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High-pressure synthesis and thermodynamic stability of PdH 1 ± ε up to 8 GPa
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.
Supplementary resource (1)
... 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 [30] and [60] for permission to quote their unpublished results.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. ...
The partial atomic volume of hydrogen, vH, is a fundamentally important thermodynamic parameter of interstitial metal hydrides in which dissociated H occupies interstices in the metal lattice. Such an important property should be able to be reliably calculated by a suitable theory or model in order to explain and understand its origin. In practice, vH is typically obtained by means of ab initio calculations founded on density functional theory (DFT), where the equilibrium lattice constant at zero temperature is found by minimising the Born-Oppenheimer energy. While the absolute lattice constants calculated in this way depend quite strongly on the DFT scheme employed, the present work showed that vH is rather robust against differing calculational approaches, thus making a meaningful comparison of theory and experiment possible. Comparing vH for PdnH (0 < n < 8) calculated with DFT and obtained from in-situ neutron diffraction measurements revealed a significant discrepancy when octahedral-only interstitial occupancy was assumed. Calculations for PdH with mixed octahedral and tetrahedral occupancy gave a value for vH in agreement with experiment assuming that PdH contains 15–20% tetrahedral H.
... In addition, the claim by Staker [61,74,75] that vacancy tubes are present in PdD is very weak because the claim by Fukai [76] on which this idea is based could not be replicated. [77] Because these tubes would have the same chemical properties as a Pd atom vacancy, their occupancy would suffer from the same limitation. ...
... In addition, the claim by Staker [51][52][53] that vacancy tubes are present in PdD as the NAE is very weak because the claim by Fukai [54] on which this idea is based could not be replicated. [55] Because these tubes would have the same chemical properties as a Pd atom vacancy, their occupancy would suffer the same limitation. ...
A brief description of selected information presently known about the fusion process called cold fusion is provided and used to support a general description of how the process is proposed to work. The nuclear process results from an unusual chemical condition in which a structure forms that can cause nuclear fusion when isotopes of hydrogen are present. This fusion process has two important features. It may be a source of clean, inexhaustible, and inexpensive energy. In addition, it has revealed a new kind of nuclear process. Although this proposed mechanism could not yet be described in mathematical detail, its major features can be identified and used as a guide for future studies.
... Even the proposed metal atom vacancy model, called superabundant vacancies (SAV) [6], is not consistent with the observed behavior because the conditions required to form the SAV are not present when LENR is produced [7]. Indeed, if the proposed vacancy structure could be created within a crystal structure, this condition would be expected to occur throughout the crystal structure, thereby making LENR much more common and uniform at its locations in the material than is the case. ...
The cold fusion process is explained using a new understanding of the observed behavior that shows how many of the behaviors are related to each other and to the mechanism causing cold fusion. A mechanism is described that can guide a more reliable initiation of the nuclear process. Methods are suggested by which the model can be tested. The need for a new understanding of nuclear interaction is identified.
We present an ab initio study of the ternary hydride PdCuH x, a parent compound of the superconducting PdH, at different hydrogen content (x = 1, 2). We investigate its structural, electronic, dynamical, and superconducting properties, demonstrating that, at low hydrogen content, the system is not a superconductor above 1 K; however, the highly hydrogenated structure is a strongly coupled superconductor. We give a solid rationale for the unusual increase of the superconducting critical temperature in hydrogenated palladium when alloyed with noble metals (Cu, Ag, and Au), as observed in Stritzker's experiments in 1972 [B. Stritzker, Z. Phys. 268, 261-264 (1974)] but never investigated with modern experimental and theoretical techniques. We highlight the important role played by H-derived phonon modes at intermediate frequencies, dynamically stabilized by anharmonic effects, as they strongly couple with states at the Fermi level. We hope that the present results will stimulate additional experimental investigations of structural, electronic, and superconducting properties of hydrogenated palladium-noble metal alloys. Indeed, if confirmed, these compounds could be considered a novel class of superconducting hydrides, showing different coupling mechanisms, which can be exploited to engineer new ambient-pressure superconductors.
