Polycrystalline samples of chromium hydride CrH and deuteride CrD with a hexagonal close-packed (hcp) metal lattice were synthesized at a hydrogen (deuterium) pressure of 7.4 GPa and a temperature of 873 K, rapidly cooled to 100 K, recovered to ambient pressure and studied by differential scanning calorimetry at temperatures from 120 to 220-240 K. The calorimetric study and density functional theory calculations helped to determine the isobaric heat capacity CP of hcp-CrH at temperatures up to 1000 K and to clarify the shape and position of its acoustic phonon band and the coefficient of electronic heat capacity. The investigation of hcp-CrD demonstrated the approximately harmonic change in the energy of its optical vibrations compared with hcp-CrH. Using the obtained CP(T) dependence of hcp-CrH for the calculation of its Gibbs energy gave the line of the Cr + (1/2) H2--CrH equilibrium in the T-P diagram of the Cr-H system. The position of this line corroborated the debatable opinion that the pressure of thermodynamic equilibrium is much closer to the pressure of hydride decomposition than to the pressure of its formation.

Multilayer graphane (hydride of graphite) is a crystalline hydrocarbon of composition CH, which can be synthesized from graphite and molecular hydrogen at pressures above 2 GPa [V.E. Antonov et al. Carbon 100 (2016) 465]. Using X-ray diffraction, this compound was tentatively identified as the "graphane II" phase of 3D-graphane predicted by ab initio calculations [X.-D. Wen et al. PNAS 108 (2011) 6833] and consisting of layers of 2D graphane in the " chair " conformation. When heated in a vacuum, the compound does not form any intermediate hydrocarbons and reversibly decomposes back into graphite and hydrogen at 770-920 K. In the present work, almost single-phase samples of graphite hydride and deuteride were synthesized at 7.4 GPa and 870 K. Their investigation by inelastic neutron scattering supplemented by ab initio calculations gave spectra g ( E ) of the phonon density of states with a gap of about 15 meV at approx. 100 meV, which is a unique identifier for the chair form of graphane. The equation of state V ( P ) of the hydride was studied at room temperature and hydrogen pressures up to 53 GPa by synchrotron X-ray diffraction in a diamond anvil cell. The graphane II phase did not react with the surrounding hydrogen and did not undergo any phase transformations upon the compression and after heating to 1500 K at 53 GPa. The high thermal and pressure stability of this exotic phase makes it an important part of the C-H system. The obtained g ( E ) spectra of graphite hydride and deuteride were used to calculate temperature dependences of their heat capacity. Measurements of the heat capacity at temperatures 120-673 K confirmed the good accuracy of these calculations.

The luminescence and scintillation properties of YAG:Ce crystals grown from the melts in vacuum has been analysed. We have investigated absorption spectra, X-ray excited luminescence (XRL), XRL decay kinetics and scintillation light yield in a wide range of activator concentrations (from 0.0036 at.% to 1.175 at.% substitution of Y in the c-positions of garnet structure). The effective quenching of the intrinsic luminescence of antisite and vacancy defects of the crystal in the UV region with increasing activator concentration has been determined. The optimal concentration of the activator has been determined in order to increase the XRL intensity and the light output of scintillations of Сe3+ ions, taking into account the technological peculiarities of growing optically perfect single crystals with high concentration of Сe3+ ions by using the method of horizontal directional crystallisation in vacuum. The relations between the XRL kinetics and the activator concentration have been investigated. It has showed the possibility to obtain crystals with photon yield up to 25,000 ph/MeV.

Yttrium‒aluminum garnet single crystals, including cerium-doped ones, have been grown from melt by Bagdasarov’s method. A comparative spectroscopy study of garnet single crystals and specially prepared ceramics of the same composition has been carried out. The comparative analysis suggests that an increase in the concentration of cerium ions in the garnet crystals improves the spectral-luminescence and scintillation characteristics of the latter and facilitates effective quenching of the substrate luminescence. Ways of optimizing the synthesis conditions to improve the efficiency of scintillators based on Y _1-x Ce x 3 Al 5 O 12 garnet crystal have been proposed.

Understanding an interrelation between the structure, chemical composition and hydrogenation properties of intermetallic hydrides is crucial for the improvement of their hydrogen storage performance. Ability to form the hydrides and to tune the thermodynamics and kinetics of their interaction with hydrogen is related to their chemical composition. Some features of the metal–hydrogen interactions remain however poorly studied, including chemistry of Sc-containing hydrides. ZrNiAl-type ScNiSn-based intermetallic hydride has been probed in the present work using a broad range of experimental techniques including Synchrotron and Neutron Powder Diffraction, 119Sn Möessbauer Spectroscopy, hydrogenation at pressures reaching several kbar H2 and hydrogen Thermal Desorption Spectroscopy studies. Computational DFT calculations have been furthermore performed. This allowed to establish the mechanism of the phase-structural transformation and electronic structure changes causing a unique contraction of the metal lattice of intermetallic alloy and the formation of the ...H-Ni-H-Ni… chains in the structure with H atoms carrying a partial negative charge. Such hydrogen absorption accompanied by a formation of a covalent Ni-H bonding and causing an unusual behavior contracts to the conventionally observed bonding mechanism of hydrogen in metals as based on the metallic bonding frequently accompanied by a jumping diffusion movement of the inserted H atoms – in contrast to the directional Metal-Hydrogen bonding observed in the present work. At high applied pressures ScNiSnH0.83 orthorhombic TiNiSi type hydride is formed with H atoms filling Sc3Ni tetrahedra. Finally, this study shows that scandium closely resembles the behavior of the heavy rare earth metal holmium.

