A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of 580∘C using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group Pm−3n, Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a=5.510(5)Å at T=85 K. The hydrogen content determined by hot extraction was H/Ta=1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps—first, ΔH/Ta=0.2 was desorbed at around −70∘C, and then the rest of the hydrogen was desorbed between 100∘C and 390∘C. The A15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A15−TaH1.23(5) demonstrated that hydrogen atoms occupy the 24k and 16i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24k and an increase of the 16i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A15−TaH1.23(5) at 72, 135, 145, and 166meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24k and 16i sites, respectively. No superconductivity was found in A15−TaH1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K. Published by the American Physical Society 2024

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of 580 degrees C using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group Pm-3n, Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a = 5.510(5) & Aring; at T = 85 K. The hydrogen content determined by hot extraction was H/Ta = 1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps-first, AH/Ta = 0.2 was desorbed at around -70 degrees C, and then the rest of the hydrogen was desorbed between 100 degrees C and 390 degrees C. The A15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A15-TaH1.23(5) demonstrated that hydrogen atoms occupy the 24k and 16i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24k and an increase of the 16i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A15-TaH1.23(5) at 72, 135, 145, and 166 meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24k and 16i sites, respectively. No superconductivity was found in A15-TaH1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K.

Due to the small mass and anomalously large neutron scattering cross-section of proton (about 80 barns compared to a few barns for other nuclei), inelastic neutron scattering is considered as one of the most effective tools in studying optical vibrations of hydrogen atoms in metal hydrides. The current review is focused on the binary hydrides of 3d-and 4d-metals of groups VI-VIII, which were produced at high hydrogen pressures of several gigapascals in relatively large quantities of hundreds of mg, quenched to low temperature and studied by INS ex situ at ambient pressure with high statistical accuracy. One of the unusual effects revealed by INS is a strong increase in the strength of the metal-hydrogen interactions with decreasing atomic number of the d-metal accompanied by an increase in the Me-H distance. Based on the available experimental results, the spectra g(E) of the phonon density of states and temperature dependencies C-V(T) of the heat capacity at constant volume at T up to 1000 K have been derived in this paper and presented both in the figures and in digital form. This provides the reference data for the theoretical investigations of the crystal structures and compositions of new practically important hydrides giving the opportunity to validate calculation methods by comparing the calculated g(E) and C-V(T) with the accurate experimental dependencies for the binary hydrides. Recent INS studies showed [R.A. Klein et al., J. Alloy. Compd. 894 (2022) 162381] that the fingerprints of anomalously short H-H separations of 1.6 & ANGS; violating the "2 & ANGS; rule " can be easily and unambiguously identified in the complex INS spectra of quaternary hydrides (La,Ce)NiInH1+x. This makes neutron spectroscopy an attractive means for obtaining valuable data in the search for novel hydrides with a record high hydrogen capacity. (C) 2022 The Authors. Published by Elsevier B.V. CC_BY_4.0

It is currently known that three hydrides - PdHx, MoHx, and TiHx - exhibit an inverse isotope effect in superconductivity. Namely, the phase with a heavier hydrogen isotope, deuterium, has a higher critical temperature. Hydrides and deuterides of palladium have intensively been studied both experimentally and theoretically, but the origin of the isotope effect has not been established with certainty. The commonly accepted explanation is that the effect is likely to be due to the strong anharmonicity of the optical hy-drogen vibrations, which was considered to be responsible for the large deviation of the ratio of the fun-damental optical frequencies omega(H)/omega(D) = 1.51 from the harmonic value root 2 approximate to 1.41. In the present paper, powder samples of MoH1.1(1) and MoD1.07(3) were synthesized under a hydrogen/deuterium pressure of several gigapascals and studied by inelastic neutron scattering (INS) at ambient pressure and T = 10 K. The INS study demonstrated that optical vibrations of H atoms in MoH1.1 and D atoms in MoD1.07 are harmonic and the ratio of fundamental optical frequencies omega(H)/omega(D) = 1.44 is close to the harmonic value root 2 approximate to 1.41. This shows that anharmonicity is not a necessary condition for the presence of the inverse isotope effect. The MoD1.07 sample was additionally studied by neutron diffraction (ND) at ambient pressure and T = 100 K. In agree-ment with previous ND results for MoH1.2, the ND study of MoD1.07 showed that deuterium atoms occupied almost all octahedral interstitial sites in its hexagonal close-packed metal lattice and formed a NiAs-type crystal structure with the composition close to MoD. The overstoichiometric composition MoD1.07 of the deuteride is likely to result from a small fraction D/Mo similar to 0.07 of deuterium atoms partially occupying the tetrahedral interstices. (C) 2021 Elsevier B.V. All rights reserved.

