Dihydride Phase Research Articles

Phase transformations in Ti2Cu under destructive hydrogenation and recombination

The thermodynamic possibility and conditions of Ti2Cu hydrogenation and recombination are assessed. The mechanism of Ti2Cu interaction with hydrogen between 293 and 973 K at pressure 1.0 MPa is studied. The interaction of Ti2Cu with hydrogen is thermodynamically possible in the destructive hydrogenation region over a wide temperature range. X-ray diffraction shows that destructive hydrogenation products are titanium dihydride and copper-rich intermetallic phases and copper depending on reaction conditions. According to the established mechanism of Ti2Cu destructive hydrogenation, hydrogen selectively interacts with titanium leaving Ti2Cu composition as follows: β-TiHy (bct-bcc) → TiH1.92 (fcc). The initial Ti2Cu composition is recombined from destructive hydrogenation products (titanium dihydride and copper) in vacuum and hydrogen. The minimum temperature of Ti2Cu vacuum recombination is 1050 K and ensures complete TiH2 decomposition. The recombination of Ti2Cu in hydrogen can occur at temperatures above 907 K.

Volume dependent vibrational properties of cerium hydrides from first principles

We have performed an ab initio study of structural, volume-dependent elastic and lattice dynamical properties of rare-earth metal-hydrides CeH 2 and CeH 3. The calculations have been carried out within the density functional theory and linear response formalism using norm-conserving pseudopotentials and a plane-wave basis. The hydrogen incorporation into the octahedral sites of the cubic CeH 2 to obtain CeH 3 leads to stiffening of the lattice as indicated by a volume reduction and an up to 60% increase of the elastic constants of the trihydride compared to that of CeH 2. The phonon density of states has been found to show well separated and narrow two (centered at 15 and 110 meV) and three (centered at 15, 60 and 120 meV) groups of vibrations for CeH 2 and CeH 3, respectively. The transverse acoustic mode Grüneisen parameters of the dihydride phase are found to be negative in the most part of the Brillouin zone which leads to lattice dynamical unstability above a pressure of 14 GPa as indicated by imaginary phonon frequencies.

Hydrogen Ordering and Metal-Semiconductor Transitions in Rare-Earth Hydrides

AbstractAfter an introduction to the rare earth – hydrogen phase diagram, stressing the often broad existence range of the solid solution (α), dihydride (β) and trihydride (γ) phases, we are describing in detail the fluorite-type dihydride and its superstoichiometric composition, RH2+x, where the x atoms occupy the available octahedral interstitial sites. It is shown how these additional x atoms interact with each other to form ordered H superlattices (sometimes distorting the cubic CaF2structure) and how the latter influences the electronic structure of the systems modifying the magnetic properties and/or leading to metal-semiconductor transitions.

Optical properties of Pr thin film and nanoparticle layers during hydrogen loading and deloading

Changes in the optical properties of Pd capped Pr thin film and nanoparticle layers prepared by vacuum evaporation and inert gas evaporation techniques, respectively, have been studied as a function of time during hydrogen loading and deloading. These samples were characterized by transmission electron microscopy, atomic force microscopy, x-ray diffraction, and spectrophotometery techniques. Absorption spectra of Pr thin film and nanoparticles samples in metal, and hydrogen loaded and deloaded states have been reported. It is observed that filling of octahedral sites in the dihydride phase results in changes in absorption coefficient values at 1.5–2.0eV and H content in the trihydride phase causes shift of the absorption edge. Enhanced surface area, loose topography, a larger number of interparticle boundaries due to small sizes, relatively loose adhesion to the substrate, and smaller structural changes in nanoparticle layers result in larger and faster changes in the optical properties during loading in comparison to thin film.

Synthesis of the dihydride phase of vanadium

There are two constituents that determine success in synthesizing vanadium hydride: activation and supply of hydrogen to vanadium at an appropriate pressure. The most efficient activation has been found to occur, depending on the vacuum produced in a facility, in temperature ranges between 550 and 750 K or above 850 K. Dependencies of equilibrium sorption pressures of protium and deuterium over respective dihydride phases of vanadium are provided. At an atomic ratio of 0.85 ≤ X/V ≤ 1.5 (X is protium or deuterium), they take the following form, respectively, log P ( Pa ) = − 2160 / T + 12.8 and log P ( Pa ) = − 2480 / T + 13.6 . At higher hydrogen concentrations, coefficients in the Van’t Hoff equation are observed to depend on the hydrogen content. Based on the obtained experimental data on equilibrium sorption pressures and available literature data on isotopic effects, the same dependence of sorption pressure has been calculated for vanadium ditritide. For T/V = 0.85 , it is expressed as: log P ( Pa ) = − 2490 / T + 13.4 . Dependencies of equilibrium sorption pressures for all the three hydrogen isotopes at temperatures between 273 and 323 K and ratios of 0.85 ≤ ( H, D or X ) / V ≤ 1.9 are presented in the form of diagrams.

