Anomalous compressibility and metallization of deuterium and hydrogen at high pressures

The high compressibilities recorded in experiments on liquid-deuterium compression and the significant scatter of experimentally measured densities in the region of anomalous compressibilities are explained by the manifestations of an unusual dielectric-metal phase transition: from a dense molecular gas to a liquid-metal atomic gas. This transition has previously been described by us and called the dissociative phase transition (DPT). The dissociative equilibrium curves, the isotherms, the DPT binodal, and the Hugoniot adiabat have been calculated. The DPT has been shown to be a transition of a new, non-van der Waals type.     

The incoherent spinodals of hydrogen in Nb,Ta and V

The incoherent spinodals for H in Nb, Ta and V are measured for wire-shaped specimens using the Gorsky effect, and compared with incoherent spinodals as deduced from solubility experiments. For Nb the spinodal touches the phase boundary at the critical point, whereas for Ta and V the maxima of the spinodals are about 100° below the phase boundary. Within the error bars the spinodals for Nb and Ta are isotope-independent.     

Study of deuterium and hydrogen distributions in Ta|CD<Subscript>2</Subscript>|Ta,Ta|Ta|CD<Subscript>2</Subscript>|Ta|Ta,and Nb|CD<Subscript>2</Subscript>|Nb assemblies after exposure to high-temperature argon plasma

Assemblies made of Ta|CD₂|Ta, Ta|Ta|CD₂|Ta|Ta and Nb|CD₂|Nb foils are irradiated with pulses of high-temperature argon plasma created by means of a “Plasma Focus” setup. The irradiated foil samples are investigated by recording the recoil nuclei of hydrogen and deuterium. It is found that hydrogen and deuterium are redistributed in foil stacks. The ultradeep penetration of light gas impurities (hydrogen and deuterium) can be explained by the influence of shock waves on the foils and accelerated diffusion under an external force.     

Structural transitions in hydrogen and deuterium at ultrahigh pressures

Abstract

Raman measurements indicate that normal hydrogen and deuterium compressed in a diamond-anvil cell at 77 K undergo phase transformations at 145 (±l5) GPa and 190 (±l20) GPa, respectively. The new observations reported for deuterium indicate that the transition in both isotopes is a structural change not associated with a simple rotational ordering mechanism. The possibility of a structural transition driven by band-gap closure is discussed.     

Diffusion coefficients of hydrogen and deuterium in highly concentrated palladium hydride and deuteride phases

Fick's diffusion coefficients for hydrogen and deuterium were determined in palladium hydride and deuteride in the pressure range of gaseous hydrogen and deuterium from 10¹–10⁴ bar and the temperature range from 208 to 338K. This corresponds to the concentration range H, D/Pd from about 0.8 to near stochiometry. An electrical resistance relaxation method was used experimentally. Einstein's diffusion coefficients were calculated and the activation volumes and energies were evaluated. An attempt is made to interpret the anomalous behaviour of the diffusion coefficient in the low temperature range. The results are presented in terms of a compact formula.     

Diffusion of hydrogen and deuterium on the (111) plane of tungsten

The diffusion of ¹H and ²H on the (111) plane of a W field emitter has been studied by the fluctuation method at various coverages. Both activated and unactivated diffusion is observed; the latter shows very little isotope effect, suggesting that coupling to the substrate is so strong that mass renormalization makes the effective masses of ¹H and ²H nearly identical. Values of D in the tunneling, i.e. temperature independent, regime are 10^?13?5 × 10^?14 cm²/s depending on coverage. For activated diffusion at high coverages, corresponding to population of the β₁ state E = 2.4?3.2 kcal/mol and D₀ = 2 × 10^?8 ?5 × 10^?7 cm²/s, depending on coverage. For lower coverages, corresponding to β₂ population, E = 7–9 kcal/mol, D₀ = 9 × 10^?6 ?2 × 10^?3 cm²/s, again depending on coverage. Similar values are obtained for ²H, with E and D₀ values slightly reduced. An exponentially decaying correlation signal for clean W was also seen and interpreted in terms of flip-flop of W atoms.     

Unusual softening of the Nb and V lattice due to dissolved hydrogen (deuterium)

We report on X-ray Bragg intensity measurements in the high concentration range of NbH _x and VD _x as a function of temperature andH(D) concentrationx. The mean square thermal displacements of the metal atoms decrease with increasingH(D) concentration. For concentrations higher thanx0.7 a sharp increase was observed. The metal lattice first stiffens withH(D) dissolution and then becomes very soft.     

Pionic hydrogen and deuterium

The ground-state level shifts and broadenings of the hydrogen isotopes caused by the strong interaction have been redetermined by using a high-resolution crystal spectrometer. An additional measurement of muonic hydrogen reveals properties of the de-excitation cascade of such electrically neutral exotic atoms, in particular Coulomb de-excitation, the understanding of which is essential for the analysis of the hadronic-atom data.     

