Rare Earth Hydrides Research Articles

Hydrogen-desorbing behavior of the hydrides in 15 rare earth elements measured by temperature swing column chromatography

Hydrogen-desorbing behavior of the hydrides in 15 rare earth elements (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) was examined by temperature swing column chromatography from room temperature to 1273 K. As a result, it was found that hydrogen-desorbing profiles of the rare earth hydrides except Eu had two peaks. The first peaks were located on low-temperature zone of about 600–700 K, the second peaks were located on high-temperature zone of about 700–900 K, and the peaks were separated. The cause of separation was the bonding type between hydrogen and metals, not from hetero-reaction of dehydrogenation. The bonding between hydrogen and metals seemed to be two types. In addition, the first peaks had two types, sharp one and broad one. The first peaks of Y and heavy rare earth Tb–Lu showed sharp one and those of light rare earth La, Ce, Pr, Nd showed broad one. Sm, Eu and Gd showed complex hydrogen-desorbing behavior. Molar ratio of the first bonding hydrogen atom dissociated at low-temperature zone to the second bonding hydrogen atom dissociated at high-temperature zone was approximately 1–2.

Prediction of thermodynamic stability and electronic structure of novel ternary lanthanide hydrides

We theoretically examine four hypothetical ternary lanthanide hydrides, CsLnIIH3 and Cs2LnIIH4, where LnII = Yb, Tm. We optimize their crystal unit cells in the BaTiO3 and K2NiF4 structures, respectively, and compute their electronic band structures. Our calculations indicate that the novel hydrides should be unstable with respect to decomposition into binaries (CsH and LnIIH2); ternaries would form only under elevated pressure (>7–22 GPa). We predict that significant perturbation of the electronic and magnetic properties of CsYbIIH3 and Cs2YbIIH4 will take place via a progressive exchange of f14 YbII for f13 TmII, while promoting magnetic ordering and valence fluctuations. Analysis of the phonon dispersion for these hydrides suggests that metallic forms of doped CsLnIIH3 and Cs2LnIIH4 would exhibit substantially high Debye temperatures of ∼1800 K. This is likely to prompt moderate-TC superconductivity in these as yet unknown materials.

High-pressure synthesis of novel hydrides in Mg–RE–H systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, Dy)

The high-pressure synthesis of new hydrides of Mg–RE–H systems, where RE = Y, La, Ce, Pr, Sm, Gd, Tb and Dy, were conducted by using a cubic-anvil-type apparatus, and their crystal structure, thermal stabilities and hydrogen contents were investigated. In Mg–Y–H system, newly found MgY 2H y with a FCC-type structure has been prepared. In MgH 2– x mol% REH (REH = LaH 3, CeH 2.5 and PrH 3), new hydrides with primitive tetragonal structure were synthesized around x = 25–33 under GPa-order high pressures. The lattice constants were a = 0.8193 nm, c = 0.5028 nm, a = 0.8118 nm, c = 0.4979 nm and a = 0.8058 nm, c = 0.4970 nm at x = 25 in Mg–La, Ce and Pr systems, respectively. The hydrogen contents of the novel compounds were 4.1, 3.7 and 3.9 mass% in Mg–La, Ce and Pr systems, respectively, and the chemical formulae were found to correspond to Mg 3LaH 9, Mg 3CeH 8.1 and Mg 3PrH 9. The new hydrides decomposed into Mg and rare-earth hydride at about 600 K (Mg 3LaH 9: 614 K, Mg 3CeH 8.1: 609 K, Mg 3PrH 9: 630 K) with an endothermic reaction. In MgH 2– x mol% hREH (hREH = GdH 3, TbH 3 and DyH 3), new hydrides with FCC-type structure were synthesized around x = 67.

