Saturated Hydrides Research Articles

Hydrogenation of the Laves Phases of Gd(Mn, Al)2, Tb(Mn, Al)2, and Tb(Fe, Al)2Compounds

We study the formation of the AB2 Laves phases in RE–M–Al (where RE is Gd or Tb and M is Fe or Mn) systems and determine their hydrogen-sorption properties. The crystal structures of the original compounds and their saturated hydrides are studied by the X-ray powder diffraction method. It is shown that these compounds absorb hydrogen at pressures of 0.1–0.12 MPa without amorphization and their hydrogen-sorption ability decreases as the amount of aluminum in the compound increases.

Hydrogenation of oxygen-stabilized Zr 3NiO x compounds

It is shown that oxygen-stabilized compounds Zr 3NiO x ( x=0.4, 0.6, 0.8, 1.0) interact with hydrogen at ambient temperature and pressure forming saturated hydrides with a filled Re 3B-type structure. The hydrogen storage capacity decreases with increasing oxygen content from 6.65 H/f.u. for Zr 3NiO 0.4 down to 5.58 H/f.u. for Zr 3NiO 1.0. A slight decrease of the crystal lattice parameters of the parent compounds and a substantial increase of these parameters for the saturated hydrides were observed with increasing oxygen content. The partial hydrogen-induced lattice expansion, Δ V/at. H, increases from 2.333 Å 3 for Zr 3NiO 0.4H 6.65 to 3.047 Å 3 for Zr 3NiO 1.0H 5.58. Joint Rietveld refinement using X-ray and neutron powder diffraction data showed a distribution of deuterium atoms on similar positions as in oxygen-free Zr 3FeD x and Zr 3CoD x . The oxygen atoms move during deuteration from the octahedral site to one trigonal bi-pyramidal and two tetragonal interstices that are fully occupied in the saturated deuterides jointly by deuterium and oxygen. After deuterium desorption the oxygen atoms fully return to the initial octahedral site.

Kinetics of suprathermal hydrogen atom reactions with saturated hydrides in planetary and satellite atmospheres

The kinetics of saturated hydrides methane (CH 4), silane (SiH 4), germane (GeH 4), ammonia (NH 3), phosphine (PH 3), arsane (AsH 3), water (H 2O), and hydrogen sulfide (H 2S) in the low-temperature atmospheres of Jupiter, Saturn, Uranus, Neptune, Pluto, Titan, and Triton reacting with suprathermal hydrogen atoms were investigated computationally to extract suprathermal rate constants k( E) via an inverse Laplace transformation from experimentally available thermal rate constants k( T). Our data reveal that all suprathermal rate constants range up to 10 −10 cm 3 s −1 , whereas the thermal counterparts are as low as 8×10 −73 cm 3 s −1 . These data demonstrate explicitly a significantly enhanced reactivity of photolytically generated suprathermal hydrogen atoms in the low-temperature planetary and satellite atmospheres and suggest that this hitherto unaccounted reaction class should be included by the planetary modeling community into future photochemical networks of atmospheres of outer solar system planets and their moons.

Short hydrogen-hydrogen separation inRNiInH1.333(R=La,Ce, Nd)

First-principle studies on the total energy, electronic structure, and bonding nature of $R\mathrm{NiIn}$ $(R=\mathrm{La},$ Ce, and Nd), and their saturated hydrides ${(R}_{3}{\mathrm{Ni}}_{3}{\mathrm{In}}_{3}{\mathrm{H}}_{4}=R{\mathrm{NiInH}}_{1.333})$ are performed using a full-potential linear muffin-tin orbital approach. This series of phases crystallizes in a ZrNiAl-type structural frame-work. When hydrogen is introduced in the $R\mathrm{NiIn}$ matrix, anisotropic lattice expansion is observed along [001] and lattice contraction along [100]. In order to establish the equilibrium structural parameters for these compounds we have performed force minimization as well as volume and $c/a$ optimization. The optimized atomic positions, cell volume, and $c/a$ ratio are in very good agreement with recent experimental findings. From the electronic structure and charge density, charge difference, and electron localization function analyses the microscopic origin of the anisotropic change in lattice parameters on hydrogenation of $R\mathrm{NiIn}$ has been identified. The hydrides concerned, with their theoretically calculated interatomic H-H distances of \ensuremath{\sim}1.57 \AA{}, violate the ``2-\AA{} rule'' for H-H separation in metal hydrides. The shortest internuclear Ni-H separation is almost equal to the sum of the covalent radii. H is bonded to Ni in an H-Ni-H dumbbell-shaped linear array, with a character of ${\mathrm{NiH}}_{2}$ subunits. Density of states, valence charge density, charge transfer plot, and electron localization function analyses clearly indicate significant ionic bonding between Ni and H and weak metallic bonding between H-H. The paired and localized electron distribution at the H site is polarized toward La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H-H separation in these materials. The calculations show that all these materials have a metallic character.