While most of the rare-earth metals readily form trihydrides, due to increased stability of the filled 4f electronic shell for Yb(II), only YbH2.67, formally corresponding to YbII(YbIIIH4)2 (or Yb3H8), remains the highest hydride of ytterbium. Utilizing the diamond anvil cell methodology and synchrotron powder X-ray diffraction, we have attempted to push this limit further via hydrogenation of metallic Yb and Yb3H8. Compression of the latter has also been investigated in a neutral pressure-transmitting medium (PTM). While the in situ heating of Yb facilitates the formation of YbH2+x hydrides, we have not observed clear qualitative differences between the systems compressed in H2 and He or Ne PTM. In all of these cases, a sequence of phase transitions occurred within ca. 13–18 GPa (P3̅1m–I4/m phase) and around 27 GPa (to the I4/mmm phase). The molecular volume of the systems compressed in H2 PTM is ca. 1.5% larger than of those compressed in inert gases, suggesting a small hydrogen uptake. Nevertheless, hydrogenation toward YbH3 is incomplete, and polyhydrides do not form up to the highest pressure studied here (ca. 75 GPa). As pointed out by electronic transport measurements, the mixed-valence Yb3H8 retains its semiconducting character up to >50 GPa, although the very low remnant activation energy of conduction (<5 meV) suggests that metallization under further compression should be achievable. Finally, we provide a theoretical description of a hypothetical stoichiometric YbH3.

Powder samples of TaH0.89 and TaD0.96 are synthesized under a hydrogen (deuterium) pressure of 2.8 GPa and a temperature of 250 C-circle, then quenched to the liquid nitrogen temperature, recovered to ambient pressure and studied by neutron diffraction (ND) and inelastic neutron scattering (INS). The ND study shows that both hydrogen and deuterium atoms occupy tetrahedral interstitial sites in a distorted body centered cubic (bcc) crystal structure of metal atoms, while the ordering scenarios in TaH0.89 and TaD0.96 are different. Hydrogen and deuterium atoms are ordered in a layered fashion, forming long period superstructures with space groups P (4) over bar and P222, respectively, so that the unit cells of the p ffiffi p ffiffi p ffiffi p ffiffi crystal structures of TaH0.89 and TaD0.96 are root 2 x root 2 x 7 and root 2 x root 2 x 8 supercells of the initial cubic unit cell. The INS study demonstrates a pronounced "soft" (trumpet-like) anharmonicity of the potential well for H and D atoms. (c) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Tellurium trioxide, TeO3, is the only example of a trioxide adopting at ambient conditions the VF3-type structure (a distorted variant of the cubic ReO3 structure). Here we present a combined experimental (Raman scattering) and theoretical (DFT modelling) study on the influence of high pressure (exceeding 100 GPa) on the phase stability of this compound. In experiments the ambient-pressure VF3-type structure (R3̄c symmetry) is preserved up to 110 GPa. In contrast, calculations indicate that above 66 GPa the R3̄c structure should transform to a YF3-type polymorph (Pnma symmetry) with the coordination number of Te6+ increasing from 6 to 8 upon the transition. The lack of this transition in the room-temperature experiment is most probably connected with energetic barriers, in analogy to what is found for compressed WO3. The YF3-type phase is predicted to be stable up to 220 GPa when it should transform to a novel structure of R3̄ symmetry and Z = 18. We analyse the influence of pressure on the band gap of TeO3, and discuss the present findings in the context of structural transformations of trioxides and trifluorides adopting an extended structure in the solid state.

Hydrogen sorption in urea C(NH2)2O has been probed by direct measurements in Sievert’s apparatus at 7.23 and 11.12 MPa as well as by Raman spectroscopy for the sample compressed and heated in a high-pressure gas-loaded diamond-anvil cell up to 14 GPa. Both these methods consistently indicate the occurrence of small nonstoichiometric sorption of hydrogen in urea phase I. The compression of urea in hydrogen affects the Raman shifts of the C–N bending mode δ and the stretching mode υs. The sorption affects the H2 vibron position too. The sorption of 1.3 × 10–2 at 11.12 MPa corresponds to a stochastic distribution of H2 molecules in channel pores of urea. The mechanism leading to this stochastic sorption involves strong correlations between the swollen nanodot regions around the pores accommodating H2 molecules and the squeezed neighboring pores too narrow to act as possible sorption sites. This study on the hydrogen-bonded framework (HOF) of urea marks the smallest pores capable of absorbing hydrogen documented so far. This observation also reveals a new class of compounds, which is located between those that absorb large stoichiometric amounts of certain guest molecules and those that do not absorb them at all, namely, the group of compounds that absorb the guests in a stochastic manner.

