Rare-earth Dihydrides Research Articles

Ab initio study of the structural, electronic, magnetic, mechanical, optical, and dynamical properties of the rare-earth dihydrides MH2 (M= Yb, Sc, Eu, Y, Lu and Gd)

Most of the physical properties of rare-earth dihydrides MH2 (M = Yb Sc, Eu, Y, Lu and Gd) have not yet been discovered. The aim of this work is to investigate to the structural, electronic, magnetic, mechanical and optical properties employing density functional theory (DFT) within the VASP code. The computed electronic structures, elastic constants, and lattice parameters of MH2 hydrides are in good agreement with the current experiments and other theoretical studies. The mechanical properties of MH2 (M = Sc, Y, Yb, Eu, Lu and Gd) such as: ductility, elastic moduli, and elastic anisotropy, were evaluated and conferred. The brittle/ductile properties are determined by the bulk/shear ratio B/G and the Cauchy pressure C12–C44. The current work shows that the LuH2, GdH2 and ScH2 hydrides crystallizing in the cubic phase of hydrogen exhibit good mechanical behavior that may be useful for applications in hydrogen storage in the future. In addition, the electronic properties were also investigated in order to present a thorough knowledge of their patterns and verify the metallic behavior trend of these hydrides. The Debye temperature was therefore computed using the single crystal elastic constants and the average sound velocity in accordance with the projected bulk properties of crystalline materials. The optical properties were computed by fitting the dispersion relation of the imagined component. Optical spectra showed a significant absorption coefficient in the in the ultraviolet region of these compounds. The present findings can improve our understanding of rare-earth-based materials towards potential use in the experimental research. Finally, The MH2 (M = Yb Sc, Eu, Y, Lu and Gd) compounds were comprehensively investigated, with a focus on their dynamic characteristics and identification of infrared and Raman active modes. The elastic constant and phonon spectrum calculations revealed the stability of the cubic structure of MH2 under mechanical and dynamic conditions. Furthermore, the study obtained thermal function data, including phonon contributions to internal energy, Helmholtz free energy, entropy, and specific heat at high temperatures.

Structural, elastic, dynamical and thermophysical characteristics of rare-earth dihydrides XH2 (X = Ce, Pr, Tb, Ho)

Most of the physical properties of cubic structure compounds XH2(X = Ce, Pr, Tb, Ho) have not yet been developed. This study aims to study their structure, elasticity, dynamics and thermal properties. Based on our calculated elastic constants, we calculated the bulk modulus, shear modulus, Young's modulus, the brittle/ductile properties, Poison's ratio, anisotropy factor, Debye temperature, and sound velocities of XH2(X = Ce, Pr, Tb, Ho). In particular, the dynamic characteristics of the XH2(X = Ce, Pr, Tb, Ho) were studied in detail, and the infrared and Raman active modes were determined. According to the calculated elastic constant and phonon spectrum, the cubic structure of XH2 (X = Ce, Pr, Tb, Ho) is mechanically and dynamically stable. The thermal functions such as phonon contributions to the internal energy, the Helmholtz free energy, the entropy and specific heat under high temperatures are also obtained.

DFT investigation analyzed with data mining technique of rare-earth dihydrides REH2 for hydrogen storage

Ab initio calculation and data mining are very useful methods to predict the interrelationships among structure properties. In this paper, the first-principles modelling based on the density functional theory (DFT) was used to explore the structural, mechanical, and electronic properties of ten hydrides REH2 (RE = La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm). From the present work, it can be noted that LaH2, CeH2, NdH2, and PrH2 compounds crystallizing in the hydrogen-rich cubic phase present good mechanical properties which could be of use to future applications in hydrogen storage. Furthermore, the electronic structures were also evaluated to provide a deep insight regarding their trends and confirm the metallic character of these systems. While, HoH2, DyH2, ErH2, TbH2, TmH2, SmH2 hydrides are mainly hard and rigid systems with high values of the bulk modulus (B), shear modulus (G), Young's modulus (E), micro-hardness (H), and low values of B/G ratio. More significantly, the adopted principal component analysis (PCA) provided an advantageous technique in selecting and identifying the minimum of inputs variables necessary to capture all information of the systems.

