractive index, and elastic anisotropy, as well as Cauchy deviation of fcc solid Xe have been determined up to 10 GPa at 296 K by high‐pressure Brillouin scattering spectroscopy. The characteristics of elastic properties at high pressure of rare‐gas solid Xe are investigated by comparison with the previous studies on Ne, Ar, and Kr. Above 10 GPa, the occurrence of splitting in the Brillouin signals and the direction dependence of acoustic velocities for solid Xe clearly show partial phase transformation to the hcp structure reported by the previous X‐ray diffraction and Raman scattering studies. The shear elastic modulus in the hcp phase of solid Xe has also been estimated at pressures up to 45 GPa by using the pressure dependence of the Raman wavenumber shift for the E_2g mode. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

High volumetric hydrogen density phases of magnesium borohydride at high-pressure: A first-principles study

The previously proposed theoretical and experimental structures,bond characterization,and compressibility of Mg(BH 4) 2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations.It is found that the ambient pressure phases of meta-stable I4 1 /amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure.Enthalpy calculation indicates that the ground state of F 222 structure proposed by Zhou et al.[2009 Phys.Rev.B 79 212102] will transfer to I4 1 /amd at 0.7 GPa,and then to a P-3m1 structure at 6.3 GPa.The experimental P 6 1 22 structure(α-phase) transfers to I4 1 /amd at 1.2 GPa.Furthermore,both I4 1 /amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure.Their theoretical volumetric hydrogen densities reach 146.351 g H 2 /L and 134.028 g H 2 /L at ambient pressure,respectively.The calculated phonon dispersion curve shows that the I4 1 /amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa.So the I4 1 /amd phase may be synthesized under high pressure and retained to ambient pressure.Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range.In addition,they each have an anisotropic compressibility.The c axis of these structures is easy to compress.Especially,the c axis and volume of P-3m1 phase are extraordinarily compressible,showing that compression along the c axis can increase the volumetric hydrogen content for both I4 1 /amd and P-3m1 structures.     

Phase relations in the Fe-P system at high pressures and temperatures from ab initio computations

ABSTRACT

Based on the first-principles calculations within the density functional theory and crystal structure prediction algorithms iron phosphide phases stable under pressure of the Earth’s core and temperatures up to 4000?K were determined. A new low-temperature modification FeP-P2₁/c stable above ～75?GPa was predicted. Fe₂P with the allabogdanite structure has been established to be stable in the low-temperature region at ambient conditions. At 750?K it transforms into the barringerite structure. The transition from Fe₃P with schreibersite structure to Fe₃P-Cmcm was observed at 27?GPa, and the phase transition boundary is nearly isobaric. Fe₂P and FeP are thermodynamically stable at the Earth’s inner core pressures and 0?K according to the obtained results, whereas Fe₃P stabilizes with respect to decomposition to Fe?+?Fe₂P at high temperatures above ～3200?K.     

Pressure dependence of the electron density in solid iodine by the maximum-entropy method

Abstract

A direct observation of the electron density of solid iodine has been attempted in order to study the electron-density delocalization process due to pressure-induced metallization. A high-accuracy x-ray powder diffraction measurement was carried out with a diamond anvil cell and an imaging plate on a synchrotron-radiation source. The maximum entropy method was employed to analyze the data and to obtain electron-density maps under pressures up to 20 GPa. The electron density between adjacent iodine molecules has been shown to gradually increase with increasing pressure; also, a two-dimensional network is formed at a density level of 0.2 e/ų at around 16 GPa.     

Pressure effect on structural,electronic optical and thermodynamic properties of cubic AlxIn1-xP: a first-principles study

ABSTRACT

First-principles total energy calculations have been performed using the full potential linearised augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code based on the density functional theory (DFT) to investigate the Al-doping effects on the structural, electronic and optical properties of Al_xIn_1-xP ternary alloys in the zinc-blende (ZB) phase. Different approximations of exchange-correlations energy were used such as the local density approximation (LDA), the generalised gradient approximation within parameterisation of Perdew–Burke–Ernzerhof (PBE-GGA), and the Wu-Cohen (WC-GGA). In addition, we have calculated the band structures with high accuracy using the Tran-Blaha modified Becke–Johnson (TB-mBJ) approach. The pressure dependence of the electronic and optical properties of binary AlP, InP compounds and their related ternary alloys Al_xIn_1-xP were also investigated under hydrostatic pressure for (P?=?0.0, 5.0,10.0, 15.0, 20.0, 25.0?GPa), where it is found that InP compound change from direct to indirect band gap for P?≥?9.16?GPa. Furthermore, we have calculated the thermodynamic properties of InP and AlP binary compounds as well as the Al_xIn_1-xP solid solutions, where the quasi-harmonic Debye model has been employed to predict the pressure and temperature dependent Gibbs free energy, heat capacity, Debye temperature and entropy.     