Synthesis and Crystal Structure of New Hydrides in Mg-RE Systems under High-Pressure (RE = La, Ce, Pr)

The high-pressure synthesis of new hydrides of Mg-RE-H systems, where RE = La, Ce and Pr, were conducted by using a cubic-anvil-type apparatus, and their crystal structure, thermal stabilities and hydrogen contents were investigated. In MgH2-xmol%REH (REH = LaH3, CeH2.5 and PrH3), new hydrides with primitive tetragonal structure were synthesized around x = 25 - 33 under GPa-order high pressures. The lattice constants were a = 0.8193 nm, c = 0.5028 nm, a = 0.8118 nm, c = 0.4979 nm and a = 0.8058 nm, c= 0.4970 nm at x = 25 in Mg-La, Ce and Pr systems, respectively. The hydrogen contents of the novel compounds were 4.1 mass%, 3.7 mass% and 3.9 mass% in Mg-La, Ce and Pr systems, respectively, and the chemical formulas were found to correspond to Mg3LaH9, Mg3CeH8.1 and Mg3PrH9. The new hydrides decomposed into Mg and rare-earth hydride at about 600 K (Mg3LaH9: 614 K, Mg3CeH8.1: 609 K, Mg3PrH9: 630 K) with an endothermic reaction.

Post‐Metallocene Hydridolanthanide Chemistry: [Lu{(Me3Si)2NC(NiPr)2}2(μ‐H)]2 — A Novel Lanthanide Hydride in a Non‐Cyclopentadienyl Coordination Environment; Synthesis, Structure and Catalytic Activity in Olefin Polymerization

AbstractThe reaction of anhydrous LuCl3 with a twofold molar excess of lithium guanidinate [Li{(Me3Si)2NC(NiPr)2}], obtained in situ from the amide [LiN(SiMe3)2(Et2O)] and 1,3‐diisopropylcarbodiimide in THF, yields the bis(guanidinate) “ate” complex [Lu{(Me3Si)2NC(NiPr)2}2(μ‐Cl)2Li(THF)2] (1). The alkyllutetium derivative [Lu{(Me3Si)2NC(NiPr)2}2(CH2SiMe3)] (2) was synthesized by a metathesis reaction of 1 with LiCH2SiMe3 in toluene. The treatment of 2 with an equimolar amount of PhSiH3 in hexane at room temperature results in the first known dimeric lanthanide hydride in a bis(guanidinate) coordination environment, [Lu{(Me3Si)2NC(NiPr)2}2(μ‐H)]2 (3). The crystal structures of complexes 1 and 3 have been determined by X‐ray crystallography. The hydrido complex 3 was found to catalyze polymerization of ethylene, propylene and styrene. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Electrochemically induced switching in SmHx thin films

Discovery of optical and electrical switching in yttrium and lanthanum hydride thin films in 1996, has triggered large scale investigations of rare earth hydride thin films. Both light and heavy rare earths have been shown to exhibit this so called ‘switchable mirror’ effect. Besides gas phase loading, solid as well as liquid electrolytes have been used as means of hydrogen loading. In this paper, we report on the ability of samarium films to switch between the reflecting and transparent states electrochemically by altering the hydrogen content. Samarium films of typical thickness 55nm deposited by vacuum evaporation and covered with Pd overlayer of thicknesses 5, 8 and 11nm were loaded with hydrogen and deloaded in a 1M KOH solution galvanostatically at room temperature. Our studies have shown that thickness of palladium overlayer plays the most crucial role in observing fast transition between the as deposited metallic and the semiconducting nearly samarium trihydride states as well as to obtain a very high optical contrast. The large reversible change in transparency between di and trihydride states suggests possibilities of use of samarium films as an optical shutter as well as a hydrogen sensor material.

