Article

High-pressure synthesis and thermodynamic stability of PdH 1 ± ε up to 8 GPa

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Abstract

Palladium hydride alloys are superconductors and hydrogen storage materials. One synthesis route is compression of Pd to high pressure in a hydrogen-rich environment. Here we report the evolution of the unit cell volume of PdHx synthesized by compressing Pd in a pure H2 medium to pressures from 0.2 to 8 GPa in a diamond anvil cell at room temperature. The volume of the face-centered cubic unit cell changes nonmonotonically with pressure, increasing upon compression from 0.2 to 1 GPa and decreasing upon compression from 1 to 8 GPa. Volume is reversible upon decompression and is independent of whether the sample was heated to 600 K at low pressure (P<2 GPa). The x-ray diffraction data show no evidence for a phase transition between 0.2 and 8 GPa. The volume maximum at 1 GPa must be caused by progressive hydrogenation from 0 to 1 GPa. Assuming a pressure-volume-composition equation of state derived from previously published data, the [H]:[Pd] ratio in this study increases to a maximum value of x=1±0.02 at 2±0.5 GPa and remains stable upon further compression to and from 8 GPa. These results add to a mounting body of evidence that PdH1±ε is in thermodynamic equilibrium with pure H2 at room temperature from 2 GPa to at least 8 GPa. The simplest interpretation is that H atoms occupy all octahedral sites and no tetrahedral sites in face-centered cubic PdH1.0.