Instability of the Si(1 0 0) dihydride phases accompanied by molecular emissions under pulsed-H irradiation

We have measured the transient desorption of HD and D 2 molecules from D/Si(1 0 0) surfaces during pulsed H dosing for the surface temperatures from 443 to 593 K. Contrary to the direct D abstraction by H to generate HD desorptions, the adsorption-induced-desorption (AID) of D 2 takes place preferentially on the 3 × 1 dideuteride domains, with four transient desorption components characterized with ⩽0.005, ≃0.06, ≃0.8 s, and ≃30 s lifetimes. It is considered that the excess dihydrides (dideuterides) generated at the 3 × 1 local domains or at their boundaries may receive a thermal instability towards molecular emissions, either promptly returning to the original 3 × 1 phase or enlarging the dihydride domain area. The long ≃30 s lifetime component could be attributed to the molecular emissions that occur when the 3 × 1 dideuteride domains reduce their size at off-cycle of the H beam.

Thermodesorption of Vanadium-Hydride-Based Hydrogen Isotope Sources

Hydrogen isotopes sources with pressure up to several thousand atmospheres are required to pursue some investigations in nuclear and thermonuclear processes. Investigations are performed in mixtures of all three hydrogen isotopes. Stringent requirements are placed on such sources. Primarily, this is reliability and safety, purity of supplied gas, possibility of smooth pressure control, minimal dimensions and the ease to operate. Vanadium-base thermodesorption sources meet these requirements. Literature data on equilibrium pressures of hydrogen isotopes desorption over vanadium dihydride phase have been reviewed. It has been shown that in data analysis, gas nonideality at high pressure should be taken into account. In particular, when showing temperature dependencies of desorption equilibrium pressures it is reasonable to use appropriate value of fugacity. The paper presents temperature dependencies of protium and deuterium fugacity over corresponding dihydride vanadium phases in the range of 300-635K, determined using experimental data of the authors. Temperature dependence of tritium fugacity over dihydride vanadium phase at 273-483K determined using experimental data of the authors and available literature data are also presented. A series of vanadium-base thermodesorption high-pressure hydrogen isotopes sources have been developed in RFNC-VNIIEF using of the obtained dependencies. Schemes of sources for research in fundamental science are presented.

Fermi-surface topology of rare-earth dihydrides

We report high-resolution angle-resolved photoemission experiments on epitaxial thin films of different rare-earth (RE) dihydrides $(\mathrm{RE}=\mathrm{Gd},\mathrm{La})$ and of $\mathrm{Y}{\mathrm{H}}_{2}$ and $\mathrm{Sc}{\mathrm{H}}_{2}$. It is found through ab initio calculations and confirmed by Fermi surface mapping that the electronic structure becomes very similar upon hydrogenation, rendering the studied dihydrides isoelectronic. We propose that the dihydride phase acts as a common precursor state for the formation of the insulating trihydride phase. For states with higher binding energies (which exhibit considerable H character) the agreement between calculation and measurement is less convincing. Independent of the difficulties to describe these hydrogen related states, we note in the comparison between experiment and calculation a very convincing description of the Fermi surface for the dihydrides. Therefore we trace the apparent inability of density, functional theory to describe the hygrogenation up to the trihydride phase to an insufficient description of hydrogen states in general and, in particular, involving octahedral sites.

Modulated hydrogen beam study of adsorption-induced desorption of deuterium from Si(100)-3x1:D surfaces.

We have studied the kinetic mechanism of the adsorption-induced-desorption (AID) reaction, H+D/Si(100) --> D2. Using a modulated atomic hydrogen beam, two different types of AID reaction are revealed: one is the fast AID reaction occurring only at the beam on-cycles and the other the slow AID reaction occurring even at the beam off-cycles. Both the fast and slow AID reactions show the different dependence on surface temperature Ts, suggesting that their kinetic mechanisms are different. The fast AID reaction overwhelms the slow one in the desorption yield for 300 K < or = Ts < or = 650 K. It proceeds along a first-order kinetics with respect to the incident H flux. Based on the experimental results, both two AID reactions are suggested to occur only on the 3x1 dihydride phase accumulated during surface exposure to H atoms. Possible mechanisms for the AID reactions are discussed.