Magnetic Monitoring of the Disproportionation Behavior in Sm–Fe-Based Materials

Two Sm-Fe-based alloys, with compositions Sm/sub 13.7/--Fe/sub 86.3/ and Sm/sub 13.8/--Fe/sub 82.2/--Ta/sub 4.0/, were investigated in order to determine their disproportionation reaction kinetics. The alloys were first heated in hydrogen at a rate of 4/spl deg/C/min to a temperature where the disproportionation reaction starts. At this onset temperature, which is different for the individual material, the alloy begins to decompose into a rare-earth hydride and /spl alpha/Fe. The disproportionation behavior of the systems can be explained in terms of the Johnson-Mehl-Avrami (JMA) theory of phase transformations. The ternary alloy with Ta satisfies the JMA conditions and has Avrami constants of /spl les/0.5, which implies diffusion-controlled one-dimensional growth, i.e., the thickening of particles with an appreciable initial volume. On the other hand, the growth of a newly formed iron phase in the binary alloy can only be explained in terms of mixed-transformation process involving one-dimensional and two-dimensional growth.

Rare Earth Alkyl and Hydride Complexes Bearing Silylene‐Linked Cyclopentadienyl‐Phosphido Ligands. Synthesis, Structures, and Catalysis in Olefin Hydrosilylation and Ethylene Polymerization.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Rare earth alkyl and hydride complexes bearing silylene-linked cyclopentadienyl-phosphido ligands. Synthesis, structures, and catalysis in olefin hydrosilylation and ethylene polymerization

A series of silylene-linked cyclopentadienyl-phosphido rare earth alkyl and hydride complexes of type Me2Si(C5Me4)(PR′)LnR (Ln=Y, Yb, Lu; R′=Ph, Cy, C6H2tBu3-2,4,6; R=CH2SiMe3, H) have been synthesized and structurally characterized, and their activity in ethylene polymerization and olefin hydrosilylation has been studied. These complexes represent the first examples of rare earth alkyl and hydride complexes bearing cyclopentadienyl-phosphido ligands, which are in sharp contrast both structurally and chemically with the analogous cyclopentadienyl-amido and metallocene complexes.

Magnetic polarization of the La and Ce5dstates near the interfaces ofFe/LaHxandFe/CeHxmultilayers across the metal-insulator transition in the hydrides: An x-ray magnetic circular dichroism study

Rare-earth hydrides $R{\mathrm{H}}_{x}$ show a metal-to-insulator transition for x between 2 and 3. In an ionic picture strong Coulomb interactions between the electrons on H sites are responsible for opening up a gap of \ensuremath{\sim}2 eV between the valence bands derived from $R\mathrm{H}$ and H-H hybridization and a set of bands of predominantly R-metal d character. We have studied the magnetic polarization near the interfaces of $\mathrm{Fe}/R{\mathrm{H}}_{x}$ multilayers $(R=\mathrm{La},$ Ce) across the metal-to-insulator transition in the hydrides by measurements of x-ray magnetic circular dichroism (XMCD) at the $R{L}_{2,3}$ edges. The mean Fe-induced magnetic polarization of the R $5d$ states is considerably reduced in the insulating phase but remains finite. We attribute this to the presence of $5d$ states induced into the energy gap of the insulator sublayers by Fe, as they result from recent calculations of the electronic structure of ferromagnet/insulator interfaces. Variation of the $R{\mathrm{H}}_{x}$ sublayer thickness reveals that the $5d$ polarization decays exponentially away from the interface, on a length scale of about 10 \AA{} into the volume of the $R{\mathrm{H}}_{x}$ sublayers, both in the metallic and insulating phase. To our knowledge this is the first experimental observation that metal-induced gap states evanescent into the interior of an insulator may be spin polarized. The identical decay length in both $R{\mathrm{H}}_{x}$ phases, independent of the R element, is remarkable. The $R{L}_{2,3}$ XMCD spectra themselves reveal the complex interplay between the magnetic polarization by Fe $3d$ and R $5d$ hybridizations, and the R $4f$ magnetic moment. In fact, they are not only related to the magnetic $5d$ polarization in the ground state, but are largely controlled by the exchange interaction between the $2p$ core level and the spin polarized $5d$ band and, in the case of Ce, by the difference between the radial parts of the $2p$-to-$5d$ matrix element for the $5d$ majority and minority spin channels, resulting from the $4f\ensuremath{-}5d$ exchange interaction. It induces a drastic modification of the line shape and even a change in sign when the samples are cooled to low temperature or oriented under different angles with respect to the beam. We present a detailed discussion within a simple phenomenological model.