Hydrogen in Nb/Ta superlattices

The structural and thermodynamic properties of hydrogen dissolved in Nb/Ta superlattices are studied by in-situ x-ray scattering techniques. From the x-ray satellite intensities, it is found that H induces a strain modulation exhibiting a Curie-Weiss temperature dependence. The intensities are analyzed in terms of a mean field model of the modulated lattice gas, yielding quantitative information on the hydrogen-metal and hydrogen-hydrogen interaction energies. Critical behavior associated with a gas-liquid transition is also observed. Hydrogen density fluctuations with wavelengths shorter than a superlattice period are, however, suppressed by the superlattice, which represents a novel manifestation of a coherent phase transition. These experiments provide new and fundamental insight into the role of spacially varying two body interactions in critical phenomena.     

Electric field gradients at V,Nb and Ta in A15 alloys

The efg at V, Nb and Ta in all existing binary alloys with A15 structure has been calculated with the FLAPW technique. It is generally a result of counteracting p- and d-contributions from the conduction band, with a significant effect also of semicore states. For virtually all V and Nb compounds where experimental data are available, these are well reproduced, with the exception of Nb₃Sb. A critical test would be a measurement of the quadrupole interaction in the Ta alloys. This revised version was published online in August 2006 with corrections to the Cover Date.     

Orientation of an oxygen-induced non-axially symmetric electric field gradient at Ta in Nb

The interaction of dilute Hf in Nb with interstitially dissolved oxygen has been investigated via the --TDPAC-technique applied to the 482 keV state of¹⁸¹Ta. After melting the parent radioactivity¹⁸¹Hf with Nb the trapping of oxygen at the probe atoms leads to a quadrupole interaction frequency _Q =268(1) MHz with an asymmetry parameter of =0.68(1). The temperature dependence of this frequency between 17 K and 293 K was found to be very weak whereas varies considerably. In a single crystal experiment the orientation of the three principal axes of the electric field gradient tensor leading to _Q has been determined. TheV _zz -axis points into a 110, theV _yy -axis into a 100 and theV _x -axis into a 110 direction. The results can be explained in the point charge model by assuming that one oxygen atom is trapped on the second nearest neighbour octahedral position next to the substitutional atom and the covalent bonds between the daughter Ta and the oxygen lead to a localization of charge near the probe atom.     

Hydrogen distribution in Nb/Ta superlattices

The distribution of hydrogen in Nb/Ta superlattices has been investigated by combined neutron reflectivity and x-ray scattering. We provide evidence to support that strain modulations determined with x-ray diffraction can be interpreted as modulations in hydrogen content. We show that the hydrogen concentration is modulated and favors Nb, in agreement with previous studies. We measure the concentration directly using neutron reflectivity and demonstrate no detectable change in the distribution of hydrogen with temperature, in stark contrast to previous studies.     

Effect of deuterium on phase transformations in fullerenes at high temperatures and high pressures

The effect deuterium has on phase transformations is studied for amorphous and crystalline fullerenes C₆₀ and C₇₀ at high temperatures of up to 1300°C and high pressures (2–8 GPa). Amorphous fullerene phases are obtained via long grinding in a planetary mill. Structure is studied by means of neutron diffraction. In all cases, amorphous graphite (nanographite) forms in the temperature range of 800–1100°C. This material has different diffraction spectra distinguished by the heights of the halos observed on the graphite diffraction maxima and their relative intensities. These spectra (the structure of nanographite) are affected by preliminary amorphization, the number of carbon atoms in the fullerenes (C60 or C70), and the introduction of deuterium atoms. The different spectra of amorphous (disordered) graphite testify to its varying structure.     

An els study of the oxidation of polycrystalline Ta and Nb at room temperature

ELS has been used to characterize the initial steps in the oxidation of polycrystalline Ta and Nb in an oxygen atmosphere at low pressures (~1.3 × 10^?6 Pa) and room temperature. The results are interpreted in terms of transitions to unoccupied states above E_F and plasmon losses, and are discussed in the light of previous studies. The results show important changes in the valence band. The O 2s and O 2p bands increase in intensity during oxidation.     

Formic acid in deuterium and hydrogen matrices

ABSTRACT

Formic acid (HCOOH, FA) was studied experimentally, by infrared spectroscopy, in H₂ and D₂ matrices, with focus on the preparation and characterisation in these matrix media of structures containing the higher-energy (cis) conformer. The cis-FA monomer and the cis-FA^?…?N₂ complex were successfully produced by selective vibrational excitation of corresponding trans-FA based species, and vibrationally characterised. The tunneling-induced conversion of the cis-FA^?…?N₂ complex in the studied matrices into the corresponding trans-FA complex was also investigated, and the found tunnelling properties discussed, in particular in comparison with those observed for the spontaneous conversion of cis-FA monomer into trans-FA. This article constitutes the first report on the infrared spectrum of FA conformers and stability of cis-FA monomer in a D₂ matrix, and on the structure, spectroscopy and stability of the cis-FA^?…?N₂ complex in both H₂ and D₂ matrices. Different attempts to prepare the cis-FA^?…?H₂O complex in the two investigated matrices are also described in detail, both from previously in situ generated cis-FA monomer followed by thermal mobilisation and by direct selective vibrational excitation of the trans-FA-H₂O complex.