D abstraction by H on Si( [formula omitted]) surfaces

We studied H-induced abstraction (ABS) and adsorption-induced-desorption (AID) on the D/Si(1 1 1) surface for various initial D coverages θ 0 D and surface temperatures T s. As θ 0 D was increased, the kinetics of HD ABS was found to switch from the first- to second-order rate law with respect to θ D. D 2 AID appeared to obey a nearly third-order rate law in θ D. The hot atom mechanism was ruled out for both ABS and AID. We found that AID is associated with β 2 temperature-programmed-desorption, i.e. D 2 AID proceeds as a spontaneous desorption of deuterium from a di-deuteride or di-hydride phase formed by H atoms. Rate equations for HD and D 2 desorptions at 620 K were proposed on the basis of the observed rate laws. The apparent activation energy of D 2 AID was evaluated to be ∼0.16 eV for T s⩽650 K. Taking steady state adsorption and desorption into consideration, the origin of the third-order rate law as well as the low activation energy for AID was ascribed to the process of sticking by H atoms rather than the process of D 2 desorption. An isotope effect on both ABS and AID was recognized in the rate curve analysis. The hot complex-mediated reaction model was invoked to explain the complicated kinetic phenomena appeared in ABS and AID on the Si(1 1 1) surface.

Kinetics of the Electrochemical Loading of Hydrogen into Thin Yttrium Films Covered by Palladium

The behavior of a thin-film yttrium electrode (150-200 nm) capped by a palladium layer (20 nm) toward electrochemical hydrogen uptake was studied in 1 M NaOH electrolyte under galvanostatic conditions. The system is of interest for smart optical windows/mirrors in devices with changeable transparency. The electrochemical behavior of the Y/Pd electrode depended on many factors, e.g., whether the loading is performed on a fresh or used electrode, current density, presence of reducible components in the electrolyte, time of spontaneous open-circuit potential relaxation and/or anodic discharge. Residual hydrogen kept by the system and its interaction with some electrolyte components is a reason for this behavior. A decisive role of the palladium layer in electrochemical kinetics was elucidated for the first time. We assume that the kinetics of the hydrogen uptake by underlying yttrium as well as the corresponding hydrogen release process is controlled by diffusion from/through hydrogenated palladium. The constant potential hydrogenation process realized at 0.045 V vs. the reversible hydrogen electrode (RHE) was proposed to be the α-β Pd hydride transition. Formation of the β-Y dihydride phase is completed on the way to the −0.095 V potential level (vs. RHE) and is irreversible. In contrast, the β-γ Y hydride transition realized at this potential level is reversible and could be repeated many times. The last potential drop to −0.345 V (vs. RHE) is a hydrogen evolution process and, simultaneously, a metal-semiconductor (and optical) transition in the phase. In a discharge process, a β-Y dihydride of different stoichiometry and α-Pd hydride are probably the only species that could be obtained. The kinetics depends extremely on the discharge conditions (open-circuit relaxation or anodic polarization), and the process shows two potential levels. Under anodic hydrogen release, surface Pd oxidation could also take place. We have proposed and discussed a scheme for hydrogenation/release in the Y/Pd electrode under different experimental conditions. © 2002 The Electrochemical Society. All rights reserved.

Energetics of the dihydride phases on the diamond (100) surface

It was previously claimed that the herringbone and lean-to dihydride structures possessing a $(2\ifmmode\times\else\texttimes\fi{}1)$ symmetry are more stable than other suggested dihydride structures on the C(100) surface, so that the $(2\ifmmode\times\else\texttimes\fi{}1)$ patterns frequently observed in low-energy electron diffraction (LEED) experiments at low temperatures are not sufficient to distinguish between the monohydride and dihydride phases on C(100). To clarify this situation, we investigate the herringbone and lean-to structures using the pseudopotential plane-wave method. Contrary to the claim, we find that these two $(2\ifmmode\times\else\texttimes\fi{}1):2\mathrm{H}$ structures on C(100) are higher in energy than the canted $\mathrm{C}(100)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1):2\mathrm{H}$ structure. For completeness, these two structures are also considered for the Si(100) case. Our calculations show that for both the C and Si cases, the canted $(100)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ dihydride phase is the lowest in energy among the dihydride structures considered so far. Therefore, the $(2\ifmmode\times\else\texttimes\fi{}1)$ LEED patterns can be assigned to the bare (100) surface or the monohydride (100) surface, while the $(1\ifmmode\times\else\texttimes\fi{}1)$ symmetry can be associated with the dihydride (100) surface.

