Rare Earth Trihydrides Research Articles

Deforming lanthanum trihydride for superionic conduction.

With strong reducibility and high redox potential, thehydride ion (H-) is a reactive hydrogen species and an energy carrier. Materials that conduct pure H- at ambient conditions will be enablers of advanced clean energy storage and electrochemical conversion technologies1,2. However, rare earth trihydrides, known for fast H migration, also exhibit detrimental electronic conductivity3-5. Here we show that by creating nanosized grains and defects in the lattice, the electronic conductivity of LaHx can be suppressed by more than five orders of magnitude. This transforms LaHx to a superionic conductor at -40 °C with a record high H- conductivity of 1.0 × 10-2 S cm-1 and a low diffusion barrier of 0.12 eV. A room-temperature all-solid-state hydride cell is demonstrated.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

Stabilization of Face-Centered Cubic High-Pressure Phase of REH3 (RE = Y, Gd, Dy) at Ambient Pressure by Alkali or Alkaline-Earth Substitution.

Heavy rare-earth trihydrides such as GdH3, DyH3, and yttrium trihydide (YH3) usually show a hexagonal crystal structure under ambient pressure. This structure is known to transform to a face-centered cubic (FCC) one at higher pressures (at the order of GPa), and the FCC one returns to the hexagonal structure when the applied pressure is released. In this study, we investigated the structure of alkaline or alkaline-earth (A)-substituted REH3 (RE = Y, Gd, Dy; A = Li, K, or Mg) using X-ray diffraction, and measured the phase transition pressure. We found that this FCC high-pressure phase can be stabilized by 10-33 mol % A substitution for RE in the REH3. The mechanism of phase stabilization is simply explained by the ionic radius ratio between the cation and anion ( rcat/ rani), as well as the stabilities of other ionic crystals such as perovskite materials. For all considered REH3 samples, the FCC phase becomes stable when rcat/ rani > 0.856, such as in the case of 10 mol % Li-substituted YH3.

Synergism of Rare Earth Trihydrides and Graphite in Lithium Storage: Evidence of Hydrogen-Enhanced Lithiation.

The lithium storage capacity of graphite can be significantly promoted by rare earth trihydrides (REH3 , RE = Y, La, and Gd) through a synergetic mechanism. High reversible capacity of 720 mA h g-1 after 250 cycles is achieved in YH3 -graphite nanocomposite, far exceeding the total contribution from the individual components and the effect of ball milling. Comparative study on LaH3 -graphite and GdH3 -graphite composites suggests that the enhancement factor is 3.1-3.4 Li per active H in REH3 , almost independent of the RE metal, which is evident of a hydrogen-enhanced lithium storage mechanism. Theoretical calculation suggests that the active H from REH3 can enhance the Li+ binding to the graphene layer by introducing negatively charged sites, leading to energetically favorable lithiation up to a composition Li5 C16 H instead of LiC6 for conventional graphite anode.

Phase stability of some rare earth trihydrides under high pressure

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.

Crossing-line-node semimetals: General theory and application to rare-earth trihydrides

Multiple line nodes in energy-band gaps are found in semimetals preserving mirror-reflection symmetry. We classify possible configurations of multiple line nodes with crossing points (crossing line nodes) under point-group symmetry. Taking the spin-orbit interaction (SOI) into account, we also classify topological phase transitions from crossing-line-node Dirac semimetals to other topological phases, e.g., topological insulators and point-node semimetals. This study enables one to find crossing-line-node semimetal materials and their behavior in the presence of SOI from the band structure in the absence of SOI without detailed calculations. As an example, the theory applies to hexagonal rare-earth trihydrides with the HoD3 structure and clarifies that it is a crossing-line-node Dirac semimetal hosting three line nodes.

Pressure effects on hydrogen atoms near the metal plane in the HCP phase of rare-earth metal trihydrides

Abstract Rare-earth metal trihydrides, REH3 (RE=Sc, Y, La), in the hcp phase were investigated under high pressure by the ab initio method. We concentrated on the behavior of hydrogen atoms which is affected by pressure. Two-thirds of the hydrogen atoms near the metal plane (Hm) were found to displace away from the metal plane as pressure increases. The trajectory of these squeezed hydrogen atoms is from a site near the metal plane, and moves past the plane of the tetragonal sites, and heads toward the nearest octahedral site. However, the rate of displacement depends on the local environment. LaH3 exhibits the least impediment on the Hm displacement while YH3 and ScH3 exhibit stronger impediment. Furthermore, our calculated Raman and IR active modes are in general agreement with the experimental data. The displacement of Hm can be used to explain the behavior of the Ov peak in Raman spectra, where it exists at low pressure and disappears at higher pressure in YH3 and ScH3.

