Trihydride Phase Research Articles

Low-temperature synthesis of hydride semiconductor YH3−δ using Pt capped Y films and its chemical thermodynamics analysis

To decrease the temperature at which metallic yttrium (Y) reacts with H2 to form the semiconductor trihydride phase, we employed Pt capping layers as catalysts, and compared the result with those obtained when employing Pd, Ni, or their co-capping layers. It was found that Pt capping with 5 nm thickness makes trihydride phase majority when hydrogenated at 27°C, and allows us to obtain the trihydride phase with approximately 100% molar concentration when hydrogenated at 150 – 200°C. Chemical thermodynamics analysis of the experimental results reveals that both the enthalpy and entropy differences between pure states of trihydride phase and dyhydride phase with hydrogen vapor show monotonic hydrogenation temperature dependencies with their sign inversions at approximately the same temperature around 150°C, suggesting a particular thermodynamic mechanism specific to the hydrogen – yttrium system.

Cubic erbium trihydride thin films

High-purity, erbium hydride thin films have been deposited onto α-Al2O3 and oxidized Si by reactive sputtering methods. Rutherford backscattering spectrometry and elastic recoil detection show that films deposited at temperatures of 35, 150 and 275°C have a composition of 3H:1Er. Erbium trihydride films consist of a face-centered cubic erbium sub-lattice with a lattice parameter in the range of 5.11–5.20Å. The formation of cubic ErH3 is intriguing, because previous studies demonstrate a single trihydride phase with a hexagonal metal sub-lattice. The formation of a stable, cubic trihydride phase is attributed to a large, in-plane stress resulting from ion beam sputter deposition.

Low temperature specific heat of lutetium trihydride

The low temperature heat capacity measurements for the LuH 2.84 trihydride sample are reported. The analysis of C p ( T) was performed on the basis of the Debye and Einstein approximations. The characteristic temperatures: Θ D = 227 K for Lu atoms, Θ E I = 97 K , Θ E I I = 886 K and Θ E I I I = 1380 K for hydrogen sublattice are estimated. The analysis of the C p ( T) results at the lowest temperatures indicates that the so-called model V of modified P 3 ¯ c 1 crystal lattice symmetry seems to be the best approximation for the crystallographic unit cell of lutetium trihydride. It was found that no conduction electrons contributed to the specific heat. The reported experimental results can be used as reference data for the lattice contribution in the specific heat of all the heavy rare earth trihydride phases. The polynomials function to fit the measured C p values of LuH 2.84 trihydride are given.

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.

In situ impedance study of the Y–Ni thin film under electrochemical hydrogenation

A thin film of yttrium Y (150 nm) protected by a 6 nm coating of nickel Ni on a glass substrate was completely hydrogenated in a 1 M NaOH electrolyte at a constant negative current until the transparent Y tri-hydride phase was achieved and hydrogen gas evolution from the electrode began. A series of impedance measurements were performed in situ during the electrochemical experiment to study the properties of the system as dependent on hydrogenation degree and time of relaxation. The equivalent electrical circuit (EEC) simulations were performed with a Randles-like scheme R 0[ R 1CPE 1], where R 0 is the thin film electrode resistance, R 1 the charge transfer resistance and CPE 1 is the capacitive constant phase element. The behavior of all the components of the EEC undergoes a clear transition when the hydrogenation degree of the electrode is approximated to its maximum value (H to Y ratio 2.7) and electrochemical process changes from hydrogen uptake to hydrogen evolution.

Hydrogen in Rare Earths: A Wealth of Structural and Electronic Phenomena

AbstractFor Abstract see ChemInform Abstract in Full Text.

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.

Electronic structure of the YH3 phase from angle-resolved photoemission spectroscopy.

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on angle-resolved photoemission experiments of the Y trihydride phase. Most importantly, we find the absence of metal d bands at the Fermi level and a set of flat, H-induced bands located at much higher binding energy than predicted, indicating an increased electron affinity at H sites.

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.