First principles studies of the structural stability and lattice dynamics for rare earth hydridies REHx (RE = Sm, Gd, Tb, Dy, Ho, Tm, x = 2, 3) under pressure

In the paper, the structures, pressure-induced structural transitions (from cubic to hexagonal), lattice dynamics and the possible transition mechanisms for rare earth dihydrides REH2 and trihydrides REH3 (RE = Sm, Gd, Tb, Dy, Ho, and Tm) are investigated comparatively and systematically using first-principles calculations. The transition pressures from the cubic CaF2-type structure to the hexagonal REB2-type structure for REH2 (RE = Sm, Gd, Tb, Dy, Ho, Tm) and the transition pressures from the cubic BiF3-type structure to the hexagonal LaF3-type structure for REH3 (RE = Sm, Gd, Tb, Dy, Tm) are predicted reasonably. It is concluded that the two kinds of structural phase transitions are not sensitive to RE 4f electrons. In the two kinds of structural phase transitions under pressure, we note that the ‘‘2.1 Å rule’’ (the purposed minimum HH distance for the stable metal hydirde at 0 K and 0 GPa) doesn't work. While lattice dynamics show that the dynamical stable pressure ranges for the cubic REH2 (RE = Sm Gd, Tb, Dy, Ho, Tm) are about in 0–52, 0–60, 0–70,0-87, 0–94 and 0–108 GPa, respectively. Ones of the cubic REH3 (RE = Sm, Gd, Tb, Dy, Tm) are around 17–147, 31–182, 13–200, 16–224 and 14–280 GPa, respectively. In contrast to the corresponding structural transition pressure as well as the lattice dynamics of the hexagonal structure under high pressure, the phonon modes softening behaviors under pressure might be responsible for the pressure-induced structural phase transition in the cubic REH2, but not in the cubic REH3.

Electronic structure of the rare-earth superstoichiometric dihydride GdH2.25

With the WIEN2k simulation code, ab initio calculations of electronic structure and of equilibrium properties for the superstoichiometric rare-earth dihydride GdH2.25 using the full-potential linearized augmented plane-wave (FP-LAPW) method within density functional theory (DFT) in generalized gradient approximation (GGA) were performed. The lattice parameter, bulk modulus, pressure derivative, density of states (DOS) and energy band structures were determined. The GGA-optimized lattice parameter agrees much better with the experimental findings than the local density approximation (LDA) one. The non-negligible electronic DOS at Fermi level confirms that GdH2.25 has a metallic character. The Fermi energy (E F) falls at a level where most of the states are rare-earth 5d conduction states while negligible contributions of both interstitial (tetrahedral and octahedral) H s states are observed near E F. It is found that hybridization exists between s electronic orbital of both interstitial H atoms and d electronic orbitals of Gd.

A datamining approach to predict the formation enthalpy for rare-earth dihydrides REH2 (RE = Ce,Pr,Dy)

Different theoretical methods estimating the formation enthalpy for hydrides have been adopted, among them the density functional theory (DFT) calculations. In this paper, an attempt has been made to study and estimate the formation enthalpy for CeH2, PrH2 and DyH2 hydrides, by mean of a datamining approach. Based on the data-sets selected from different published work, a principal component analysis (PCA) is applied to select the significant modeling inputs and simplify the model structure. An artificial neural network (ANN) with 3-13-1 architecture structure has been developed to estimate ΔH. The structural parameter and elastic properties of Ce, Pr, Dy, CeH2, PrH2 and DyH2 were calculated by the density functional theory (DFT+U). Also, the formation enthalpy of CeH2, PrH2 and DyH2 are determined. The results of ΔH calculated with both ANN and DFT+U are basically consistent and are in good agreement with the experimental data. The adopted ANN structure provided an advantageous technique in predicting the non-linear relationship between the formation enthalpy and its influences factors (a, R, χ) which were previously selected by PCA.