Local switching in epitaxialYHxswitchable mirrors

All the essential switchable mirror properties of ${\mathrm{YH}}_{x}$ exhibit a strong hysteresis that is related to the structural \ensuremath{\beta}-dihydride to \ensuremath{\gamma}-trihydride phase transition. This structural transition in ${\mathrm{YH}}_{x}$ has so far been difficult to investigate since polycrystalline ${\mathrm{YH}}_{x}$ films are optically homogeneous on the length scale of visible light wavelengths. However, in epitaxial ${\mathrm{YH}}_{x}$ films the relation between local structural changes and local optical transmission can be investigated using in situ atomic force microscopy and optical microscopy, in hysteretic yttriumhydride mirrors exhibiting the earlier reported, domainwise pixel switching. We find widely varying local switching kinetics during the whole hydrogenation process. As a result, an epitaxial mirror forms essentially an ensemble of individual microcolumns, each of which can be in three regimes: (i) a single-phase regime of the \ensuremath{\beta} dihydride phase, where the optical transmission lowers with increasing hydrogen concentration, (ii) a layered two phase configuration in which the optical transmission rises with the fraction of high-concentration \ensuremath{\gamma} phase, and (iii) a single-\ensuremath{\gamma}-phase regime in which the transmission still increases significantly. The effect of a wide distribution of local properties on the global mirror properties are discussed within a simple model.

ANGLE-SCANNED PHOTOEMISSION ON YbHx: RELEVANCE FOR SWITCHABLE MIRRORS

Yttrium, lantanum and rare earth elements can be loaded with hydrogen inducing a metal–insulator transition and giving rise to optical switching from reflecting to transparent. We present angle-scanned photoemission experiments characterizing thin YbH x films grown on W(110) at room temperature. Hydrogen loading is performed in an ultrahigh-vacuum-compatible high pressure (1 bar) reaction cell. Via full-hemispherical X-ray photoelectron diffraction and low energy electron diffraction, it is demonstrated that these films grow well-ordered and single-crystalline. Ultraviolet photoemission reveals a gap for the dihydride phase confirming a transition from reflecting to transparent as seen by visual inspection. Ion implantation through additional H+ sputtering allows one to increase the hydrogen content to x ≈ 2.4.

The initial stage of the hydriding of gadolinium metal at 100°C and sub-ambient pressure

The development of di- and trihydride phases on gadolinium metallic samples during the initial stage of the hydriding reaction at 100°C and under 0.5 atm of hydrogen was studied using combined microscopic examination, X-ray diffraction (XRD), and hydrogen absorption measurements. Samples were exposed to hydrogen for various times and the amount of hydrogen absorbed was measured. The reaction was then stopped and the samples were removed and examined microscopically to determine the degree of hydride development on the samples surfaces. XRD measurements were performed on the partially hydrided samples. The sequential formation of GdH 2 and GdH 3 was identified for the first time. The topochemical development of the hydride phases on gadolinium is discussed, based on the experimental results. It was found that following a short nucleation and growth of the dihydride phase, at a relatively early stage of the hydriding reaction, the trihydride is formed on top of the dihydride growing nuclei, so that when the hydride layer formation on the sample is completed, it is already constructed of both hydrides.

Variations of the infrared transmission properties with the metal–insulator transition in thin films of the yttrium-hydride system

This work investigates the variations of the infrared transmission spectra of yttrium-hydride films YHx during the hydrogen loading process for the frequency range 500–4000 cm−1. The results indicate that the transmittance slightly decreases in the dihydride phase, followed by a significant increase in the trihydride phase. In addition, the carrier concentration decreases, whereas the carrier relaxation time increases with hydrogen content. The hydrogen vibration modes at interstitial sites are completely screened in the dihydride phase. The screening effect decreases as the system goes through the metal–insulator transition. Moreover, the screening effect can be continuously tuned by simply varying the hydrogen content in the yttrium-hydride system. Analysis indicates that the absorption intensity of the vibration mode depends on the carrier concentration. This effect can be used as a diagnostic tool for estimating the carrier concentration and hydrogen content in rare-earth hydrides.

High-Pressure Hydrogen Isotopes Sources Based on Vanadium Hydride

Equilibrium pressures of protium and deuterium desorption over a two-phase area of monohydride-vanadium dihydride were measured. Temperature measurement range was 300–635 K and the pressure range 1–500 MPa. Obtained temperature-dependences of fugacities within the given measurement range are: lgf(MPa) = −2152/T+ 6.6 and lgf(MPa) = −2575/T + 7.4 for protium and deuterium, respectively. The values of enthalpy and entropy for vanadium dihydride phase formation were calculated from obtained relations. Using expressions obtained for fugacities and literature data on hydrogen imperfection the pressures, which can be obtained with vanadium dihydride employed in thermodesorption hydrogen sources, were estimated. Taking into account that due to deterioration in strength properties of the used structural materials, the heating temperature of the load-bearing body is limited to ∼973 K, maximal calculated pressure, which can be obtained with such sources as ≈1820 MPa for protium and ≈2220 MPa for deuterium.