Magic-angle spinning NMR study of deuterium site occupancy and dynamics inZrNiD1.0andZrNiD3.0

Both static and magic-angle spinning (MAS) ${}^{2}\mathrm{H}$ nuclear magnetic resonance spectroscopy indicate that two inequivalent sites are occupied in $\mathrm{ZrNi}{D}_{3.0},$ in a 2:1 ratio, in agreement with previous work. The sites occupied in the trihydride phase have been previously identified as ${\mathrm{Zr}}_{3}\mathrm{Ni}$ and ${\mathrm{Zr}}_{3}{\mathrm{Ni}}_{2}.$ In $\mathrm{ZrNi}{D}_{1.0},$ two well-resolved lines of equal intensity are observed in the MAS spectrum at $\mathrm{temperatures}<220\mathrm{}\mathrm{K},$ indicating that two other inequivalent sites are occupied in a 1:1 ratio, in contrast with previous reports that only one type of site $({\mathrm{Zr}}_{4}{\mathrm{Ni}}_{2})$ is occupied in the monohydride (\ensuremath{\beta}) phase at room temperature and above. The temperature dependences of both MAS and static $\mathrm{ZrNi}{D}_{1.0}$ spectra indicate that no phase transitions occur over the entire temperature range studied, 160--500 K. The deuterium hopping rate in $\mathrm{ZrNi}{D}_{1.0}$ is determined from characteristic changes in the MAS spectra as a function of temperature; the motion is consistent with an activation energy ${E}_{a}\ensuremath{\cong}0.44\mathrm{eV}$ assuming a rate prefactor $1/{\ensuremath{\tau}}_{0}\ensuremath{\sim}{10}^{13}{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ In $\mathrm{ZrNi}{D}_{3.0},$ the rate of deuterium motion is determined from signatures of motion in the static and MAS spectra. The motion in $\mathrm{ZrNi}{D}_{3.0}$ is consistent with an activation energy ${E}_{a}\ensuremath{\cong}0.62\mathrm{eV}$ assuming a rate prefactor of $\ensuremath{\sim}{10}^{13}{\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$

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.

Ion beam analysis of oxidation and hydrogenation of switchable mirrors

Thin films of Y, La or rare earth (RE) hydrides exhibit a metal insulator transition between their di- and trihydride phase. After H loading lateral diffusion samples of these materials contain an overview of all hydride phases present in the thin film phase diagram. In this paper the thin film YH x system will be investigated. The unexpected presence of a lateral oxygen profile in the YH x sample necessitates a careful interpretation of local hydrogen concentration differences. In this paper the potential of ion beam analysis will be discussed with respect to the investigation of oxidation and hydrogenation of YH x switchable mirrors. The results of the measurements will be discussed in terms of differences between bulk- and thin-film-phase diagrams of YH x . The experimental methods used are 1H ( 4He, 1H) 4He elastic recoil detection at 5 MeV and 16O( 4He, 4He) 16O resonant backscattering around 3.036 MeV.

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.

Preparation and characterization of clean, single-crystalline YHx films (0⩽x⩽2.9) on W(110)

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the preparation of clean, single-crystalline YHx films (0⩽x⩽2.9). The films have been characterized in situ combining angle-resolved photoelectron spectroscopy (ARPES) and low energy electron diffraction. Direct Y dihydride growth, i.e., Y evaporation under a H2 partial pressures of ≈5×10−6 mbar at 500 K on W(110), is the most convenient starting point for the preparation of clean single-crystalline Y hydride films covering H concentrations from the “clean metal” (x≈0) up to the lower boundary of the pure trihydride phase (x≈2.9). Upon annealing Y dihydride films the desired H concentration can be adjusted within the α-phase or the (α+β) two-phase regime. On the other hand, the extension of our photoelectron spectrometer with an homemade ultrahigh vacuum (UHV) compatible hydrogenation system allows to induce the transition from Y dihydride to Y trihydride within a few minutes. The hydrogenation system combines a high-pressure reaction cell with hydrogen permeation through a Pd–24%Ag tube. The overall design is such that the sample never gets in contact with non-UHV compartments. For direct Y dihydride growth on W(110) two equally populated face-centered- cubic(111) domains rotated by 180° with respect to each other are observed. In the α- and γ-phase the Y atoms form a hexagonal-close-packed(0001) oriented lattice. Furthermore, the previously established model for in situ H concentration estimation in Y [J. Hayoz et al., Phys. Rev. B 58, R4270 (1998)] is extended successfully from the α to β to the β to γ-phase transition. Ultraviolet photoemission spectroscopy data unequivocally reveal the opening of a gap extending as far as 1 eV below EF for normal electron emission upon the phase-transformation from Y dihydride to Y trihydride. It also appears that the H absorption rate strongly depends on the H2 purity. Our experimental results demonstrate the capability of this setup for in situ preparation and investigations on the geometrical and electronic structure of Y hydride films and, more generally, rare-earth hydride films using ARPES.