First-principles study of the electronic structure of a superstoichiometric rare earth dihydride GdH2.25

AbstractWe performed ab initio calculations of electronic structure and equilibrium properties for the rare earth superstoichiometric dihydride GdH2.25 (with the space group Pm−3m (No. 221)) using the full-potential linearized augmented plane wave method (FP-LAPW) approach within the density functional theory (DFT) in the generalized gradient approximation (GGA) and local density approximation (LDA) as implemented in the WIEN2k simulation code at 0 K. The equilibrium properties were determined, the density of states, electronic density and the energy band structures were studied in details. It was concluded that the GGA optimized lattice parameter agreed much better with the experimental findings than the LDA one. The non negligible electronic density of states at the Fermi level confirmed that the GdH2.25 had a metallic character. The Fermi energy EF fell at a level where most of the states were rare-earth 5d-eg conduction states while negligible contributions of both interstitial (tetrahedral and octahedral) H s-states were observed near EF. From electronic density, the bonding between Gd and tetrahedral H atoms was of prominent covalency, while was ionic between Gd and octahedral H atoms.

Ab initio study of the structural, mechanical, and dynamical properties of the rare-earth dihydrides XH2 (X=Sc, Y, and La)

The structural, electronic, mechanical, dynamical and thermodynamic properties of rare-earth dihydrides XH2 (X=Sc, Y, and La) with the cubic fluorite structure have been systematically investigated by using the projector augmented wave (PAW) method within the generalized-gradient approximation. The calculated lattice parameters, electronic structures and elastic constants of XH2 agree well with available experimental data and other theoretical results. Both the lattice constants and the equilibrium unit cell volumes increase in accordance with heavier rare-earth atom. From the obtained bulk modulus, shear modulus and Young's modulus, the resistance to volume change, shear deformations, and even tensile and compressive deformations weakens from ScH2 to LaH2 successively. The brittle/ductile properties of the three compounds are determined by the ratio of B/G and the Cauchy pressure C12−C44. Poisson's ratio, elastic anisotropy factor, Debye temperature, and sound velocities of XH2 are also predicted. Especially, the phonon dispersion curves and corresponding phonon density of states of XH2 are determined using a linear-response approach to density functional perturbation theory (DFPT) successfully, the phonon frequencies (in THz) obtained at the zone-center (Γ-point) for infrared-active modes T1u and Raman-active modes T2g are ScH2: T1u=29.294, T2g=35.279; YH2: T1u=27.295, T2g=32.624; and LaH2: T1u=24.286, T2g=28.004. Our results show that the three compounds can be stabilized dynamically and mechanically in the ground state phase of the cubic fluorite structure. The thermodynamic functions such as phonon contributions to the internal energy E, the Helmholtz free energy F, the entropy S, the changes of H–H298.15 and G–G298.15 and the constant volume specific heat CV under high temperatures are also obtained based on phonon density of states.

Rare-earth dialkyl and dihydride complexesbearing monoanionic ancillary ligands

This paper gives a brief overview on recent progress in the chemistry of rare-earth dialkyl and dihydride complexes with monoanionic supporting ligands. The dialkyl complexes serve as excellent catalysts for the polymerization and copolymerization of a wide range of olefins to give novel polymer materials difficult to prepare by other means. The dihydride species show unique features in terms of both their structure and reactivity, which are in sharp contrast with those of the monohydride analogues bearing two supporting ligands.