The properties of pulsed laser deposited YH2 films for switchable devices

In-situ deposited YH2 films ranging from epitaxial to nanocrystalline were prepared by pulsed laser deposition (PLD). PLD has the advantage that the yttrium target acts as a hydrogen source. Comparing the switching behavior to that of MBE-grown, ex-situ loaded films, the PLD-films show a reduced optical and electrical contrast in switching, which is probably due to the formation of Y(OH)3. On the other hand, we found that in the nanocrystalline PLD-films the hysteresis in switching is absent. Moreover, these films have a smoother surface morphology. Both effects are important for the implementation of switchable rare earth hydride films in all-solid-state devices.

Dimeric n-Alkyl Complexes of Rare-Earth Metals Supported by a Linked Amido−Cyclopentadienyl Ligand: Evidence for β-Agostic Bonding in Bridging n-Alkyl Ligands and Its Role in Styrene Polymerization

The dimeric rare-earth hydrides [Ln(η5:η1-C5Me4SiMe2NCMe3)(THF)(μ-H)]2 (Ln = Y, Yb) react with excess α-olefin H2CCHR (R = Et, nPr, nBu) in a 1,2-insertion to give the series of THF-free dimeric n-alkyl complexes [Ln(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2R)]2 as isolable crystals. Single-crystal X-ray diffraction studies on the five derivatives [Y(η5:η1-C5Me4SiMe2NCMe2R‘)(μ-CH2CH2R)]2 (R‘ = Me, R = Et, nBu; R‘ = Et, R = Et, nPr) and [Yb(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2nBu)]2 revealed that the centrosymmetric dimeric complexes consist of two trans-arranged [Ln(η5:η1-C5Me4SiMe2NCMe2R‘)] fragments connected by two μ-alkyl ligands. Most strikingly, there is an agostic interaction of the n-alkyl groups' β-CH2 hydrogen atoms with the formally 12-electron lanthanide metal center. Variable-temperature NMR spectroscopic data suggest a fluxional process that interconverts the diastereotopic protons of the α-CH2 group and a dynamic β-agostic interaction. Addition of >10 equiv of THF per yttrium to a solution of [Y(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2Et)]2 results in the formation of the highly reactive, nonisolable, monomeric THF adduct [Y(η5:η1-C5Me4SiMe2NCMe3)(CH2CH2Et)(THF)]. Reaction of 1,2-dimethoxyethane (DME) with [Y(η5:η1-C5Me4SiMe2NCMe3)(μ-CH2CH2Et)]2 forms the crystalline compound [Y(η5:η1-C5Me4SiMe2NCMe3)(CH2CH2Et)(DME)] with a terminal n-butyl group that contains a slightly distorted α-carbon atom according to a crystallographic study. α-Olefins having two or more substituents on the γ-carbon do not react with the hydride complexes. The role of these n-alkyl complexes in the controlled polymerization of styrene is discussed.

Preparation of Lanthanide Hydrides of Nanometric Size by the Catalytic Method

AbstractRare‐earth metals were hydrogenated in the presence of TICI4 catalyst in tetrahydrofuran (THF) at 45 °C under normal pressure. Transmission electron micrographs showed that the resulting lanthanide hydrides were in the form of nanoparticles. The rate of hydrogenation decreased with increasing atomic number of the rare‐earth elements.

Ultraviolet Triggered Switchable Mirrors

Trivalent rare earth hydrides stabilized in thin film form demonstrate spectacular optical and electronic properties. A thin film of YH_x or LaH_x can be transformed rapidly from metal to insulator, from shiny mirror to transparent window, simply by changing the surrounding hydrogen gas pressure or an electrolytic cell potential at room temperature (RT). At low temperatures, in-situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination permits tuning through the T = 0 metal–insulator transition and reveals the important role played by strong electron correlations. We discuss the optical, electronic, magnetic, and structural properties of switchable mirrors from both the technological and scientific perspectives.