Raman and UV-Visible Absorption Spectroscopy of Rare-Earth Hydride under High Pressure

Raman and visible absorption spectra of rare earth trihydrides have been measured at high pressures, in order to investigate the structural and electronic phase transitions. The successive phase transitions of hcp-intermediate-fcc phase were confirmed by Raman measurements. From the visible absorption experiments, the band gap energy was estimated. The gap closure was found to occur on the transition from the intermediate to the fcc phase.

Raman spectroscopy study of REH 3 under pressure

The Raman spectroscopic studies of two rare earth trihydrides: Y H 3, HoH 3, have been performed in the pressure range from ambient up to 16 GPa and 25 GPa respectively. For the first time samples of REH 3 in the form of powder have been studied by Raman spectroscopy using the Diamond Anvil Cell (DAC) technique. A rapid decrease of Raman activity has been observed for the hydrides under pressure values in the vicinity of structural phase transition. Metallization as a possible reason for the observed dramatic change of the REH 3 Raman activity has been discussed.

Pressure induced phase transformation of REH 3

High-pressure X-ray diffraction studies of the number of rare earth trihydrides (REH 3) have been carried out in a diamond anvil cell up to 30 GPa at room temperature. This includes the family of rare earth trihydrides that possess hexagonal structure at ambient conditions. A reversible structural phase transformation from the hexagonal to cubic phase has been observed for the Sm, Gd, Er, Ho and luthetium trihydrides. The same type of phase transition has been found also for yttrium trihydride. The lattice parameters of the new cubic phases and the volume changes at transition points were determined for the all compounds investigated. The parameters of the equation of state for all the hexagonal and cubic phases have been determined in the pressure range up to 30 GPa. The role of H–H repulsive interaction as a probable promoter of the transition and a systematics of transition are discussed.

Pressure-induced structural phase transition in rare-earth trihydrides. Part II. SmH 3 and compressibility systematics

High-pressure studies of samarium trihydride have been performed in a diamond anvil cell at pressures up to 30 GPa at room temperature. As a result, a reversible structural phase transformation from the hexagonal to the cubic phase has been observed. The transition pressure and the lattice parameter follow the tendency of the other lanthanide trihydrides investigated earlier [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 477; T. Palasyuk, M. Tkacz, Solid State Commun. 133 (2005) 481]. The compressibility systematics of lanthanide and yttrium trihydrides has been established.

Pressure-induced structural phase transition in rare-earth trihydrides. Part III. Systematics: General and geometric approach

Systematics of hcp–fcc phase transition observed in a series of lanthanide and yttrium trihydrides has been established. Based on the established systematics the prediction of the hcp–fcc transition pressure for the rest of the lanthanide trihydrides has been made. The role of H–H repulsive interaction as a probable transition promoter is discussed.

Neutron scattering studies of the structures and Vibrational Dynamics of the HCP Rare-Earth Trihydrides

ABSTRACTBoth neutron-powder-diffraction and neutron-vibrational-spectroscopy studies have been undertaken to characterize the structures and vibrational dynamics of a variety of bulk hcp rareearth trihydrides RH3 (where R = Y, Th, Dy, Ho, Er, and Lu). The results are consistent with P3c1 as the prevalent structural symmetry for all trihydrides investigated except LuH3. Corresponding results for LuH3 are suggestive of an atypical arrangement of H atoms. Preliminary analyses of LuD3 diffraction patterns indicate that the main peaks can be satisfactorily modeled using P63/mmc symmetry with a reduced unit cell, although numerous minor diffraction features and a relatively unusual vibrational spectrum hint at either a more complex structure or an unknown secondary phase. The deviations from P3c1 symmetry are likely a consequence of the relatively smaller metal-hydrogen bond distances for LuH3.

Unusual behaviour of the band structure and electronic-induced phase transitions in metallic hydrides

Progress in theoretical and experimental investigations aimed at achieving a better understanding of the structural transformations in the transition metal hydrides is reviewed. Tetragonal distortion in the titanium group dihydrides and the light rare earth trihydrides and the metal-to-semiconductor transition in the trihydrides are discussed. Both theoretical band structure calculations and experimental investigations indicate that the phase transitions are electronically induced. The theoretical description was based on the excitonic dielectric model.