Significance
Superhydrides are a materials system where near–room-temperature superconductivity has been achieved but only at very high (megabar) pressures. This work proposes an approach that combines pressure and electrochemistry to stabilize superhydrides at moderate pressures. Through a computational study of the palladium–hydrogen system, we construct electrochemical phase diagrams and show that electrochemically synthesizing superhydrides may be possible when combined with moderate pressures. We generalize this to other binary metal superhydrides of interest for superconductivity, including La, Y, and Mg hydrides.
The diamond anvil cell (DAC) technique combined with laser heating is one of the major methods for studying materials at high pressure and high temperature conditions. In this work, we present a transferable double-sided laser heating setup for DACs with in situ temperature determination. The setup allows precise heating of samples inside a DAC at pressures above 200 GPa and could be combined with synchrotron beamline equipment. It can be applied to X-ray diffraction and X-ray transmission microscopy experiments. In the setup, we use high-magnification and low working distance infinity corrected laser focusing objectives that enable us to decrease the size of the laser beam to less than 5 µm and achieve the maximum optical magnification of 320 times. All optical components of the setup were chosen to minimize chromatic and spatial aberrations for accurate in situ temperature determination by multiwavelength spectroscopy in the 570–830 nm spectral range. Flexible design of our setup allows simple interchange of laser sources and focusing optics for application in different types of studies. The setup was successfully tested in house and at the high-pressure diffraction beamline ID15B at the European Synchrotron Radiation Facility. We demonstrate an example of application of the setup for the high pressure–high temperature powder diffraction study of PdH and X-ray transmission microscopy of platinum at 22(1) GPa as a novel method of melting detection in DACs.
The dynamics of hydrogen-induced vacancies are the key for understanding various phenomena in metal–hydrogen systems under a high hydrogen chemical potential. In this study, a novel dry-electrolysis experiment was performed in which a hydrogen isotope was injected into a Pd cathode and time-resolved in situ monochromatic X-ray diffraction measurement was carried out at the Pd cathode. It was found that palladium-hydride containing vacancies forms multiple phases depending on the hydrogen chemical potential. Phase separation into vacancy-rich, vacancy-poor, and moderate-vacancy-concentration phases was observed when the input voltage was relatively low, i.e., ∼0.5 V. The moderate-vacancy-concentration phase may be attributed to Ca7Ge or another type of super-lattice Pd7VacH(D)8. Transition from the vacancy-rich to the moderate-vacancy-concentration phase explains the sub-micron void formations without high temperature treatment that were observed at the Pd cathode but have never been reported in previous anvil experiments.
Hydrogen isotopes and metal interaction with respect to Pd under high hydrogen isotope potential at moderate temperature region around 300 °C was studied. A dry electrolysis technique using BaZr1−x YxO3 solid state electrolyte was developed to generate high hydrogen isotope potential. Hydrogen or deuterium was loaded into a 200 nm thick Pd cathode. The cathode is deposited on SiO2 substrate and covered with the solid state electrolyte and a Pd anode layer. Time resolved in situ monochromatic x-ray diffraction measurement was performed during the electrolysis. Two phase states of the Pd cathodes with large and small lattice parameters were observed during the electrolysis. Numerous sub-micron scale voids in the Pd cathode and dendrite-like Pd precipitates in the solid state electrolyte were found from the recovered samples. Hydrogen induced super-abundant-vacancy may take role in those phenomena. The observed two phase states may be attributed to phase separation into vacancy-rich and vacancy-poor states. The voids formed in the Pd cathodes seem to be products of vacancy coalescence. Isotope effects were also observed. The deuterium loaded samples showed more rapid phase changes and more formation of voids than the hydrogen doped samples.
