Conductive dense hydrogen

Molecular hydrogen is expected to exhibit metallic properties under megabar pressures. This metal is predicted to be superconducting with a very high critical temperature, T(c), of 200-400 K, and it may acquire a new quantum state as a metallic superfluid and a superconducting superfluid. It may potentially be recovered metastably at ambient pressures. However, experiments carried out at low temperatures, T<100 K, showed that at record pressures of 300 GPa, hydrogen remains in the molecular insulating state. Here we report on the transformation of normal molecular hydrogen at room temperature (295 K) to a conductive and metallic state. At 200 GPa the Raman frequency of the molecular vibron strongly decreased and the spectral width increased, evidencing a strong interaction between molecules. Deuterium behaved similarly. Above 220 GPa, hydrogen became opaque and electrically conductive. At 260-270 GPa, hydrogen transformed into a metal as the conductance of hydrogen sharply increased and changed little on further pressurizing up to 300 GPa or cooling to at least 30 K; and the sample reflected light well. The metallic phase transformed back at 295 K into molecular hydrogen at 200 GPa. This significant hysteresis indicates that the transformation of molecular hydrogen into a metal is accompanied by a first-order structural transition presumably into a monatomic liquid state. Our findings open an avenue for detailed and comprehensive studies of metallic hydrogen.     

Characterization and investigation of the mesogenic, thermo-morphologic and thermotropic properties of new chiral (S)-5-octyloxy-2-[{4-(2-methylbuthoxy)-phenylimino(methyl]phenol liquid crystalline compound

Synthesis, characterization and investigation of the mesogenic, thermo-morphologic and thermotropic properties of a new chiral liquid crystalline compound are presented in this work. This new compound has prolate molecules and exhibits the chiral smectic C* mesophase in a sufficiently large temperature interval. Two types of solid crystalline phases have been found in this compound. Typical textures and temperatures of the direct and reverse phase transitions, taking place in the compound, are given. Typical peculiarities for the first-order transition between the smectic C* mesophase and isotropic liquid have been observed.     

On the Order–Disorder Surface Phase Transition and Critical Temperature of Pure Metals Originating from BCC, FCC, and HCP Crystal Structures

The excess surface Gibbs energy and surface tension of pure liquid metals (originating from bcc, fcc, and hcp solid metals) of ordered and disordered surface structures are compared in this paper. It is shown that at a special temperature T ^* an order–disorder surface phase transition is expected in all liquid metals from a low-temperature ordered surface state to a high-temperature disordered surface state. This surface phase transition is similar to the first-order bulk solid–liquid phase transition (melting). The values of T ^* appear in the temperature interval between the melting point and the critical point of metals. Critical temperatures of metals are estimated from the equation for high-temperature disordered surfaces.     

Hydrogen under extreme conditions

Some recent advances in our understanding of the properties of hydrogen and deuterium at megabar pressures are reviewed. The emphasis is on recent spectroscopic experiments to elucidate the nature of the H-A phase (Phase III), the high-pressure phase diagram, and on the reported generation of liquid metallic hydrogen under shock compression conditions. Density Functional and Quantum-Monte Carlo calculations have proved a useful guide to interpreting the experimental results: these will also be reviewed.     

Charged Surface of Liquid Hydrogen at Near Zero Gravitation

The results of investigations of the evolution of the shape of the equipotentially charged surface of a liquid hydrogen layer in the external electric field in the case when the effective gravitation acceleration is near to zero are presented. The reconstruction phenomenon—the formation of a solitary wave at the flat charged surface of liquid hydrogen at some critical value of the external electric field—has been studied under the conditions of a total screening of the electric field in the bulk of a liquid by a surface charge. From the results obtained it follows that the transition of the flat surface into the reconstructed state is close to a second order phase transition. A voltage-temperature U–T phase diagram of the charged surface has been determined. The statement of a soft character of the transition has been confirmed also by the results of studies of the oscillations spectrum of the charged surface in electric fields lower and higher the critical value.     

Experimental study of phase transitions in mercury

The results of sound velocity measurements in mercury, performed at temperatures from 300 up to 2(150 K and pressures from 30 up to 1900 bar by a precise pulsed phase-sensitive technique for a frequency of 10 MHz, are presented. The explored range of state parameters includes liquid and gaseous phases, the coexistence curve up to the critical point, and the supercritical region. The data obtained indicate the existence of two first-order phase transitions in mercury that take place in the vapor near saturation and in the supercritical fluid. The positions of the critical points of these transitions were estimated. An interpretation of the observed phenomena is given: It leads to the new approach to the nature of the critical point of liquid-gas transition in mercury. It is shown also that the fourth derivative of the thermodynamic potential of mercury has a special feature in the metal-nonmetal transition region.Invited paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

A new constant-pressure ab initio/classical molecular dynamics method: simulation of pressure-induced amorphization in a Si35H36 cluster