A single-phase sample of $\mathrm{Ta}{\mathrm{H}}_{2.2(1)}$ with a hexagonal close-packed metal lattice (hcp; space group $P{6}_{3}/mmc$) was synthesized under a hydrogen pressure of 9 GPa and a temperature of 100 \ifmmode^\circ\else\textdegree\fi{}C; quenched to the liquid nitrogen temperature; recovered to ambient pressure and studied by neutron diffraction (ND) and inelastic neutron scattering (INS). The ND study showed that hydrogen atoms occupied one half of the tetrahedral (T) and all octahedral (O) interstitial sites in the hcp lattice of Ta atoms. The arrangement of the H atoms over the T sites was proven to be ordered, which lowered the symmetry of the full crystal structure of the dihydride to $P{6}_{3}mc$. Due to the resulting asymmetry in the local environment of the O sites, the H atoms were considerably displaced from the centers of these sites along the $z$ axis, away from the H atoms occupying the neighboring T sites. The INS study demonstrated that the potential wells for H atoms at both the T and O sites are highly anharmonic and anisotropic. The potential wells at the O sites are softer along the $z$ axis than in the $x, y$ plane, while the T sites show opposite anisotropy.

The discovery of superconductivity at 203 K in H3S brought attention back to conventional superconductors whose properties can be described by the Bardeen-Cooper-Schrieffer (BCS) and the Migdal-Eliashberg theories. These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals possessing certain favorable parameters such as lattice vibrations at high frequencies. However, these general theories do not suffice to predict real superconductors. New superconducting materials can be predicted now with the aid of first principles calculations based on Density Functional Theory (DFT). In particular, the calculations suggested a new family of hydrides possessing a clathrate structure, where the host atom (Ca, Y, La) is at the center of the cage formed by hydrogen atoms. For LaH10 and YH10 superconductivity, with critical temperatures Tc ranging between 240 and 320 K is predicted at megabar pressures. Here, we report superconductivity with a record Tc ~ 250 K within the Fm-3m structure of LaH10 at a pressure P ~ 170 GPa. We proved the existence of superconductivity at 250 K through the observation of zero-resistance, isotope effect, and the decrease of Tc under an external magnetic field, which suggests an upper critical magnetic field of 120 T at zero-temperature. The pressure dependence of the transition temperatures Tc (P) has a maximum of 250-252 K at the pressure of about 170 GPa. This leap, by ~ 50 K, from the previous Tc record of 203 K indicates the real possibility of achieving RTSC (that is at 273 K) in the near future at high pressures and the perspective of conventional superconductivity at ambient pressure.

The reaction of ZrH2 and HfH2 with molecular hydrogen was studied by in situ X-ray diffraction in a diamond anvil cell at room temperature at high pressures of up to 39 GPa. The formation of novel zirconium and hafnium hydrides was observed at 8.2(5) and 12.4(5) GPa, respectively. These new hydrides had the cI16-type structure of a metal lattice, identical to that of Th4H15, and the compositions of new hydrides were estimated to be Zr4H15 and Hf4H15 on the basis of their lattice volumes. The decomposition of these hydrides back into ZrH2 and HfH2 during decompression was observed at 3.5(3) and 3(1) GPa, respectively. Electrical resistance measurements indicated that Hf4I-H-15 is a superconductor with T-c approximate to 4.5 K at 23 GPa.

The stability field of the C0 hydrogen hydrate in the pressure–temperature phase diagram of the water–hydrogen system is studied by volumetry under hydrogen excess. This stability field is confined by the sII hydrate stability field at the low-pressure side; by the liquid stability field at the high-temperature side; and by the C1 hydrate stability field at the high-pressure side. The pressure of the C0 ↔ C1 phase equilibrium is temperature-independent in the studied temperature range from −20 to +18 °C. The corresponding equilibrium line in the phase diagram terminates at the nonvariant quadruple point “C0 hydrate + C1 hydrate + liquid H2O + H2 gas” (or Q3) at 7.7(3) kbar and +22(2) °C. The volume change accompanying the C0 → C1 phase transition, and the dTdPH2 slopes of the melting lines of the C0 and C1 hydrates are measured by volumetry. Thermodynamic considerations are used to estimate the hydrogen content H2/H2O ≈ 0.53(5) and 0.23(5) of the C0 and C1 hydrates, respectively, near the Q3 point.