The discovery of superconductivity in palladium-hydrogen (PdH) and its isotopes (D,T) at low temperature, brought about extensive study of this system. These studies have shown that the critical transition temperature is a function of the H concentration x in the PdHx system with Tc=9K for x=1. In the last decade we defined a room temperature and room pressure technique to load H and maintain stable the stoichiometry in Pd lattice at levels higher than unit. Several magnetic and electric transport measurements have been performed showing transition temperature in the range of [18K< Tc < 273K]. Moreover in a typical critical current measurement configuration, current density greater than 6*10⁴Acm⁻²⊐ has been measured at liquid nitrogen temperature. The 263.5K superconducting transition after a week of sample storage at room pressure and temperature, decreased down to 261.5K and after 2 years it became 160.5K, demonstrating a fairly good stability of the sample. Evidences of the flux exclusion (ZFC measurements) and the flux expulsion (FC measurements) have been found at very high transition temperature (Tc=235K) for the PdH system.
Since the discovery of superconductivity in palladium-hydrogen (PdH) and its isotopes (D,T) at low temperature, several efforts have been made to study the properties of this system. Superconductivity of PdH system has been initially claimed by resistance drop versus temperature and then confi rmed by dc magnetic susceptibility measurements. These studies have shown that the critical transition temperature is a function of the hydrogen concentration x in the PdHx system. In all these experiments, the highest concentration x of hydrogen in palladium was lower than the unit. In the last decade we defi ned a room temperature and room pressure technique to load hydrogen and its isotopes into palladium at levels higher than unit, using electrochemical set-up, followed by a stabilization process to maintain the hydrogen concentration in palladium lattice stable. In the meanwhile, several measurements of resistance versus temperature have been performed. These measurements have shown several resistive drops in the range of [18K
We are facing accelerated global warming due to the accumulation of greenhouse gases. A hydrogen-based economy is one potential approach toward maintaining our standard of living while lowering carbon dioxide emissions. Palladium is a unique material with a strong affinity to hydrogen owing to both its catalytic and hydrogen absorbing properties. Palladium has the potential to play a major role in virtually every aspect of the envisioned hydrogen economy, including hydrogen purification, storage, detection, and fuel cells. Major aspects of current research and potential applications of palladium-based nanomaterials in various hydrogen technologies are presented in this review.
Experimental data on PdHx(D) at 300 K shows electrical resistivity ρ lower than that of the pure Pd for stoichiometry x=H/Pd close to the unit. At this stoichiometric value x=1 a Tc of 9 K and higher Tc for x>1 have been measured. In these systems Tc increases with stoichiometry x, hence a phenomenological description of the ρ for highly loaded PdH(D) system at 300 K has been developed. This approach uses a parallel model of two concurrent electrical transport processes: (i) ρ has a linear raise with the x, due to the increase of Pd lattice relative volume, (ii) ρ has an exponential decrease versus x due to superconducting fluctuations at very high x in PdH(D). Under the assumption of our model, PdHx could be considered as a possible room temperature superconductor. Inverse isotopic effect for 0.6>x>0.96 changes to normal isotopic effect at stoichiometry xΨ>1.
Electron transport and magnetic properties have been studied in a deformed 12.5-μm-thick Pd foil with a thermally-grown oxide and a low residual concentration of hydrogen. This foil was deformed by cycling across the Pd hydride miscibility gap and the residual hydrogen was trapped at dislocation cores. Anomalies of both resistance and magnetic susceptibility have been observed below 70 K, indicating the appearance of excess conductivity and a diamagnetic response that we interpret in terms of filamentary superconductivity. These anomalies are attributed to a condensed hydrogen-rich phase at dislocation cores near the Pd-Pd oxide interface.
The dependence of critical temperature on hydrogen (deuterium) content in palladium hydride (deuteride) was measured on samples hydrogenized under equilibrium conditions with H(D) at high pressure. The superconductivity transitions can be observed for atomic ratios of hydrogen (deuterium) to palladium H(D)/Pd above 0.83 (0.77). The critical temperatures of deuteride are about 2.5K higher than those of hydride (at the same atomic ratios). The highest critical temperatures so far observed for the hydride and deuteride are 9.1K and 10.7K respectively. For both phases the effect of hydrostatic pressures is to decrease the critical temperature. The observed isotope effect does not support a metallic behaviour of hydrogen in palladium but is in reasonable agreement with Ganguly's model (see abstr. A6687 of 1974) for the superconductivity of palladium hydride or deuteride.
