Rare-earth Dihydrides Research Articles

Properties of rare earth dihydrides as reflected in the magnetic behavior of TmH2, 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.

Spin-disorder resistivity in the paramagnetic state of the heavy rare-earth dihydrides.

We present systematic measurements of the paramagnetic spin-disorder resistivity ${\ensuremath{\rho}}_{m}$ in Gd, Tb, Dy, Ho, and Er dihydrides. We observe that ${\ensuremath{\rho}}_{m}$ is strongly affected by crystalline-field effects especially as concerns ${\mathrm{DyH}}_{2}$ and ${\mathrm{ErH}}_{2}$. The absolute values of ${\ensuremath{\rho}}_{m}$ are much smaller than in the corresponding pure metals, an effect which can be attributed in part to modifications, with hydrogen, of the electronic band structure, but it implies also a reduction of the fundamental exchange interaction between conduction and localized electron spins.

On the temperature dependence of hydrogen site occupancy in rare earth dihydrides

Previous work claimed that the observed anomalous temperature dependence of octahedral(o)-site occupation in f.c.c. rare earth dihydrides could be explained by interaction effects not involving optic mode vibrations. We show that this conclusion was based on an erroneous assumption about the role of the chemical potential in determining the thermal equilibrium solution to lattice-gas-model equations. A correct derivation is given, and we conclude that such interactions cannot be the source of the anomaly. We examine some other possibilities.

Fermi surface nesting in rare earth dihydrides

Abstract A new type of Fermi surface nesting is proposed for the dihydrides of rare earths. It is now possible to explain the antiferromagnetic spin structure of most systems based on the energy bands and Fermi surface calculated by Gupta and Burger.

Temperature and concentration dependence of hydrogen site occupancy in several rare earth dihydrides

Abstract Neutron inelastic scattering and diffraction techniques were used to study the hydrogen site distribution in several rare earth dihydrides as a function of temperature and concentration. For Y, La, and Ce the fraction of hydrogen on octahedral sites is approximately a constant from 15 to 200 K followed by a decrease with increasing temperature. A model is presented which qualitatively explains this behavior.

Temperature and concentration depedence of hydrogen site occupancy in several rare earth dihydrides

Neutron inelastic scattering and diffraction techniques were used to study the hydrogen site distribution in several rare earth dihydrides as a function of temperature and concentration. For Y, La, and Ce the fraction of hydrogen on octahedral sites is approximately a constant from 15 to 200 K followed by a decrease with increasing temperature. A model is presented which qualitatively explains this behavior.

Hydride formation on polishing rare earth alloys and reflected electron loss maps

Standard metallographic preparation of a two-phase alloy, RFe 2 plus R, where R represents a Dy + Tb metal phase, has shown that one often finds a blue phase in the regions suspected to be the Dy + Tb metal phase. By examining chemical shifts of the rare earth metals in both the Auger and electron loss spectra it is demonstrated that the blue phase is a rare earth dihydride. A new technique, reflected electron loss mapping, is shown to provide an excellent method for revealing the location of the dihydride phase. Sputtering experiments confirm that the blue phase is located only near the polished surface and it is concluded that this rare earth dihydride forms upon mechanical polishing. Other evidence is presented which shows that dihydride formation can be a common problem in the metallographic preparation of rare earth alloys.

Crystal growth of ytterbium dihydride and the phase relations in the Yb-H system

Orthorhombic YbH 1.94 and cubic YbH 2.56 were the only two phases found in the range 250 ° C ⩽ T ⩽ 800 ° C and 0 atm ⩽ P H 2 ⩽ 120 atm. The metastable cubic phase YbH 2.02 reported previously was not detected. Single crystals of YbH 1.75 with dimensions up to 5 mm × 4 mm × 3 mm were grown for the first time in sealed tungsten crucibles by sublimation. A plot of the lattice constants of the cubic rare earth (RE) dihydrides and trihydrides versus the atomic number shows a much larger value for YbH 2.56. Therefore, we conclude that of all the binary cubic RE hydrides only this phase has strong contributions from RE 2+ ((4f) n (5d)° configuration). The possible mixed valence of this phase is discussed. Optical re-emission measurements show that YbH 1.94 is a semiconductor and that YbH 2.56 is metallic. This supports the existence of mixed valence in the latter phase.