We present here a new constant-pressure ab initio molecular dynamics method, suitable, e.g., for studying pressure-induced structural transformations in finite non-periodic systems such as clusters. In order to apply external isotropic pressure on the cluster, we immerse an ab initio treated cluster in a model classical liquid, described by a repulsive soft-sphere potential, which acts as a pressure reservoir. The extended system cluster + liquid is simulated by a coupled Car–Parrinello and classical molecular dynamics. The pressure is varied by tuning the parameter of the liquid potential. We apply the method to a Si₃₅H₃₆ cluster, which undergoes a pressure-induced amorphization at 35 GPa, and remains in a disordered state even upon pressure release. The properties of cluster at different pressures are analyzed by means of maximally localized Wannier functions method. In the high-pressure phase, a considerable reduction of the Kohn–Sham energy gap as well as an increase of electronic delocalization is observed, which represents an analogue of metallization of bulk Si upon transition from diamond to β-tin phase.     

Amorphous silicon exhibits a glass transition

Amorphous silicon is a semiconductor with a lower density than the metallic silicon liquid. It is widely believed that the amorphous-liquid transition is a first-order melting transition. In contrast to this, recent computer simulations and the experimental observation of pressure-induced amorphization of nanoporous silicon have revived the idea of an underlying liquid-liquid phase transition implying the existence of a low-density liquid and its glass transition to the amorphous solid. Here we demonstrate that during irradiation with high-energy heavy ions amorphous silicon deforms plastically in the same way as conventional glasses. This behaviour provides experimental evidence for the existence of the low-density liquid. The glass transition temperature for a timescale of 10 picoseconds is estimated to be about 1,000 K. Our results support the idea of liquid polymorphism as a general phenomenon in tetrahedral networks.     

Magnetic properties and first-order magnetic phase transition in single crystal FeRh thin film

To understand the mechanism of first-order magnetic phase transition in ordered FeRh thin films, the magnetic properties and first-order antiferromagnetic (AFM)–ferromagnetic (FM) phase transition behavior of single-crystal FeRh thin film are investigated in detail. The first-order magnetic phase transition is seen at a temperature of around 120 °C during heating and 145 °C cooling processes in perpendicular direction. The M–H loops measured isothermally amidst the AFM–FM transition regime show an opening at high magnetic field, which indicate a reversible AFM–FM transition induced by magnetic field. The clusters of the FM phase nucleate in the AFM matrix heterogeneously and vice versa during the first-order phase transition and the mechanism of nucleation and growth kinetics of the first-order magnetic phase transition in ordered FeRh thin film is quite similar to that of the crystallization of solids described by the Avrami model.     

Preparation of metallic glass powders by hydrogen charging

Metallic glass powders were produced by exposing metallic glass alloy ribbons to hydrogen at pressures of 12–15 MPa. A list of powders prepared by this method was given. Some metallic glass powders, such as CuTi, were difficult to remove hydrogen for them, indicating strongly bound hydrogen in them.     

Perspective on Structural Evolution and Relations with Thermophysical Properties of Metallic Liquids

The relationship between the structural evolution and properties of metallic liquids is a long‐standing hot issue in condensed‐matter physics and materials science. Here, recent progress is reviewed in several fundamental aspects of metallic liquids, including the methods to study their atomic structures, liquid–liquid transition, physical properties, fragility, and their correlations with local structures, together with potential applications of liquid metals at room temperature. Involved with more experimentally and theoretically advanced techniques, these studies provide more in‐depth understanding of the structure–property relationship of metallic liquids and promote the design of new metallic materials with superior properties.     

Photopyroelectric Determination of the Order of Liquid Crystal Phase Transitions

The photopyroelectric technique has been used to measure simultaneously the specific heat, the thermal conductivity, and the thermal diffusivity of 9CB liquid crystal in the temperature range 35 to 60°C, where the sample undergoes a weakly first-order phase transition and a second-order one. Measurements of the anisotropy of the thermal conductivity have also been performed, and the data have been used to establish the order of the above-mentioned phase transitions. Pretransitional effects in the isotropic phase in the thermal diffusivity have been found, and they have been associated with similar effects reported for the specific heat.     

Rotational molecular motion in solid H2 and D2 under pressure

The rotational state of a H₂ or D₂ molecule in solid hydrogen or deuterium is calculated over the range of densities for which the molecular phases are expected to be stable. For para-H₂ and ortho-D₂ we predict sudden transitions as the pressure is increased from states where the molecules are rotating freely to states where they are librating about equilibrium orientations. These transitions are predicted to occur in the region of several hundred megabars of pressure. Although in principle observable, these first-order transitions are expected to lead to only small changes in the equation-of-state curves. These transitions from rotating to librating molecular motion are expected to be accompanied by changes in the crystal structures from hcp to fcc.Supported by NSF Grant No. 22553.     

Record‐High Superconductivity in Niobium–Titanium Alloy

The extraordinary superconductivity has been observed in a pressurized commercial niobium–titanium alloy. Its zero‐resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record‐high pressure up to which a known superconducting state can continuously survive. Remarkably, at such an ultra‐high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (T_C) increases to ≈19.1 K from ≈9.6 K, and the critical magnetic field (H_C2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both the T_C and the H_C2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high‐pressure superconducting properties observed in the niobium–titanium alloy not only expand the knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.     