In previously published papers, preliminary results on highly hydrogen loaded palladium (PdH x) samples with H/Pd stoichiometric ratio x>1, have shown probable multi-phase HTSC superconducting states. In this letter, the stability of the new phases in the sample has been verified. After long-term sample storage of two years at room temperature and room pressure, ac susceptibility measurements in temperature have been done. The sample, that two years earlier exhibited a high temperature phase at T c =261.5K with x~1.56, presented now a T c shift down to T c =160.5K with a x~1.46 and T c =82K with a x~1.34. The H/Pd decreases by x~0.1 and ~0.21 while the T c diminishes down to T c ~101K and 180K. In very high x ratio phase, slight hydrogen leakage causes significant T c drop. Moreover, another previously existing low temperature phase T c =9K with x~1, shifts down to 6K with x~0.85. In this case the decrease of x ~0.15 causes the T c drop by 3K. In both phases, similar calculated x decrease percentage with subsequent reduction of the T c estimated percentage shows a like behavior. This suggests that a single process takes place in the formation of HTSC and low T c superconducting state in PdH x . The measured PdH sample, before and after long-term storage and trough the thermal cycles, maintains high H loading value with high transition temperatures.
The magnetic characterization of Pd single crystals deformed by cycling in a hydrogen atmosphere has been performed. Based
on evidence obtained from thermal desorption analysis, it is shown that the condensed hydrogen phase formed inside deep dislocation
cores in PdH
x
(x = H/Pd ≈ 4.5 × 10−4) is tightly bound with a Pd matrix. The activation energy of hydrogen desorption from these cores was found to be as high
as e = 1.6eV/H-atom, suggesting the occurrence of a strong band overlapping between Pd and H atoms. SQUID measurements carried
out in a weak magnetic field (H < 5.0 Oe) showed an anomalous diamagnetic contribution to the DC and AC magnetic susceptibilities of the PdH
x
sample at T < 30 K resulting in the presence of the hydrogen phase. It is suggested that the anomalous diamagnetic response in PdH
x
is caused by the presence of a hydrogen dominant phase, tightly bound with a Pd matrix inside the dislocation cores (nanotubes).
We have studied a sample of deuterated palladium at pressures up to 105 kbar and temperatures from 4.3 to 400 K in an effort to detect cold fusion. A [D]/[Pd] concentration of 1.34+/-0.1 was achieved. No evidence of cold fusion was found with a limit of 2.5×10-18 fusions (DD pair)-1 s-1 from neutron detection and 1.5×10-8 fusions (DD pair)-1 s-1 from detection of possible heating; a gamma-ray detector showed no evidence of gamma rays differing from background.
Palladium hydride was discovered more than 150 years ago and remains one of the most-studied interstitial metal hydrides because of the richness of its physical behaviours, which include ordered phases and anomalous properties at temperatures below 100 K, a superabundant-vacancy (SAV) phase with stoichiometry Pd3H4 formed at high temperature and pressure, and quenching of the enhanced Pauli paramagnetism of palladium. One of the most fascinating properties of palladium hydride is superconductivity at about 10 K without external pressure, in contrast to the newly-discovered polyhydride room-temperature superconductors that require megabar pressures. Moreover, the superconductivity exhibits an inverse isotope effect. Remarkably, modern first-principles approaches are unable to accurately predict the superconducting transition temperature by calculating the electron–phonon coupling constant within Migdal-Eliashberg theory. Anharmonicity of the hydrogen site potential is a key factor and poses a great challenge, since most theoretical approaches are based on the harmonic approximation. This review focuses on the electron and phonon band structures that underpin all such calculations, with palladium as a reference point. While the electron band structures of palladium and its monohydride are uncontroversial, the phonon band structure of palladium hydride in particular is problematic, with a realistic treatment of anharmonicity required – and largely yet to be achieved – to reproduce the results of inelastic neutron scattering experiments. In addition to the monohydride and SAV phases, possible higher hydrides are surveyed and the origin of the famous “50-K” anomaly in specific heat and other physical properties is critically reviewed.