Electrical resistivity and Raman spectra of ErH3 were studied in a diamond anvil cell under high pressure up to 140 GPa in the temperature range 4–300 K. A crossover from a semiconductor-like to a metallic temperature dependence of resistivity at fixed pressures was observed at about 50 GPa. In the pressure range 80–140 GPa a resistivity maximum was observed at the R(T) dependencies. The temperature corresponding to this maximum linearly increased with pressure increase, reaching 26 K at 140 GPa. No superconductivity was observed in the studied pressure-temperature range.

Phase stability of lanthanum, gadolinium and erbium trihydides have been studied at high pressure in diamond anvil cell. Raman high pressure studies up to 40 GPa of lanthanum trihydride and deuteride have shown more complex spectra than one can expect from the factor group analysis for stoichiometric LaH3. Our studies have shown that the stoichiometric lanthanum trihydride and deutride undergo pressure induced phase transformation presumably of hydrogen ordering origin. Investigations of the phase stability of erbium and gadolinium trihydrides have been performed using argon as pressure transmitting medium in Raman scattering method. The same medium has been used for X-ray diffraction analysis under pressure for erbium trihydride. Significant difference in the transition pressure as compared to earlier results obtained in quasi hydrostatic conditions has been noticed both in X-ray and Raman scattering measurements. Deviatoric stresses present in earlier nonhydrostatic measurements can be blamed for its discrepancy. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

Synchrotron X-ray and Raman scattering investigations have been performed on dysprosium trihydride at high pressure up to 40 GPa. Structural phase transformation from original hexagonal to cubic phase has been observed confirming predictions from other studies and theoretical calculations. Dysprosium trihydride is the last one that has not been studied so far among the heavier lanthanide trihydrides family. These studies completed the overall picture of hexagonal to cubic phase transition for the whole ReH3 compounds. Equation of state and Gruneisen mode parameters have been calculated. High pressure Raman scattering performed on dysprosium deuteride unveiled significant deviation from the harmonic approximation in specific modes most probably due to the anharmonicity of hydrogen potential in the hydride lattice. (C) 2017 Elsevier B.V. All rights reserved.

The reaction of tantalum with molecular hydrogen was studied by x-ray diffraction in a diamond-anvil cell at room temperature and pressures from 1 to 41 GPa. At pressures up to 5.5 GPa, a substoichiometric tantalum monohydride with a distorted bcc structure was shown to be stable. Its hydrogen content gradually increased with the pressure increase, reaching H/Ta = 0.92(5) at 5 GPa. At higher pressures, a new dihydride phase of tantalum was formed. This phase had an hcp metal lattice, and its hydrogen content was virtually independent of pressure. When the pressure was decreased, the tantalum dihydride thus obtained transformed back to the monohydride at P = 2.2 GPa. Single-phase samples of tantalum dihydride also were synthesized at a hydrogen pressure of 9 GPa in a toroid-type high-pressure apparatus, quenched to the liquid-N-2 temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with a = 3.224(3) and c = 5.140(5) angstrom at T = 85 K. The hydrogen content determined by thermal desorption was H/Ta = 2.2(1).

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 angstrom(3) 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 angstrom(3) 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. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Ruthenium hydride was synthesized at a hydrogen pressure of about 14 GPa in a diamond-anvil cell. Energy-dispersive x-ray diffraction was used to monitor the ruthenium crystal structure as a function of hydrogen pressure up to 30 GPa. The hydride formation was accompanied by phase transition from the original hcp structure of the pristine metal to the fcc structure. Our results confirmed the theoretical prediction of ruthenium hydride formation under hydrogen pressure. The standard Gibbs free energy of the ruthenium hydride formation reaction was calculated assuming the pressure of decomposition as the equilibrium pressure.

The molybdenum-hydrogen system was studied in a diamond anvil cell at high hydrogen pressure up to 30 GPa at room temperature by X-ray diffraction. At pressure around 4 GPa a phase transformation was observed of a bcc metal to a hydride with a hcp metal lattice and H/Mo approximate to 1.1. Further hydrogen pressure increase resulted in a continuous increase of the hydrogen content of the hydride. At about 15 GPa the hydrogen content reached saturation, and no further hydrogen absorption occured up to the maximal reached pressure. The saturation composition H/Mo = 1.35 (10) was estimated from volumetric considerations. (C) 2016 Elsevier B.V. All rights reserved.