DFT study of CH4 activation by d0 Cl2LnZ (Z = H, CH3) complexes

DFT(B3PW91) calculations of the activation of CH4 by models (Cl2LnZ) of Cp*2LnZ (Z = H, Me) have been carried out for the entire lanthanide series. Cl2LnZ appears to be a good model for Cp*2LnZ. It reproduces well the coordination around the lanthanide. The energetics of the transformation X2LnH + CH4 → X2LnCH3 + H2 are fairly close for X = Cl and Cp and the difference in behavior can be attributed to the stronger electron donating ability of Cp. Formation of the lanthanide hydride complex is calculated to be exothermic in agreement with experimental evidence. The energy profiles of the reactions Cl2LnH + CH4 → Cl2LnCH3 + H2; Cl2LnH* + CH4 → Cl2LnH + H*CH3; Cl2LnCH*3 + CH4 → Cl2LnCH3 + H–CH*3 have been calculated. The transition states for the first and third transformations are energetically accessible, in good agreement with the known experimental data. The second reaction has a transition state of very high energy indicating an unfeasible reaction. The geometry of the transition stuctures are suggestive of a proton transfer between two anionic species (Z and CH3−; Z = H− and CH3−) in the field of the lanthanide fragment.

Design and operational conditions of small electrochemical cell for optical switching employing hydrogenation of Pd/Y structure

A reversible optical transition in rare-earth metal hydrides can take place in a gas phase (when H 2 is injected into the chamber with rare-earth metal film and pumped out periodically) as well as in an electrolyte solution (when the electrode of active metal is polarised cathodically and depolarised). The liquid hydrogenation process, though more reliable from technological point of view, still presents some undesirable side effects and inferior longevity due to high reactivity of metals and their hydrides towards electrolyte solutions and enhanced corrosion. In this paper, we present our results on electrochemical behaviour of the Y/Pd cathode and its reversible optical switching paying attention to some common problems that might affect the device work and even cause its destruction. Based on these data, we have discussed a possible design and optimum operational conditions of the planar electrochemical cell: type of electrolyte used in the cell (liquid, solid or polymeric); role and design of the anodic electrode and the anodic process accomplished on it; sealing problems due to the necessity to obtain a hermetically closed cell with a liquid electrolyte inside and a possible gas evolution under the operation.

Low-temperature specific heat of non-stoichiometric β-ytterbium deuteride

Heat capacity measurements on a β-ytterbium deuteride sample with the composition D/Yb=2.71 have been carried out over a temperature range from 1.9 to 260 K. Two anomalies were observed at 4 K and 230 K, respectively. On the grounds of the separated Schottky contribution to the specific heat, the influence of the crystal field on the splitting of the (2J+1)-fold degenerate ground state J=7/2 of the Yb3+ ion is analysed. The existence of two sublattices (sort of an electronic phase separation) of hydrogen atoms arranged by the two different ytterbium atom valencies is suggested. The sharp discontinuity in the specific heat at 230 K in YbD2.71 resembles that observed in the light rare earth hydrides REHx in the composition range 2.7<x<2.92 and attributed to a metal-semiconductor transition. X-ray diffraction measurements in the range 50–300 K indicate that the anomaly in Cp(T) at 230 K is accompanied by the probable tetragonal distortion of the cubic structure at low temperatures. The metal-insulator/semiconductor transformation is likely induced when the deuterium/hydrogen atoms order in the structure.

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.

Light-induced metal-insulator transition in a switchable mirror.

Rare earth hydride films can be converted reversibly from metallic mirrors to insulating windows simply by changing the surrounding hydrogen gas pressure at room temperature. At low temperatures, in situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination offers an attractive possibility to tune yttrium hydride through the T = 0 metal-insulator transition. Conductivity and Hall measurements are used to determine critical exponents. The unusually large value for the product of the static and dynamical critical exponents appears to signify the important role played by electron-electron interactions.

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.