Electronic properties and electron-phonon coupling in zirconium and niobium hydrides

The electronic structure of the two 4d transition-metal cubic dihydrides ZrH/sub 2/ and NbH/sub 2/ is investigated by means of an augmented-plane-wave (APW) band-structure calculation. The position of the Fermi level in a peak of the density of states (DOS) in the case of cubic ZrH/sub 2/ leads to an interpretation of the tetragonal distortion observed in all the group-IV transition-metal dihydrides in terms of a Jahn-Teller effect which lifts the degeneracy of the doubly degenerate band in the GAMMAL direction; these results are in agreement with magnetic susceptibility, electronic specific heat and thermoelectric power data. The main features of the DOS of the metal 4d states are similar for ZrH/sub 2/ and NbH/sub 2/, while the position and width of the low-lying bands are found to depend substantially upon the compound under study. These two bands which result from metal-hydrogen and hydrogen-hydrogen interactions overlap the metal d bands located at higher energies, contrary to the case of rare-earth dihydrides for which an energy gap was obtained. Our theoretical results are in good agreement with photoemission and x-ray emission spectra. We have evaluated the electron-phonon coupling constant lambda using the results of our APW calculation in conjunction with neutron scatteringmore » data. For both ZrH/sub 2/ and NbH/sub 2/, and contrary to the case of superconducting PdH, the electron-optical phonon contribution is small, due essentially to the low value of the DOS of s type at the H site at the Fermi energy and to the hardening of the optic modes. Moreover, the electronic term of the electron--acoustic-phonon contribution is reduced from its value in the pure metal. The two compounds are not found to be superconducting, in agreement with experimental results.« less

Dimensional analysis of phases with the A15 β-W structure

It is found that the method of dimensional analysis of binary metallic phases M x N y , recently developed by the author, can be applied successfully to a tetrahedrally close-packed structure, e.g. the A15 or β-W type MN 3, Linear equations are developed for the cell edge a in terms of D M and D N , the atomic diameters for coordination number (CN) 12 of the atoms, and of such valency effects as are apparent; these reproduce the observed a values of 63 phases to within an average of a fraction of a per cent. Differences are found between those phases which have transition metal M components and those which do not. In the former it is the M-N contacts in the icosahedron of N atoms surrounding M that control a, whereas in the latter it is the contacts in the CN 14 polyhedron of four M and ten N atoms surrounding the N atoms that together control a. A pseudo-valency effect occurs for the N atoms in phases with transition metal M components. This appears to result in part from valency considerations and in part from geometrical constraints related to the two close N-N contacts. In this paper we compare phases with the A15 structure, rare earth dihydrides with the fluorite structure and rare earth nitrides with the rock salt structure, in each of which two or more sets of atomic contacts act simultaneously to control the cell dimension. A lemma is provided for such cases to indicate how the individual expected cell edge dependences on D M or D N are to be combined to give the overall expected dependence.

Electronic structure of cerium hydrides: Augmented-plane-wave linear-combination-of-atomic-orbitals energy bands

Electronic energy bands have been calculated for Ce${\mathrm{H}}_{2}$ and Ce${\mathrm{H}}_{3}$ using the augmented-plane-wave method and have been fitted by the linear-combination-of-atomic-orbitals interpolation scheme. The partial densities of states and the numbers of electrons on atomic orbitals indicate that hydrogen in Ce${\mathrm{H}}_{2}$ is almost anionlike. When going from Ce${\mathrm{H}}_{2}$ to Ce${\mathrm{H}}_{3}$, shallow bonding levels are found to form between the third hydrogen state and conduction electrons of Ce${\mathrm{H}}_{2}$, other features of Ce${\mathrm{H}}_{2}$ being little affected by it. Thus the rare-earth dihydrides are regarded as ionic compounds similar to the saline-element dihydrides except for the presence of $d$-like conduction electrons.

Electronic and magnetic properties of some rare-earth dihydrides and dideuterides

Electronic and magnetic properties of some rare-earth dihydrides and dideuterides

The growth and structure of epitaxial films of the rare-earth dihydrides

Thin films have been grown on the cubic, dodecahedral and octahedral faces of rock salt by vacuum evaporation, at 10-6 and 10-10 Torr, of Gd, Tb, Dy, Er and Ho, and their structures have been examined by electron microscopy and diffraction. Conditions have been established for the epitaxial growth of the rare-earth dihydride films in the thickness range up to approximately 25 nm. Between 25 and 20 nm a mixed phase of FCC dihydride and HCP metal is formed, and above 90 nm the films are substantially metallic. Reflection electron diffraction and hydrogen annealing experiments suggest that the formation of the dihydride may be influenced by adsorbed hydrogen on the substrates.

Phase transformations in hydrides of rare‐earth metals under pressure

AbstractAn analysis of the specific volume dependences on atomic number is carried out for rare earth dihydrides and trihydrides. It is shown that under high pressures the following phase transformations are possible in these compounds: a) structural phase transitions h.c.p.→f.c.c., the ion valency being unchanged, b) phase transitions accompanied by a change of metal ion valency. The dependences of volume versus pressure to 24 kbar and resistivity versus pressure to 20 kbar are measured for NdH2.82 at 20 °C. A reversible resistivity anomaly and an abrupt change of the volume by 1.6% are detected under high pressures. These anomalies are explained to be due to the phase transformation h.c.p.⇄f.c.c.