Amorphous Vortex Phase in Bi2Sr2CaCu2O8 After the First Order Liquid-Solid Phase Transition

It is widely accepted that the first-order vortex liquid-solid phase transition is associated with a crystalline solid phase and the second order transition with an amorphous one. The combination of a technique that determines the order of the transition with the visualization of the vortex structure has allowed the detection, for the first time, of a first-order liquid-solid transition without structural symmetry change. The results show that the quasi-long range order of the solid phase is not a necessary condition for the first-order phase transition to occur. This opens an important question on the microscopic origin of the liquid-solid phase transition in vortex matter.     

金属氢研究进展

综述１９８８年以来关于金属氢研究的重要实验与理论成果，研究动向及讨论了问题，主要涉及Ｈ－Ａ相的发现与讨论，Ｈ－Ａ相附近区域的相图，高压下固体氢的结构转变及氢分子的内产中状态的变化。     

<Emphasis Type="Italic">T</Emphasis>–<Emphasis Type="Italic">P</Emphasis> Phase Diagram of Nitrogen at High Pressures

By employing a mean field model, calculation of the T–P phase diagram of molecular nitrogen is performed at high pressures up to 200 GPa. Experimental data from the literature are used to fit a quadratic function in T and P, describing the phase line equations which have been derived using the mean field model studied here for N₂, and the fitted parameters are determined. Our model study gives that the observed T–P phase diagram can be described satisfactorily for the first-order transitions between the phases at low as well as high pressures in nitrogen. Some thermodynamic quantities can also be predicted as functions of temperature and pressure from the mean field model studied here and they can be compared with the experimental data.     

Phase Diagram of Hydrogen in Palladium

Hydrogen in palladium, Pd-H(D), is an interesting system because of the highly mobile hydrogen and the presence of a phase boundary below 100 K. Experimentally, however, the nature of this transition has not been established. Historically this transition around 55 to 100 K has been thought to be an order-disorder transition. Such a transition would produce a phase boundary with anomalies at specific hydrogen concentrations corresponding to the specific ordered structures. In order to check this phase boundary we have performed a detailed study of the hydrogen concentration dependence of the specific heat of PdH _x over the temperature range from below 0.5 K to above 100 K using PdH _x specimens with x up to 0.8753. The measured heat capacity has been analyzed as the sum of contributions due to the lattice specific heat of Pd, the electronic specific heat of PdH _x , and the excess contribution caused by hydrogenation of the specimen. The excess specific heat result shows a sharp peak which indicates a phase boundary with transition temperature T ₁=55 K to 85 K depending linearly on the hydrogen concentration from x=0.6572 to 0.8753. We do not observe anomalies at specific x values as would be expected for the specific ordered structures.     

A new apparatus for the measurement of the isobaric heat capacity of liquid refrigerants

A new automated adiabatic flow calorimeter was developed which enables one to measure the isobaric heat capacity, C _p, of pure fluids and their mixtures in the liquid phase. The calorimeter has been carefully designed to keep the heat loss from the sample fluid as small as possible being regarded as negligible. The experimental apparatus constitutes a closed circuit of the sample circulation using a combination of two mounted metallic bellows and a metering pump. The present apparatus is designed to measure C _p at temperatures to 500 K and pressures to 15 MPa and is also applicable to measurements in the critical region as well as the region near the saturated liquid state because of its excellent mass flow rate control stability and the high adiabatic efficiency of the calorimeter. The C _p of liquid refrigerant 114 (R114) has been measured at temperatures from 275 to 415 K and pressures up to 3.2 MPa including the critical region with experimental uncertainty of less than ±0.4%. The heat capacity of saturated liquid R114 has also been derived from the data measured in the single phase.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Solid-solid transition temperatures in SrCl2 from differential thermal analyses to 0.6 GPa

The reversible phase transition between the high-temperature, cubic C1 and the low-temperature, orthorhombic C23 polymorphs of SrCl₂ has been investigated by differential thermal analysis under hydrostatic pressure to 0.63 GPa. The C23→C1 transition temperature varies linearly with pressure at the rate of 0.42₄ μK(Pa)^?1 from the highest pressures down to ca. 0.34 GPa, and also linearly with slope 1.73 μK(Pa)^?1 at pressures ?0.26 GPa; the reverse, C1→C23 transition is not observed or deduced ?0.21 GPa. The observed curvature for the C1–C23 phase boundary over the range ?0.26–0.34 GPa, 1000–1050 K can be attributed to intersection with the “diffuse” transition in C1; the latter transition, however, could not be observed unambiguously. Linear extrapolation to 0.1 MPa places the C23→C1 transition near 553 K, which implies that C23 - not C1 - is the stable low-temperature polymorph. The recently-investigated transitions in PbF₂ closely parallel these in in SrCl₂.