Short hydrogen–hydrogen separations in novel intermetallic hydrides, RE 3Ni 3In 3D 4 (RE=La, Ce and Nd)

Crystal structure data for deuterides RENiInD x (RE=La, Ce and Nd) are provided on the basis of high-resolution powder X-ray and neutron diffraction data. The materials retain the hexagonal ZrNiAl type structure on deuteration. The formation of saturated deuterides is connected with anisotropic expansion along [001]. In the saturated hydrides, RE 3Ni 3In 3D 4, hydrogen atoms are located inside RE 3Ni tetrahedra that share a common face, thereby forming a RE 3Ni 2 trigonal bipyramid. This results in extraordinary short H–H separations of around 1.6 Å. This feature is unique among well characterised metal hydride materials and is in striking contrast with the generally obeyed empirical rule of 2.0 Å for H–H separations. On heating, the saturated materials release half of their hydrogen content at low temperatures, thereby statistically filling just one out of the two neighbouring tetrahedra. The remaining, more strongly bonded hydrogen, is released below 500°C under dynamic vacuum.

Experimental Results of Transmutation of Elements Observed in Etched Palladium Samples by an Excimer Laser

This work reports on the transmutation of elements discovered in saturated Pd hydride samples processed by an excimer laser. A high zinc concentration was recorded in almost all the samples analyzed. The element as well as the isotopic concentrations were measured using inductively coupled plasma mass spectrometry. During the experiment, excess heat was also monitored.

Effect of oxygen content on hydrogen storage capacity of Zr-based η-phases

The hydrogen storage capacity of η-phases Zr 3 V 3 O x and Zr 4 Fe 2 O x has been found to depend substantially on the oxygen content. Increase of the hydrogen storage capacities to H/f.u. ∼10.5 and 9.4 with the decrease of O-content have been observed for Zr 3 V 3 O x and Zr 4 Fe 2 O x , respectively. The hydrogen concentration in the title phases is higher than that in the corresponding ZrV 2 H 4.9 and Zr 2 FeH 4.5 saturated hydrides. The maximum hydrogen storage capacity is observed at x =0.6 for η-Zr 3 V 3 O x and x =0.3 for η-Zr 4 Fe 2 O x . Based on these results, it is believed that other Zr-based oxygen-stabilised η-phases could also display large hydrogen storage capacity due to partial occupation of 16(c) or 8(a) sites by oxygen atoms.

Hydrogen-induced changes in TbNiAl

Neutron diffraction was used to determine the deuterium sites in TbNiAlD1.28 and TbNiAlD0.8. Both samples were found to crystallize in the orthorhombic Amm2 space group. Our analysis reveals that deuterium in TbNiAlD1.28 occupies positions at x=0 and x=1/2 only, while TbNiAlD0.8 shows the formation of two different phases. For TbNiAlD1.28, additional magnetic reflections are observed below 16 K indicating antiferromagnetic order. On the other hand, at 10 K, only one of the TbNiAlD0.8 phases was found to be magnetic. In addition, our study of the localized vibrations of hydrogen in TbNiAlH1.39 and in its non-magnetic analog YNiAlH1.35 using inelastic neutron scattering supports the existence of two different hydrogen sites in the saturated hydrides.

The vibrations of C60H60 and the unidentified infrared emission

A theoretical study is presented of the vibrations of C 60 H 60 , the saturated hydride of the soccerball molecule C 60 , and the results are compared with features in the spectrum of the unidentified infrared emission. A simple force-field model with standard constants is adopted for C 60 H 60 , and also for the laboratory species C 20 H 20 as a check. The properties of the nine active vibrations of the model of C 60 H 60 , and the distribution over frequency of all the vibrations, are given. The wavelengths of several of the active modes are found to lie within a few per cent of those of features known in the unidentified infrared emission. An outstanding characteristic of the model is the large number of modes associated with the rocking of C-H units; the number density of these modes has a broad peak between 6 and 10 μm that is not unlike the 6-9 μm plateau, the strongest feature in the unidentified infrared emission

Comparison of a calculated spectrum of C60H60 with the unidentified astronomical infrared emission features

INFRARED emission features consisting of narrow lines and broad plateaux, ranging from 3 μm in wavelength to over 12 μm, are seen in a wide variety of astronomical objects in which interstellar or circumstellar gas is illuminated by ultraviolet radiation from a star. Several candidates have been proposed as the source of this emission, but none is widely accepted. Here I calculate the vibrational spectrum of the fullerane C60H60, the saturated hydride of the soccerball-shaped molecule C60, using a force-field model. Six of the infrared active frequencies match unidentified emission lines to within 4%, and a seventh differs from an observed line by 8%. The calculation suggests why the astronomical feature at 7.7 μm is the strongest, and the observed variation of the 3.4 μm and 3.28 μm features with astrophysical environment is consistent with the idea that the former is attributable to stretching of the C–H bond in heavily hydrogenated fulleranes, whereas the latter is due to the same transition in lightly hydrogenated fulleranes.

What is the mechanism of hydrogen absorption in rare-earth intermetallics?