The lattice vibrations of CeD 2.12

The complete phonon dispersion relation including both optic and acoustic modes has been measured along the major symmetry axes of a single crystal of CeD 2.12 at 295 K by coherent neutron scattering. The results show the inadequacy of simple models previously used for the analysis of neutron incoherent scattering studies of rare earth dihydrides. The interstitial mode of vibration due to the excess deuterium (above CeD 2 in octahedral sites has also been observed.

Optical vibrations of hydrogen in metals

Neutron energy loss measurements provide a powerful method of studying the modes of vibration of hydrogen in metal lattices. The crosssection for neutron scattering is proportional to the amplitude-weighted frequency distribution. Both the frequency distribution and the amplitudeweighting function can be obtained from the interatomic force constants by the standard methods of lattice dynamics. These calculations have been performed for rare earth dihydrides and it is found that hydrogen-nearesthydrogen forces have to be introduced to obtain agreement with experiment.The extension of this method directly to the case of β-PdH shows considerable differences between the calculated frequency distribution obtained from the dispersion curves for β-PdD measured by Rowe et al., and the measured cross-section over the optical modes. However, the latter is in satisfactory agreement with a recent extension of the lattice dynamical methods by Sköld et al., which allow for the defect nature of the structure.Based on this approach, a number of further measurements have been made to examine the effect of changes in the system. Thus, the measured cross-section has been shown to have a significant temperature dependence which must be due either to changes in the local order or to anharmonic effects. Further, attempts have been made to produce the structural change which has been reported to arise from a 50 K anneal, but no corresponding change in the cross-section was observed. Finally, the effect of alloying palladium with silver and cerium was examined. In the former case, little change is observed whereas, in the latter case, it is evident that the hydrogen-metal force has increased substantially while the hydrogen-second hydrogen force becomes correspondingly smaller.

Dy160andEr166Mössbauer studies of concentrated and diluted rare-earth dihydrides: Single-line compounds and crystal-field effects

M\"ossbauer-effect measurements on the cubic intermetallic rare-earth dihydrides are reported utilizing the 86.8-keV and the 80.6-keV ${2}^{+}$ \ensuremath{\rightarrow}${0}^{+}$ $\ensuremath{\gamma}$ transitions of $^{160}\mathrm{Dy}$ and $^{166}\mathrm{Er}$, respectively. All studies were carried out at liquid-helium temperatures on pure and diluted (with Y or Sc) paramagnetic samples. It is found that in the more concentrated alloys spin-spin ($4f\ensuremath{-}4f$) relaxation rates at 4.2 K are sufficiently fast to allow for the use of these systems as narrow-single-line M\"ossbauer sources and absorbers. Measurements on highly dilute $^{160}\mathrm{Dy}$ and $^{166}\mathrm{Er}$ impurities in Y${\mathrm{H}}_{2}$ reveal the resolved paramagnetic hyperfine structure of a ${\ensuremath{\Gamma}}_{7}$ crystalline electric field (CEF) ground state in both cases. The results are discussed in terms of CEF theory.

Rare earth dihydride formation by reaction of rare-earth metal thin films with water vapour

Rare-earth dihydride LnH x ( x ≅ 2) formation was observed by the exposure of rare-earth -metal thin films to water vapour at fairly low temperatures. In the case of neodymium, this is accompanied by the slow formation of the hydroxide but, in the case of dysprosium, the hydride alone is obtained at about 413 K. There are no hydrated contaminants and the dysprosium thin films do not oxidize until a higher temperature (about 473 K) is reached.

Magnetic properties of rare earth hydrides

AbstractMagnetic measurements on fully hydrogenated rare earth metals give no indication of magnetic ordering at temperatures down to 4 °K, whereas the corresponding elemental rare earth metals, except Pr, order at temperatures ranging from 19°K for Nd to 289 °K for Gd. The results are consistent with electrical resistivity measurements which indicate that these hydrides are not metallic conductors. Since the RKKY interaction is inoperative in the hydrides, ordering temperatures are depressed below 4°K. Detailed analysis of the paramagnetic behavior of PrH2 shows that experiment is consistent only with the anionic model for hydrogen in rare earth hydrides. Rare earth dihydrides, which are metallic conductors, do show magnetic ordering at temperatures ranging from 8°K, for HoH2 and DyH2, to 40°K, for TbH2, EuH2, alone among the dihydrides which order, becomes ferromagnetic.Rare earth intermetallics of the formula RT2 and RT5 (T = Co or Ni) absorb hydrogen with noticeable effect on their magnetic and electrical properties. Virgin samples of PrCo2 and NdCo2 order at 45 and 116 °K, respectively. After degassing under ultrahigh vacuum conditions at elevated temperature, these materials fail to exhibit magnetic ordering down to 4°K. A discussion is given of factors which probably bring about this striking sensitivity to hydrogen concentration.