Electroresistance of some semiconductors in the phase transition region at high pressure

Abstract

In apparata of liquid type piston-cylinder up to 2GPa and toroid with solid anvil up to 25 GPa pressure (P) electrial resistance R(P) in sphere of phase transition (PT) CdSnAs₂,SnTeInP,GaAs is investigated, and GaP at T = 300K, and also R(T,P) VTSP YB₂Cu₃0_6+x in sphere of superconductive transition. In order to describe the substance behaviour in the vicinity of PT, that is, the region of gapped change of R the approximation geterophased structure is used - the effective environment (model GSEE) which checks different configuration of phases inserts and their dependence from P. In order to picture the solid substance behaviour in the vicinity of point PT at high pressure (HP) different methods are used [1–8].     

DFT study on the effect of relative positions of methyl-, nitro- and N→oxide groups on the molecular structure,thermal/kinetic stability,crystal density,heat of decomposition and performance characteristics of triazolones

Methyl-, nitro- and N→oxide substituted triazolones are of interest in the contest of high-energy density compounds and have been found to have true local energy minima at the B3LYP/aug-cc-pVDZ level. The optimised structures, harmonic frequencies and thermodynamic values for all the model molecules have been obtained in their ground state. The velocity of detonation (D) and detonation pressure (P) have been evaluated by the Kamlet–Jacob equations using the crystal density and the heat of explosion. The estimated performance properties are higher (D = 9.92–10.27 km/s, P = 48.10–52.52 GPa) compared with 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (D = 9.20 km/s, P = 42.0 Gpa). The higher densities are possibly due to the intramolecular hydrogen bonds and the layered structures in the crystal lattice. We speculate that the calculated heat of explosion and the density are for the gas phase compounds and in the reality they should be for the solid phase which would diminish the magnitude of the calculated values. The –N→O and –NO₂ group leads to the desirable consequences of higher heat of explosion and diminished sensitivities. The substituting of N–H hydrogen atom(s) of triazolones for a –CH₃ group decreases melting point, heat of formation and density; however, the methyl group increases the thermal stability.     

Evidence of maximum in the melting curve of hydrogen at megabar pressures

Hydrogen at high pressures of ∼400 GPa might be in a zero-temperature liquid ground state (N. Ashcroft, J. Phys.: Condens. Matter A 12, 129 (2000), E. G. Brovrnan et al., Sov. Phys. JETP 35, 783 (1972)). If metallic hydrogen is liquid, the melting T _melt(P) line should possess a maximum. Here we report on the experimental evaluation of the melting curve of hydrogen in the megabar pressure range. The melting curve of hydrogen has been shown to reach a maximum with T _melt = 1050 ± 60 K at P = 106 GPa and the melting temperature of hydrogen decreases at higher pressures so that T _melt = 880 ± 50 K at P = 146 GPa. The data were acquired with the aid of a laser heating technique where diamond anvils were not deteriorated by the hot hydrogen. Our experimental observations are in agreement with the theoretical prediction of unusual behavior of the melted hydrogen [S. Bonev et al., Nature 481, 669 (2004)]. The article is published in the original.     

立方(Ba0.5Sr0.5)TiO3高压诱导带隙变化的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算研究了 (Ba_0.5Sr_0.5)TiO₃ (BST) 晶体在高压下的电子结构及能带变化行为. 研究结果发现,随着压强的增加,BST能带间隙先增加,在压强为55 GPa时达到最大值,然后减小,这些有趣的结果将有助于开发与设计新的BST铁电器件. 进一步地,通过电子态密度和密度分布图的研究分析可知:在低压区域(055 GPa),则是出现的离域现象占主导(电子的离域作用超过键态的作用),从而使带隙减小. 关键词： 钛酸锶钡 第一性原理 高压 能带间隙