Isotope Effects in Switchable Metal-Hydride Mirrors

Measurements of optical reflectance, transmittance, and electrical resistivity on the switchable mirror systems ${\mathrm{YH}}_{x}$ and ${\mathrm{YD}}_{x}$ show that the absorption of hydrogen induces the same variations as that of deuterium. In both cases there is a weak transparency window for the metallic dihydride (dideuteride) phase and a yellowish transparency in the insulating trihydride (trideuteride) phase. The slightly higher electrical resistivity of the deuterides is related to the lower energy of their optical phonons. The absence of significant isotope effects in the optical properties of ${\mathrm{YH}}_{x}({\mathrm{YD}}_{x})$ is at variance with Peierls-like theoretical models. It is, however, compatible with strong electron correlation models.

Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect

In contrast to the binary switchable mirror films (YHx, LaHx, REHx with RE:rare earth) which have a weak red transparency window in their metallic dihydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resistivity measurements on a series of Y1−yMgyHx, films we show that this is due to a disproportionation of the alloy. While the yttrium dihydride phase is formed, Mg separates out, remaining in its metallic state. Upon further loading, insulating MgH2 is formed together with cubic YH3−δ. In this way Mg acts essentially as a microscopic optical shutter, enhancing the reflectivity of these switchable mirrors in the metallic state and increasing the optical gap in the transparent state.

Hydrogen in thin epitaxial metal films and superlattices: structure, magnetism, and transport

Hydrogen in thin epitaxial films and superlattices has gained renewed interest because of the hydrogen-induced novel optical and magnetic switching properties that have recently been unraveled. In this contribution we present new results on the effect of hydrogen on the structural properties of epitaxial Y-films and the H–D isotope exchange in the trihydride phase. We also report on magneto-optical Kerr effect measurements of the magnetic hysteresis in Fe (3)/V ( x) superlattices as a function of the hydrogen pressure. Here x refers to the number of V monolayers varying from 11 to 16 and the Fe layer thickness fixed at three monolayers. Without hydrogen the superlattice is antiferromagnetic (AFM) for x between 12 and 14 and ferromagnetic (FM) in all other cases. With hydrogen loading the coupling can be switched from AFM to FM and vice versa.

Hydrogen and deuterium in epitaxial Y(0001) films: Structural properties and isotope exchange

Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator transitions including switchable optical properties from reflecting to transparent in the visible region. We report on the structural properties of hydrogenated and deuterated thin, monocrystalline Y(0001) films grown by molecular beam epitaxy on Nb/${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates. X-ray diffraction reveals the response of the host metal lattice upon hydrogen loading. The structural coherence in all three spatial directions as well as the epitaxial relation to the substrate are maintained, though the sample undergoes structural phase transitions between the different hydride phases. With neutron reflectivity measurements we have determined the hydrogen and deuterium content since the critical angle for total reflection depends on their concentration in the sample. Measurements on hydrogenated and deuterated films show that H and D are completely interchangeable within the trihydride phase. Neutron scattering also allows us to determine the position of the deuterium atoms within the yttrium matrix. All structural information gained on thin films is in agreement with the space group $P3\ifmmode\bar\else\textasciimacron\fi{}c1$ which was previously determined from powder samples.

Logarithmic Divergence of the Electrical Resistivity in the Metal HydrideYH3−δ

As reported earlier, the metal hydride YH{sub x} reveals spectacular switchable optical properties. In this Letter we report on another remarkable property of the hydrogen deficient YH{sub 3{minus}{delta} } trihydride phase. A logarithmic temperature dependence of the electrical resistivity is observed within a large temperature range (20{le}T{le}200 K ) for hydrogen deficiencies 0.01{lt}{delta}{lt}0.15 . This dependence may be related to two dimensional weak localization or Kondo scattering of the conduction electrons. {copyright} {ital 1997} {ital The American Physical Society}