The diamond anvil cell (DAC) technique is a powerful method for the synthesis and studying of novel materials at extreme conditions. In this work we report on high-pressure high-temperature (HPHT) synthesis of palladium carbides (PdCx) and palladium hydride (PdH) in a laser heated diamond anvil cell. Formation of PdCx with a face-centered cubic (fcc) structure resulted from a chemical reaction of Pd with carbon from the diamond anvils at a pressure of about 50 GPa and temperature of 2500–3000 K. The samples were analyzed in situ using synchrotron X-ray diffraction. The compressional behavior of PdC0.19(3) and PdC0.17(3), was studied on decompression. The fit of the pressure-volume data using the 3rd order Birch-Murnaghan equation of state gave the following parameters: V0 = 65.1 (1) ų, K0 = 241 (9) GPa and K0' = 2.1 (3) for PdC0.19(3), and V0 = 64.51 (5) ų, K0 = 189 (8) GPa and K0' = 4.5 (4) for PdC0.17(3). The palladium hydride PdH was synthesized at P = 39 (2) GPa and T = 1500 (200) K through a direct reaction of Pd with paraffin oil.
The stable forms of palladium hydrides up to 100GPa were investigated using the direct reaction of palladium with hydrogen (deuterium) in a laser-heated diamond anvil cell. The structure and volume of PdH(D)x were measured using synchrotron x-ray diffraction. The Pd atoms remain on a fcc lattice. The stoichiometry of the hydride is inferred from the volume expansion due to the hydrogen solubility in the Pd lattice. No evidence for hydrogen to palladium ratio greater than 1 is observed for both isotopes. An inverse isotope effect on the formation enthalpy of the stoichiometric Pd hydride is disclosed by measuring the equilibrium formation pressure of PdH and of PdD, 1.9GPa and 2.7GPa, respectively. An isotopic shift between the compression curves of PdH and of PdD is also measured, in good agreement with ab initio calculations quantifying the contribution of the hydrogen zero-point vibrational energy.
Superstoichiometric hydrides PdHx have been synthesized by the electrochemical method. The H concentrations, x = H/Pd = 1.13~1.97, have been determined directly by thermal desorption. From X-ray diffraction, the structure is found to be face-centered cubic, with the lattice parameter changing smoothly over the concentration range, x = 0.7 ~ 2.0. Surprisingly, however, the lattice parameter goes through a maximum at x ~ 1.0 and decreases at higher concentrations. It is suggested from electronic and Monte Carlo calculations that these unique features of the structure and formation process should be the consequence of partial replacement of Pd atoms with H2, namely, the formation of superabundant vacancies filled with H2 molecules.
High pressure x-ray diffraction of PdHx and PdDx demonstrate that these materials remain in a face-centered cubic (fcc, Fm3 ̅m) structure to these pressures at room temperature. The volumes indicate stoichiometric compositions under pressure with x = 1 for both materials. No indications of phase transitions were observed up to the highest pressures reached in the experiments. A third-order Birch-Murnaghan equation of state used to fit the pressure-volume data gives V0 = 10.73 (±0.03) cm³/mol, K0 = 147 (±11) GPa, and K0' = 4.7 (±0.5), whereas a Vinet fit gives V0 = 10.74 (±0.03) cm³/mol, K0 = 143 (±11) GPa, and K0' = 5.1 (±0.5), for both PdHx and PdDx. The results are used to obtain the pressure dependence of the effective volume of H and D atoms in PdHx and PdDx to megabar pressure for comparison with other hydrides, with implications for superconductivity in this class of materials.
Palladium, rhodium and their alloys were studied in a diamond anvil cell in a high pressure hydrogen atmosphere up to 20 GPa by X-ray diffraction at room temperature. Formation of a monohydride with H/(Rh + Pd) = 1 ratio was observed for each alloy at hydrogen pressures less than 3 GPa. The corresponding volume expansion of the metal lattice was 2.2–2.8 ų per metal atom. A formation of the dihydride with H/(Rh + Pd) = 2 was observed at a hydrogen pressure around 10 GPa for the alloys with rhodium content Rh/(Rh + Pd) ≥ 50 at.%. The dihydride formation is accompanied by a volume expansion of metal lattice by 3.2–3.9 ų per metal atom. For Pd-rich alloys with Rh/(Rh + Pd) ≤ 25 at.% and pure Pd the dihydride formation was not observed up to maximum reached hydrogen pressure. All observed phases had a fcc structure of metal lattice.