Zirconium trihydride and, for comparison, trideuteride with hexagonal close-packed (hcp) metal lattices were synthesized at a hydrogen/deuterium pressure of 9 GPa and a temperature of 873 K using toroid-type high-pressure chambers. After a rapid cooling (quenching) to 100 K and lowering the pressure to atmospheric, the quenched sample was removed from the chamber and studied by hot extraction analysis, powder x-ray diffraction, and ac magnetic susceptometry. The hot extraction showed that thermal decomposition of both hcp-ZrH3 and hcp-ZrD3 in vacuum begins after heating at a rate of 10 K/min to similar to 200 K and almost ceases at 270 K, leaving tetragonal epsilon-ZrH2/epsilon-ZrD2 as a result. The x-ray diffraction study at 85 K gave lattice parameters a = 3.443(2) angstrom and c = 5.964(4) angstrom for hcp-ZrH3 and somewhat smaller values of a = 3.430(5) angstrom and c = 5.947(5) angstrom for hcp-ZrD3. The measurements of magnetic susceptibility revealed superconductivity with an onset at 11.6(1) K for hcp-ZrH3 and 9.5(2) K for hcp-ZrD3.

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 shown that the T-x projection of the miscibility gap in solid interstitial MeHx solutions will be symmetric and the pressure vs. temperature dependence of the corresponding isomorphic phase transformation will be determined by the reaction Me + (z/2)H2 = MeHz with a fixed z value attained at T / 0 K, if the mixing Gibbs energy of the solutions is a symmetric two-well function of the H content with a maximum at z/2. Based on these findings and using the literature data for the pure metal Me and the hydride MeHz, it is explained why the experimental dependences of the standard Gibbs energy for the isomorphic transformation between two solid phases of variable composition in the Pd-H and Pd-D systems (z = 0.63) and in the Ni-H and Ni-D systems (z = 1) are close to linear in the entire investigated temperature range. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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.

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.

This paper is a brief summary of the results of long-term experimental studies of C60Hx hydrofullerites with a hydrogen content of up to x ~ 60 or more, obtained by loading C60 fullerites with hydrogen at pressures up to 9 GPa and temperatures up to 500°C. Basically, this is an overview of already published data. Some results for hydrofullerites with compositions of x ~ 60 and x ~ 90 are presented for the first time.

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.

A C15 AB(2) Laves-type Ti0.15Zr0.85La0.03Ni1.2Mn0.7V0.12Fe0.12 alloy was prepared by arc melting and annealing. Phase-structural composition, microstructure, hydrogen absorption-desorption properties, thermodynamic and electrochemical performances were characterized by X-ray diffraction, scanning electron microscopy, hydrogen absorption-desorption measurements and electrochemical characterization and were related to the use of the alloys as metal hydride battery anodes. The alloy contains a C15 FCC intermetallic compound as the main phase and a LaNi secondary phase as the minor constituent (similar to 1 wt%). During the electrochemical tests, the anode electrodes quickly, after just a few activation cycles, reached a maximum discharge capacity. This was related to the catalytic effect of the La-rich secondary phase which acted as a catalyst of hydrogen absorption-desorption. Annealing resulted in increase of the maximum discharge capacity from 345 mAh/g for the as cast alloy to 370 mAh/g. Furthermore, the annealed alloy showed a better high rate dischargeability and a higher cyclic stability. After 100 cycles with 100% DOD at discharge current density of 1C, the discharge capacity of the annealed alloy was very high, at a level of 90% of the initial capacity. The rates of hydrogen diffusion have been characterized by Potentiostatic Intermittent Titration Technique and Electrochemical Impedance Spectroscopy. With increasing an extent of transformation into the hydride, the H diffusion rate in the bulk of the alloy particles decreased. The maximum value of D-H measured by PITT for the annealed alloy was observed for the nearly fully discharged electrode, (SOC 2%). (C) 2019 Published by Elsevier B.V.

Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The “Magnesium group” of international experts contributing to IEA Task 32 “Hydrogen Based Energy Storage” recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures, kinetics and thermodynamics of the systems based on MgH2, nanostructuring, new Mg-based compounds and novel composites, and catalysis in the Mg based H storage systems. Finally, thermal energy storage and upscaled H storage systems accommodating MgH2 are presented.