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... GPa in ref. [60], both at room temperature), the simplest interpretation is that PdH 1 has the oct structure. If so, the problem becomes one of understanding why LDA-and GGA-level DFT is so wrong, which if true would suggest that most of the published DFT studies on PdH x should be set aside. ...
... We thank the authors of refs. [30] and [60] for permission to quote their unpublished results. N.A. thanks Pole Empire, RSA for financial support, and Sci-Hub, LibGen for providing reprints. ...
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Magnetic susceptibility measurements have been performed with deformed single-crystal Pd loaded with a low concentration of hydrogen. A diamagnetic contribution to the paramagnetic susceptibility of Pd has been observed below 30 K that is attributed to a condensed hydrogen phase at low temperature along dislocation cores in the deformed, single crystal Pd matrix.
Article
It has been shown that hydrogen–metal reactions operated at high pressures (3–5 GPa) may lead to hydrogen-induced lattice migration. The occurrence of fast diffusion processes that take place within the metal lattice has been established. Under these conditions, modifications of the diffusion kinetics and of the phases equilibria allow to produce vacancy-ordered phases with high vacancy concentrations (20%). An alternative route which leads to such phases that are stable at ambient pressure and temperature is presented. The structural properties of the Pd-(vacancy, H) system which have been studied by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy will be discussed. In the case of palladium, the vacancy-ordered state is characterized by the loss of superconductivity with respect to the Pd hydride. This spectacular modification of the physical properties will be presented and discussed in the light of band structure calculations that have been performed modeling different types of decorated vacancies with octahedral coordination.
Article
The effect of high pressure (3.5 GPa) on the Pd and Pd–H systems has been investigated. We have been able to induce a cubic–monoclinic structural transformation in the case of pure Pd treated at 450°C for 5 h. Hydrogen has been introduced at high pressures using an alternative hydrogen source (C14H10). It is shown that such a route can be operated to produce vacancy-ordered phases that are stable at ambient pressure and temperature.
Article
This is a discussion of three articles by Tripodi et al. [P. Tripodi, M.C.H. McKubre, F.L. Tanzella, P.A. Honnor, D.Di Gioacchino, F. Celani, V. Violante, Phys. Lett. A 276 (2000) 122; P. Tripodi, D.Di. Gioacchino, R. Borelli, J.D. Vinko, Physica C 388–389 (2003) 571; P. Tripodi, D.Di. Gioacchino, J.D. Vinko, Physica C 408–410 (2004) 350] on the possibility of high temperature superconducting phases in PdH. The experimental results as well as their interpretations are discussed. Critical remarks concerning the definition of superconductivity and the possibility of preparation of PdHx (for x ≫ 1) by electrochemical method are presented.
Article
Consecutive measurements of the magnetization curves and resistive transitions in magnetic fields have been made on PdH xT (x=H/Pd=0.9801–0.9957) foils between 2 and 10.4 K. The interpretation of the results is complicated by the fact that the magnetization curves are extremely irreversible and the hydrogen is distributed inhomogeneously in the samples. However, an analysis of the results shows PdH to probably be a type I superconductor with aT c of between 10.2 and 10.4 K, an HC(0) somewhat less than 900 G, and a of around 0.6 at absolute zero. Forx below about 0.995, PdH x becomes a type II superconductor due to the increasing resistivity of the material.
Article
Deuterons of 10 keV energy and protons with the same mean projected range have been implanted into several metals at a temperature of approximately 35 K and at dose rates of approximately 2 1014 cm–2 s–1. The amount of retained deuterium saturates at fluences larger than roughly 2 1018 cm–2. After implantation of deuterons and subsequent bombardment with protons, the deuteron depth profiles show characteristic double peak structures, which indicate a replacement process. The experimental data are in good agreement with a simple model of local saturation and mixing. The possible implications of this model are discussed.
Article
The pressure–volume–temperature equation of state (EOS) of gold is fundamental to high-pressure science because of its widespread use as an internal pressure standard. In particular, the EOS of gold has been used in recent in situ multi-anvil press studies for determination of phase boundaries related to the 660-km seismic discontinuity. These studies show that the boundaries are lower by 2 GPa than expected from the depth of the 660-km discontinuity. Here we report a new P–V–T EOS of gold based on the inversion of quasi-hydrostatic compression and shock wave data using the Mie–Grüneisen relation and the Birch–Murnaghan–Debye equation. The previously poorly constrained pressure derivative of isothermal bulk modulus and the volume dependence of Grüneisen parameter (q=d lnγ/d ln V) are determined by including both phonon and electron effects implicitly: K′0T=5.0±0.2 and q=1.0±0.1. This combined with other accurately measured parameters enables us to calculate pressure at a given volume and temperature. At 660-km depth conditions, this new EOS yields 1.0±0.2 GPa higher pressure than Anderson et al.’s EOS which has been used in the multi-anvil experiments. However, after the correction, there still exists a 1.5-GPa discrepancy between the post-spinel boundary measured by multi-anvil studies and the 660-km discontinuity. Other potential error sources, such as thermocouple emf dependence on pressure or systematic errors in spectroradiometry, should be investigated. Theoretical and experimental studies to better understand electronic and anharmonic effects in gold at high P–T are also needed.
Article
The dynamics of stoichiometric palladium hydride have been investigated using high-resolution inelastic-neutron-scattering experiments at 25 K in the energy-transfer region 2-800 meV. Strong anisotropy of the high-energy part of the spectra was observed. These features, which are similar to those seen previously in beta-V2H, have energies remarkably close to the predictions of recent first-principles calculations.
Article
Deuterium (D) was introduced into Pd at atomic ratios greater than one by ion implantation at cryogenic temperatures. The rise and saturation of the D concentration at these low temperatures, together with the decay of the superstoichiometric state during annealing, were observed by detecting charged particles from the nuclear reaction D(d,p)T when a deuteron beam impinged on the specimen. At implantation temperatures of 41 and 81 K, a saturation concentration ratio [D]/[Pd] of 1.6+/-0.2 was reached, substantially above the limit of 1.0 observed in gas-phase charging. As the temperature was subsequently ramped upward, [D]/[Pd] abruptly decreased to approximately 1.0 near 120 K, reflecting a process of accelerated transport unique to the superstoichiometric state. The responsible diffusion mechanism was theoretically examined using analytical modeling and molecular-dynamics simulations, leading to a picture of correlated D hopping among octahedral and tetrahedral interstitial sites. Cold fusion was not detected in the absence of external ion bombardment, implying an upper bound on the reaction rate in the high-concentration deuteride of approximately 10-21 events/s D atom.
Article
In situ x-ray diffraction on Pd hydride under 5 GPa of hydrogen pressure show that lattice contraction due to vacancy formation occurs in 2-3 h at 700-800 °C, and two-phase separation into PdH and a vacancy-ordered phase of Cu3Au structure (Pd3VacH4) on subsequent cooling. After recovery to ambient conditions and removal of hydrogen, the vacancy concentration in Pd metal was determined by measuring density and lattice parameter changes to be 18+/-3 at. %. This procedure provides a new method of introducing super- abundant vacancies in metals.
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