We present an overview of hydrogen-containing superconductors, including metallic, cuprate, pnictide and carbon-based materials. Particularly in BCS-like superconductors, hydrogen introduces new phonon modes and these may lead to better or worse superconducting properties, with anharmonicity of the H potential being a decisive factor. The availability of the deuterium isotope, in addition to protium, makes hydrogen an extremely useful extra probe of mechanisms of superconductivity. While hydrogen modification has not so far led to any confirmed remarkable improvement in already existing metallic superconductors, hydrogen plays an important role in the synthesis of novel iron-based superconductors and as a substitutional impurity appears to increase the transition temperature. Most recently, a new record for the superconducting transition temperature of 203 K has been claimed for a metallic hydride under high pressure, H2S. We survey elemental hydrides and superconductors, discuss techniques for introducing hydrogen into materials, then review and summarise published knowledge about hydrogen-containing superconductors.
20 nm thick Pd coatings deposited on Si substrates with 800 nm SiO2 and 1 nm Cr buffer layers were treated in a 2.45 GHz microwave plasma source at 700 W plasma power and 40 Pa working pressure without substrate heating. For obtaining information on the effect of energy influx due to ion energy on the palladium films the substrate potential was varied from Usub = 0 V to − 150 V at constant gas flow corresponding to mean ion energies Ei from 0.22 eV ∙ cm− 2 ∙ s− 1 to 1.28 eV ∙ cm− 2 ∙ s− 1.
In contrast to high pressure reactions with metallic Pd, under plasma exposure we do not observe solid solutions over a wide range of hydrogen concentration. The hydrogen incorporation in Pd films takes place discontinuously. At 0 V substrate voltage palladium hydride is formed in two steps to PdH0.14 and PdH0.57. At − 50 V substrate voltage PdH0.57 is formed directly. However, substrate voltages of − 100 V and − 150 V cause shrinking of the unit cell. We postulate the formation of two fcc vacancy palladium hydride clusters PdHVac(I) and PdHVac(II). Under longtime plasma exposure the fcc PdHVac(II) phase forms cubic PdH1.33.
The fcc PdH0.57 phase decomposes at temperatures > 300 °C to form metallic fcc Pd. The hydrogen removal causes a decrease of lattice defects. In situ high temperature diffractometry measurements also confirm the existence of PdHVac(II) as a palladium hydride phase. Stoichiometric relationship between cubic PdH1.33 and fcc PdHVac(II) becomes evident from XR measurements and structure considerations. We assume both phases have the chemical composition Pd3H4. Up to 700 °C we observe phase transformation between both the fcc PdHVac(II) and cubic PdH1.33 phases. These phase transformations could be explained analog to a Bain distortion by displacive solid state structural changes.
Diffraction experiments at high pressures provide measurement of the variation of the unit-cell parameters of the sample with pressure and thereby the variation of its volume (or equivalently its density) with pressure, and sometimes temperature. This last is known as the ‘Equation of State’ (EoS) of the material. It is the aim of this chapter to present a detailed guide to the methods by which the parameters of EoS can be obtained from experimental compression data, and the diagnostic tools by which the quality of the results can be assessed. The chapter concludes with a presentation of a method by which the uncertainties in EoS parameters can be predicted from the uncertainties in the measurements of pressure and temperature, thus allowing high-pressure diffraction experiments to be designed in advance to yield the required precision in results.