We have studied the intermetallic compounds EuPd and EuRh2, and the corresponding saturated hydrides, via Eu Mössbauer spectroscopy, magnetic measurements, and x-ray diffraction. In both intermetallics, the Mössbauer parameters, indicative of the environment about the Eu ion, change substantially in the hydriding process and approach those of EuH2. We interpret those changes as evidence that the hydrogen absorption occurs via the formation of EuH2 ’’complexes’’. For the EuRh2, a simple thermodynamic argument supports this approach. The EuRh2 maintains the same crystal structure under H2 absorption, but the Eu3+ becomes Eu2+, and the lattice volume increases by 40%. EuPd changes structure under H2 absorption, but the Eu remains divalent. Results of magnetic and crystallographic measurements are presented. With partial hydriding, only the pure intermetallic and saturated hydride phases are observed to occur.

Magnetic Characteristics of Dysprosium, Erbium, and Thulium Hydrides

Magnetization—temperature data are presented for the hydrides DyH1.97 to DyH2.97, ErH1.96 to ErH3.02 and TmH1.99 to TmH2.95 and for the three parent metals. Of the 15 hydrides examined only the Dy hydrides exhibited magnetic ordering. These hydrides showed a Néel point at nearly the same temperature as that found earlier for HoH2, 8°K. The susceptibilities followed the Curie—Weiss law over most of the paramagnetic region with an effective moment nearly that of the free tripositive ion. Deviations from the Curie—Weiss law noted at low temperatures are attributed to crystal-field effects. The absence of magnetic ordering in the saturated hydrides is ascribed to the fact that these hydrides lack conduction electrons, which are essential for the magnetic coupling.

Magnetic Characteristics of Praseodymium, Neodymium, and Samarium Hydrides

Magnetization-temperature data are presented for the hydrides PrH2.02 to PrH2.71, NdH2.02 to NdH2.75, and SmH1.99 to SmH2.88 and for the three parent metals from 4.2° to 300°K. Of the twelve hydrides examined only NdH2.02 and NdH2.03 exhibited magnetic ordering. These hydrides exhibited a Curie point at 9.5°K and the saturation moment measured at 4.2°K was 1.36 μB per Nd atom. Assuming the applicability of a Brillouin function with J=9/2 the moment extrapolated to 0°K is 1.51 μB. These values are considerably lower than the value for the free tripositive ion (3.27 μB) or the moment observed in the paramagnetic region (3.40 μB) but are in reasonably good agreement with the theoretical value (1.58 μB) computed for 4.2°K taking into account crystal field effects. The susceptibility behavior of the four Sm hydrides studied is generally similar. The trend with temperature is in reasonably good agreement with theory which treats Sm as a free tripositive ion. Susceptibility of the Nd hydrides follow the Curie-Weiss law above about 100°K with the expected slope but deviate negatively from it at lower temperatures. The deviations are attributed to either incipient ferromagnetic ordering or crystal field effects or a combination of the two. The susceptibility behavior of the Pr hydrides is very unusual. The highest hydrides exhibit Curie-Weiss behavior above about 80°K but show positive deviations from linearity in 1/χ vs T at lower temperatures. That the deviations may be due to crystal field effects is considered possible but unlikely. PrH2.03 and PrH2.04 exhibit essentially identical susceptibility behavior and do not follow the Curie-Weiss law. Their susceptibilities are anomalously high near room temperature, exceeding the values for the element and the saturated hydride by a factor of about five.

Ferromagnetism in EuH2

Magnetic measurements have been made on a Eu-H sample of composition represented by the formula Eu${\mathrm{H}}_{1.86}$ over a temperature range extending from 4.2\ifmmode^\circ\else\textdegree\fi{} to 300\ifmmode^\circ\else\textdegree\fi{}K. The magnetization-temperature data for the sample, which for simplicity is termed Eu${\mathrm{H}}_{2}$, indicate a Curie point at 24\ifmmode^\circ\else\textdegree\fi{}K. Field dependency studies in the liquid helium range reveal saturation effects and remanence, confirming that Eu${\mathrm{H}}_{2}$ has become ferromagnetic at these temperatures. The observed saturation moment at 4.2\ifmmode^\circ\else\textdegree\fi{}K is $\ensuremath{\sim}6.0{\ensuremath{\mu}}_{B}$ (Bohr magnetons) per Eu atom. For $T>30\ifmmode^\circ\else\textdegree\fi{}$K the susceptibility shows a Curie-Weiss dependence on temperature and the effective moment is ${8.4}_{\mathrm{\ensuremath{\mu}}\mathit{B}}$ per Eu atom. Eu${\mathrm{H}}_{2}$, the saturated hydride of Eu, is unlike the highest (i.e., saturated) hydrides of the other lanthanide metals which have been studied---Gd${\mathrm{H}}_{3}$, Tb${\mathrm{H}}_{3}$ and Ho${\mathrm{H}}_{3}$---in that it alone exhibits magnetic ordering at low temperatures. As the hydrogen-saturated metals lack conduction electrons, the coupling between the localized $4f$ electrons is expected to be very weak and magnetic ordering is expected to occur only at very low temperatures if at all. Thus the behavior of the several trihydrides is in keeping with expectation whereas that of Eu${\mathrm{H}}_{2}$ is not. The coupling mechanism in this case is not well understood; it may be due to direct overlap of the $4f$ orbitals.