At room and lower temperatures, hydrogen atoms occupy octahedral (O) interstitial sites in all known monohydrides of d-metals with close-packed metal lattices (fcc, hcp or double hcp) (see [1] and references therein). On the other hand, the ground-state energy of the H atom sitting on an alternative tetrahedral (T) interstitial site is not much higher. This opens the possibility of a partial Boltzmann occupancy of the Tsites at elevated temperatures. A considerable T-occupancy reaching about 1/3 of all D atoms has for the first time been found in fcc deuterides of palladium by in situ neutron diffraction at 310 °C and deuterium pressures up to 9 MPa [2, 3]. A few years later, another in situ ND investigation showed that about 1/6 of D atoms are likely to occupy the T-sites in fcc deuteride of iron at T = 715 °C and P = 6.3 GPa [4]. The subtle difference between the profile fits using the O-model and O+T model was not however sufficient to establish the presence of D atoms on the T-sites with certainty. At the same time, modeling this pattern assuming that D atoms could fill only the O-sites and allowing them to occupy both Oand T-sites gave noticeably different total D/Fe ratios of 0.47 and 0.64, respectively [4]. In order to examine which of the two predicted D/Fe values better agrees with experiment, we constructed an isobar of deuterium solubility in iron at P = 6.3 GPa and temperatures from 100 to 800 °C using a quenching technique [5]. The point (6.3 GPa, 715 °C) chosen for the experiment in Ref. [4] and the temperature interval of the isobar constructed in the present work are shown on the T-P diagram of the Fe-H system (Fig. 1) copied from Ref. [6]. The isobar is presented in Fig. 2. As one can see from Fig. 2, our experiment confirms the value of D/Fe = 0.64(3) resulting from the O+T model [4]. Such a value corresponds to the deuteriuminduced volume expansion of fcc iron dVa/dx = 2.21(4) Ǻ3/atom D [4], and therefore this estimate of Ref. [4] is also confirmed. The isobar in Fig. 2 also demonstrates a step-wise decrease in the deuterium solubility in iron from D/Fe =1 to D/Fe ≈ 0.9 at T0 ≈ 260 °C due to the transition from the low-temperature stoichiometric dhcp (ε′) FeD phase to the high-temperature fcc (γ) Fe-D phase with a variable composition. Since the direct synthesis of single-phase samples of ε′-FeD from α-Fe at a pressure of 6.3 GPa is not possible for kinetic reasons, the points shown by the solid blue triangles were obtained with the samples first transformed to γ-FeDx at 500 °C. Figure 1. T-P diagram of the Fe-H system [4]. α – dilute H solutions in bcc Fe; γ – hydrogen solutions in fcc Fe with the H/Fe ratio varying from x = 0 to x ≈ 1 depending on the temperature and pressure; ε ́ a stoichiometrc hydride FeH with a double hcp metal lattice.

An isobar x(T ) of deuterium solubility in iron is constructed at P = 6.3 GPa and 100 T 800 ◦C based on the results of thermal desorption analysis of FeDx samples produced by quenching under high D2 pressure to the temperature of liquid nitrogen. The experiment confirms the value of x = 0.64 at T = 715 ◦C proposed previously in a neutron diffraction work [Machida et al., Nature Commun. 5, 5063 (2014)] for γ iron deuteride under the assumption that deuterium atoms occupy both octaand tetrahedral interstices in its fcc metal lattice. An estimate of V/x = 2.2 Å/atom D made in that work for the deuterium-induced volume expansion V (x) of fcc iron is also confirmed. To prove that the absorption of protium leads to a similar volume expansion, we constructed an isotherm x(P) of hydrogen solubility in fcc iron at T = 600 ◦C and H2 pressures from 4.3 to 7.4 GPa. The available V (P, T ) data of in situ x-ray diffraction studies of iron hydrides [T. Hiroi et al., J. Alloys Compd. 404–406, 252 (2005); H. Saitoh et al., J. Alloys Compd. 706, 520 (2017)] agree with this isotherm under the assumption that V/x = 2.2 Å/atom H. The transformation between the high-temperature fcc (γ ) and low-temperature dhcp (ε′) deuterides of iron is shown to occur at 260 °C, which is approximately 100 °C lower than the temperature of the γ ↔ ε′ transformation in the Fe-H system at the same pressure of 6.3 GPa.

The possibility of formation of molybdenum and tungsten polyoxides in the Mo–W–Al 2 O 3 –H 2 system at T = 2400 K and P = 1 bar in a controlled Ar + H 2 atmosphere has been investigated by the method of thermodynamic analysis. The formation of polyoxides is found to occur both due to the processes involving Al 2 O 3 melt and in the absence of the latter. It is established that metals (Mo and W) and their mono-, di-, and even trioxides (in the latter case, mediated polymerization occurs) can be used as initial components to form polyoxides. It is shown that polyoxides themselves may interact with one of their main sources: Al 2 O 3 melt.