The variation of the volume of a solid with pressure is characterised by the bulk modulus, defined as K = − V ∂ P /∂ V . Measured equations of state are usually parameterized in terms of the values of the bulk modulus and its pressure derivatives, K′ = −∂ K /∂ P and K″ = −∂2 K /∂ P 2, evaluated at zero pressure. These zero-pressure (or, almost equivalent, the room-pressure values) are normally denoted by a subscript “0,” thus: K = − V (∂ P /∂ V ) P =0, K ′ = −(∂ K /∂ P ) P =0, and K ″ = −(∂2 K /∂ P 2) P =0. However, throughout this chapter a number of notational conventions are followed for ease of presentation. Unless specifically stated, the symbols K′ and K ″ (without subscript) refer to the zero-pressure values at ambient temperature, all references to bulk modulus, K …
Periodic table of elements starts with hydrogen, a simplest element of all. The simplicity is lost when the element is compressed to high densities or participates in a chemical bonding in compounds, being subjected to “chemical pressure” of surrounding atoms or molecules. The chemical nature of hydrogen is dictated by its simplest electronic shell, which has only one electron. Hydrogen can donate this electron and behave like alkali metal, or accept an extra electron and form a hydride ion with closed shell resembling a group VII element. The complexity of hydrogen goes beyond these simplest configurations, when hydrogen is involved in a multicenter bonding or in hydrogen bonds. This complex behavior is tightly related to the ability of hydrogen to participate in the process of electronic transport in solids and potentially be able to contribute to the superconductivity in a material. Hydrogen by itself when compressed to immense pressures of 400–500 GPa may form a simple atomic phase with very high critical superconducting temperatures (Tc) well above room temperature. While this theoretical insight awaits confirmation at pressures at the limit of current experimental capabilities, a variety of other hydrogen-rich materials have been suggested recently to have record high Tc values. The very existence of many of these materials still lacks experimental confirmation. In this review article, we will present an extensive list of such predicted materials. We will also review superconductivity in classical hydrides (mostly metal hydrides) and current theoretical understanding of relatively low Tc‘s in metal hydrides of transition and noble metals.
Lattice constants of the β phase of PdHx and
PdDx for the concentration range 0.8<x<0.98 have been
measured at 77 K using a powder x-ray diffraction technique. The lattice
constant a0 of PdHx is slightly larger (~ 0.1%)
than that of PdDx and dlna0dx=0.044 for both
PdHx and PdDx; extrapolated a0 values
for x=1.0 are 4.090 and 4.084 A for PdH and PdD, respectively. This
study shows that the inverse isotope effect in the superconducting
transition temperature is not simply a result of the relative volume of
PdHx and PdDx.
During the past five years, major progress has been made in the experimental study of solid hydrogen at ultrahigh pressures as a result of developments in diamond-cell technology. Pressures at which metallization has been predicted to occur have been reached (250-300 Gigapascals). Detailed studies of the dynamic, structural, and electronic properties of dense hydrogen reveal a system unexpectedly rich in physical phenomena, exhibiting a variety of transitions at ultrahigh pressures. This colloquium explores the study of dense hydrogen as an archetypal problem in condensed-matter physics.
In the present work palladium thin foils have been hydrogenated at high pressure. In situ monochromatic X-ray diffraction performed under 5 GPa of hydrogen pressure has allowed to follow the formation of hydrogen-rich Pd–H phases as well as the presence of ordered superabundant vacancy phases. The occurrence of hydrogen-rich Pd–H phases as well as the presence of ordered superabundant-vacancy phase is discussed. Sme discrepancies with the early experiments of Fukai are observed. In particular, our results suggest the occurrence of a superstoichiometric defect hydride related to the γ′ phase. A structural model is proposed and needs to be confirmed by using neutron diffraction.
An alternative route for hydrogenation has been investigated: plasma-based ion implantation. This treatment applied to the Pd–H system induces a re-ordering of the metal lattice and superstructure lines have been observed by grazing incidence X-ray diffraction. These results are similar to those obtained by very high-pressure hydrogenation of palladium and prompt us to suggest that plasma-based hydrogen implantation is likely to induce superabundant vacancy phase generation.
The temperature dependent phonon resistivity of the nearly stoechiometric PdH compound has been measured. The results can be explained in terms of two phonon branches, an acoustical one primarily determined by the Pd lattice and an optical one determined by the hydrogen lattice. Some implications concerning the occurence of superconductivity in this system have also been drawn.