The manifestation of oxygen contamination in ErD2 thin films

Erbium dihydride Er(H,D,T)2 is a fluorite structure rare-earth dihydride useful for the storage of hydrogen isotopes in the solid state. However, thermodynamic predictions indicate that erbium oxide formation will proceed readily during processing, which may detrimentally contaminate Er(H,D,T)2 films. In this work, transmission electron microscopy (TEM) techniques including energy-dispersive x-ray spectroscopy, energy-filtered TEM, selected area electron diffraction, and high-resolution TEM are used to examine the manifestation of oxygen contamination in ErD2 thin films. An oxide layer ∼30–130 nm thick was found on top of the underlying ErD2 film, and showed a cube-on-cube epitaxial orientation to the underlying ErD2. Electron diffraction confirmed the oxide layer to be Er2O3. While the majority of the film was observed to have the expected fluorite structure for ErD2, secondary diffraction spots suggested the possibility of either nanoscale oxide inclusions or hydrogen ordering. In situ heating experiments combined with electron diffraction ruled out the possibility of hydrogen ordering, so epitaxial oxide nanoinclusions within the ErD2 matrix are hypothesized. TEM techniques were applied to examine this oxide nanoinclusion hypothesis.

Hydrogen ordering induced resistivity anomalies in the superstoichiometric light rare-earth dihydrides

The ordering superstoichiometric light rare-earth dihydrides MH 2+c (M–La, Ce; H–hydrogen atoms) characterized by two long-range order parameters, η 1 and η 2, are considered. The electroresistivity R H associated with the redistribution of hydrogen atoms during the ordering processes is investigated. An analytical expression describing dependence of R H on the equilibrium values of order parameters is obtained. It is shown that in the disorder–order and order–order transition points, the resistivity temperature dependences R H( T) reveal the discontinuity- or kink-like anomalies. Residual resistivity and the possible quenching effect concentration dependences are given.

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.

H–H interactions in the superstoichiometric rare-earth dihydrides RH2+c

Chemically similar compounds LaH2+c and CeH2+c reveal qualitatively different scenario of the ordering process, that can be associated with different values of the energy parameter p. There is made an attempt to understand the reasons of the different behavior of the ordering subsystems.

Heat capacity anomalies associated with phase transition type change in CeH2+c-like superstoichiometric rare-earth dihydrides

In a number of superstoichiometric rare-earth dihydrides RH2+c, the subsystem of octa-hydrogen atoms undergoes an order–disorder phase transition. Previously it was shown that in CeH2+c, a phase transition type change occurs at some concentration c0: the structural transformation, discontinuous for c<c0, becomes continuous for c>c0. In the present article, the temperature dependence of the ordering subsystem heat capacity C(T) is calculated in a wide range of concentrations c and it is shown that the discontinuity of C(T) at the structural transformation point Ttr, ΔC(Ttr), becomes extremely large when c∼c0. The obtained results are analyzed from the point of view of previous theoretical considerations and compared with the available experimental data.

Phase Transition Type Change in Order-Order and Order-Disorder Transformations of LaH2+c-Like Superstoichiometric Dihydrides

Phase transformations in metal-hydrogen systems associated with the ordering processes in the subsystem of interstitial hydrogen atoms are considered. It is shown that in a particular class of superstoichiometric rare-earth dihydrides, where the ordering subsystem is characterized by two long-range-order parameters η 1 , η 2 and the energy parameter p < 1, the order-order transition can change its type: being continuous for concentrations c within the range c 0 (1) (p) ≤ c ≤ c 0 (2) (p), it becomes discontinuous outside of this region. Dependence of critical concentrations c 0 (1) and c 0 (2) upon the energy parameter p is deduced. It is shown, as well, that there exists a second sequence of critical concentrations, c 1 (1) (p) and c 1 (2) (p). denoting the boundaries between the regions of continuous and discontinuous order-disorder transformations.

Effect of the pulverization processes on the magnetostrictive characteristics on giant magnetostrictive Tb 0.3Dy 0.7Fe 2 thin film

Two different processes by ball milling and cyclic hydriding-dehydriding were examined for the preparation of the pseudo-binary Tb 0.3Dy 0.7Fe 2 powder sample with the C-15 Laves phase. The cyclic treatment resulted in both the reduction of the particle size and the disproportionation of the alloy into rare earth dihydrides RH 2 (R=Tb, Dy) and α-Fe. However, subsequent vacuum annealing treatment of the disproportionated sample at 700 K in a vacuum of 5×10 −4 Pa yielded a clean and less strained stoichiometric powder sample compared with ball milled samples. The magnetostrictive characteristics of Tb 0.3Dy 0.7Fe 2 films prepared by the flash evaporation process were found to be influenced by the prior processes for the powder preparation.