Role of hydrogen for adsorption and diffusion of a Si adatom on monohydride and dihydride Si(0 0 1) surfaces

We investigate adsorption and diffusion of a Si adatom on monohydride and dihydride Si(0 0 1) surfaces using first principles calculations. We find that adsorption and diffusion of a Si adatom on hydrogenated Si(0 0 1) surfaces are drastically altered from those on a clean surface in several ways. Both positions and number of adsorption sites vary with different coverages of hydrogen, leading to completely different diffusion behavior from the clean surface. The Si adatom is stabilized by capturing surface hydrogen without potential barrier during adsorption. The H atom hops back and forth exothermally to the adatom during diffusion and thus, the diffusion species is no longer Si adatom itself. It reveals different diffusion features that typical potential energy surface with a single adatom fails to describe. Differences in the adsorption and diffusion of Si adatom between monohydride and dihydride phases are discussed. The role of hydrogen as a surfactant on hydrogenated surface will be further discussed based on our calculations.

Temperature dependence of atomic hydrogen-induced surface processes on Ge(100): Thermal desorption, abstraction, and collision-induced desorption

The temperature effect on the atomic hydrogen-induced surface processes on Ge(100) has been studied from a desorption point of view. The experiments are carried out for temperature-programmed-desorption (TPD) of H2 and D2 and collision-induced desorption (CID) of D2 and abstraction of HD in the reaction system H(g)+D/Ge(100). The D2 and H2 TPD spectra exhibit two clear peaks, assigned as β1 and β2 TPD arising from a monohydride and a dihydride phase, respectively. There are isotope effects on the TPD spectra; D2 TPD peaks shift to higher surface temperature (Ts) compared to the peaks of H2, and the ratio of the β2 to the β1 TPD peak intensity is smaller for H2 than for D2. It is found that the kinetics of the abstraction and CID reactions are similar to those on Si(100), indicating that the mechanism for the abstraction and CID on Ge(100) is same to that on Si(100). The observed D2 rate curves show up a strong Ts dependence. The CID of D2 versus Ts curve exhibits a peak at Ts≃480 K corresponding to the leading edge of the β2TPD spectra. For Ts⩾530 K, CID of D2 is fully replaced by the spontaneous desorption ascribed to the β1 TPD. This fact suggests that the mechanism of CID is same to that of the β2 TPD. For Ts∼480 K, the D2 rate curves can be fitted with a fourth-order kinetics in a momentary D adatom coverage. The transiently created dihydride species that are considered to be mobile across the surface via a dihydride–monohydride isomerization reaction to exchange their sites, are invoked to explain the observed fourth-order kinetics. Probing H atoms reveals that the thermal desorption from the isolated dideuterides is not allowed, suggesting that for the β1 TPD the concerted desorption from hydrogen-prepared Ge dimers is preferred to the desorption from isolated dihydrides.

Rate equations for collision-induced desorption and abstraction in the reaction system H(g)+D/Si(100)→D2,HD at 573 K

Collision-induced desorption (CID) and Eley–Rideal (ER) type abstraction have been investigated in the reaction system, H(g)+Dad/Si(100)→D2,HD, at surface temperature of 573 K where dihydride phase is unstable. Rate equations for CID were obtained based on a second-order kinetics with respect to doubly occupied Si dimers DSi–SiD, DSi–SiH, and HSi–SiH, which are considered as parent species of transiently formed dihydrides, DSiD, DSiH, and HSiH, respectively. The coverages of the doubly occupied dimers were determined as a function of H exposure time by means of temperature-programmed-desorption (TPD), and then the rate equations were calculated. As a result, the experimental D2 and HD rate curves were reasonably fit with the rate equations formulated. This fact indicates that CID is induced when two dihydride species encounter during their propagation via dihydride–monohydride isomerization reactions. It was found that HSi–SiH does not play a role in CID of both D2 and HD. Possible origins of this isotope effect were discussed in terms of quantum effects on associative desorption and diffusion processes. The HD rate curve due to direct ER abstraction channel was fit with a first-order kinetics in Dad coverage for a low coverage regime where dangling bonds exist. However, for a high coverage regime where the surface is saturated, the HD rate curve was fit with DSi–SiD coverages, suggesting that Dad that are paired up with Had in the same Si dimers are not abstracted.