The large reverse isotope effect in the Pd-H(D) system is confirmed over a range of composition in samples prepared by cathodic charging. It is shown that by shifting the Tc-versus-composition curve for the deuteride by about 0.05 in atomic ratio it can be made to coincide with the data for the hydride. A qualitative explanation is given in terms of proposed differences between the hydride and the deuteride in electronic structure.
Possible new superconducting phases with a high critical transition temperature (T,) have been found in stable palladium-hydrogen (PdHx) samples for stoichiometric ratio x = H/Pd greater than or equal to 1, in addition to the well-known low critical transition temperature (0 less than or equal to T-c less than or equal to 9) when x is in the range (0.75 less than or equal to x less than or equal to 1.00). Possible new measured superconducting phases with critical temperature in the range 51 less than or equal to T-c less than or equal to 295 K occur. This Tc varies considerably with every milli part of x when x exceeds unit. A superconducting critical current density J(c) greater than or equal to 6.1 x 10(4) A cm(-2) has been measured at 77 K with H-DC = 0 T.
This is the answer to the letter of Baranowski and Dębowska [Baranowski and Dębowska, J. Alloys Compd. 437 (2007) L4–L5].
From in situ observation of x-ray diffraction of Ni and Pd under a high hydrogen pressure (approximately = 5 GPa) and temperatures (less than or equal to 800 C), anomalous lattice contraction of the hydride was found to occur in 2 to approximately 3 h. This contraction, amounting to approximately 0.5 angstrom(exp 3) per a metal atom, remained in the recovered specimen even after the hydrogen was removed by heating to 400 C, but was annealed out at 800 C. The concentration of vacancies responsible for this effect is estimated at approximately 20% of metal-atom sites. Anomalous concentration dependence of the hydrogen-induced volume and enhanced diffusion of metal atoms are explained in terms of this effect.
The palladium hydride and deuteride have been investigated in the diamond anvil cell up to 35 GPa using energy dispersive x-ray diffraction method. The search for the neutrons in the case of PdD up to the highest pressure reached was unsuccessful.
The nuclear spin-lattice relaxation rates of 1H and 2D in stoichiometric PdH and PdD indicate a significant increase in the spin density at the deuteron site over that at the proton site. The results are analyzed theoretically in two distinctly different models involving the zero-point vibrations of the hydrogen. The implication of these results for the understanding of the reverse isotope effect on Tc is discussed.
From thermal expansion measurements between 10 and 250K on alpha '-PdHx and alpha '-PdDx with 0.63<or=x<or=0.70 the authors determine the average optical phonon frequencies omega H and omega D of PdHx and PdDx. The average volume Gruneisen parameter of the optical phonons is gamma opt=3.5+or-0.2 and that of the acoustic phonons is gamma ac=2.5+or-0.2. The Debye temperature for the acoustic phonons is equal to that of pure palladium within the experimental uncertainties.
Pd lattice parameter and 57Fe isomer shift (IS) measurements have been correlated in PdHx-Fe matrices (0.3 at.% Fe) over a wide range of hydrogen concentration in the alpha ' (lattice liquid), and in the beta and alpha " phases detected previously by Mossbauer measurements. The authors find that the IS is proportional to the logarithm of the unit cell volume, IS varies as 3 ln a0, just as in IS measurements under pressure. The experimental value of the slope, +3.92 mm s-1, is however much larger than that found by pressure measurements in similar matrices. This shows that the effect in PdHx is of electronic origin, and gives support to the band calculations which have predicted partial localisation of hydrogen electrons in a Pd-H bond.
The effects of hydrogen insertion by electrolytic charging in H2SO4 solution in pure nickel and palladium were investigated. The hydrogenated phases of Ni and Pd and their stability during aging were determined by X-ray diffraction. The NiH0,68 hydride and the α-Ni phase were distinguished by scanning electron microscopy (SEM) operating in the backscattered electrons mode. Interesting features about the intergranular cracks developed during and after charging are discussed.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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