Heat capacity associated with hydrogen ordering in superstoichiometric rare-earth dihydrides: The case of LaH 2 + x-like systems

In the case of β- RH 2 + x systems ( R—rare earth; 0 < x < 1), the fraction x of the hydrogen atoms located in the octahedral interstitial sites of the f.c.c. metal lattice undergoes order-disorder and order-order transformations. The strength of these transformations and the possibility of their experimental observation in the investigation of the thermal properties of metal-hydrogen compounds can be estimated by calculating the temperature dependence of the internal energy, E ̄ (T) , and of the specific heat, C(T) = ∂ E ̄ (T) ∂T . The calculation is applied for the case of the LaH 2 + x -like systems, with different concentrations x and different energy-parameter ratios p ≡ V 2 V 1 .

Hydrogen ordering and metal-semiconductor transitions in superstoichiometric rare earth dihydrides

Temperature-dependent metal-semiconductor (MS) transitions in RH2 + x systems were first noted 20 years ago by Libowitz in CeH2 + x, in the range 0.7 ≲ x ≲ 0.8, and later by Shinar and coworkers in LaH2 + x, for 0.8 ≲ x ≲ 0.9. They were associated with the breakdown (with increasing T) of a band near the Fermi level related to a superlattice of octahedral vacancies, VO, forming below room temperature in those rather substoichiometric trihydrides.We have observed similar MS transitions at TMS = 250–300K in the superstoichiometric dihydrides RH2 + x (R = Y, Gd, Ho, Er), with x near the β phase limit xmaxβ = 0.1–0.3, depending on R. The mechanism responsible for the MS transition is, this time, associated with the breakdown of a direct superlattice of octahedral hydrogen atoms, HO, forming at low concentrations x. Necessary conditions for the occurrence of MS transitions are formulated and a general relationship between TMS and the sample resistivity ρMS at this temperature is established. An additional MS transition is observed in these systems with decreasing T at low temperatures which is attributed to carrier localization due to atomic disorder.

Properties of rare earth dihydrides as reflected in the magnetic behavior of TmH 2, a van Vleck compound

It is shown that TmH 2, which is a van Vleck compound with a Γ 2 singlet ground state and a Γ 5 (2) triplet first excited state, can be used to detect variations in crystal fields of thulium atoms. These variations were deduced from a comparison of measurements of TmH 2 using susceptibility nuclear magnetic resonance of thulium, neutron diffraction, resistivity and electron spin resonance (for which gadolinium and erbium impurities were added). Analysis of the results showed a spread from 170 to 186 K in the energy ΔE, of the first excited state for different thulium atoms in TmH 2, in addition to a drastic change in ΔE at 100 K and 80 K for thulium atoms neighboring gadolinium and erbium impurities respectively. The origin of the spread in δE is attributed to defects which induce changes in the crystal field around the defect. These changes have a range of influence which decreases as a function of distance from the defect. Since all the rare earth and rare-earth-like dihydrides have inherent defects owing to the deficiency of hydrogens in TmH 2 and to the small occupation of octahedral sites by hydrogens, we can deduce that the spread in crystal fields may be common to all these dihydrides.

Semiconductor-metal-semiconductor transitions in the superstoichiometric dihydride YH2.10.

A metal-semiconductor transition has been observed in the range 230--260 K for the first time in a slightly superstoichiometric rare-earth dihydride, ${\mathrm{YH}}_{2.10}$ (as compared to the x-rich systems ${\mathrm{CeH}}_{2+\mathit{x}}$ and ${\mathrm{LaH}}_{2+\mathit{x}}$ with 0.7\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9), implying an order-disorder transformation in the octahedral sublattice of the excess hydrogen atoms as the driving mechanism. Furthermore, a resistivity minimum in the range 60--80 K was observed, indicating for the first time in such systems an additional metal-semiconductor transition towards lower temperatures.