Hydrogen Site Occupancy Research Articles

NMR studies of electronic structure and hydrogen diffusion in transition metal hydrides

Nuclear magnetic resonance spectroscopy has had extensive applications for the characterization of numerous metal-hydrogen systems. Although the greatest emphasis of proton NMR has been to evaluate diffusion behavior, increasing attention has been addressed upon the correlation of proton Knight shifts and the conduction electron contributions to proton spin-lattice relaxation times to the electronic structure properties of the hydride. The general principles of NMR, that pertain to the usual situations for most transition metal hydrides, will be briefly reviewed. Several specific examples from some recent research will be discussed in greater detail. In particular, the roles of host crystal structure and hydrogen site occupancy to hydrogen diffusion behavior are examined for the Ti1-y Cu y H x and Zr1-y Pd y H x systems. The proton hyperfine parameters in TiH x and ZrH x , as well as several related ternary hydrides, are used to qualitatively assess the character of the Fermi level electronic states. The relationship between the tetragonal distortions of the Ti and Zr dihydrides and a solid-state Jahn-Teller mechanism will also be examined.

Hydrogen absorption by the ternary borides RCo 4B (R ≡ La, Pr, Sm) and RNi 4B (R ≡ Ca, La, Pr)

The hydrogen absorption properties of the ordered hexagonal ternary borides RCo 4B (R ≡ La, Pr, Sm) and RNi 4B (R ≡ Ca, La, Pr) were investigated in order to obtain information about the parameters leading to hydride formation in CaCu 5-type intermetallics. RCo 4B compounds crystallize with the CeCo 4B structure and absorb about 4.5 hydrogen atoms per formula unit. Multiplateau behavior indicates that at least three distinct hydride phases exist around room temperature in the pressure range 0–100 atm. In contrast with the RCo 5H z phases, which are orthorhombic for z > 1, RCo 4BH z phases exhibit an orthorhombic structure only in the α phase region ( z ⩽ 1), with all higher hydrides reverting to the hexagonal structure. CaNi 4B, which also crystallizes with the CeCo 4B structure, absorbs 3.2 hydrogen atoms per formula unit at 100 atm and 324 K. The other RNi 4B compounds produce X-ray powder patterns which are similar to CeCo 4B-type patterns but which exhibit several anomalies. In the case of LaNi 4B, singlecrystal studies confirm that a long-period superstructure extends along the basal plane directions. The RNi 4 (R ≡ La, Pr) ternaries show a maximum hydrogen uptake of 1.5 hydrogen atoms at room temperature in the pressure range 0–100 atm. This appears to indicate that only built1 2 R in the formula RNi 4B is active in absorbing hydrogen (according to the subunit built1 2 RH 3 ). It is assumed that the R which is active in bonding with hydrogen in these latter materials is situated in the R-T plane, while the R in the R-B plane is inactive in this respect. Models for hydrogen site occupancy in the RCo 4B hydrides are proposed on the basis of crystallographic considerations.

Site occupation and local mode of hydrogen in ZrO 0.4 studied by time-of-flight neutron scattering

Time-of-flight measurements of elastic and inelastic neutron scattering were performed to determine the site occupation and local mode of hydrogen (deuterium) atoms dissolved in the hexagonal Zr-O solid solution. A local mode peak at 135 meV, which was consistent with hydrogen being located on the tetrahedral (t) site, was observed for ZrO 0.4H 0.1. The single-crystal neutron diffraction study of ZrO 0.4D 0.1 determined the occupation probabilities of deuterium in the specific t sites. Refinement of the crystallographic parameters of oxygen and metal atoms was performed using X-ray and neutron diffraction analyses.

On the hydrogen site occupation in hydrides of intermetallics

Abstract We present calculations for the site occupation in some ternary hydrides based on an extended version of the model of “imaginary binary hydrides”. We obtain reasonable agreement with structural data for the ZrV 2 H x system. However, the results obtained for other metal-hydrogen systems suggest that this agreement could be fortuitous. We attempt to find some reasons for the failure of the model applied here.

Hydrogen site occupancy and hydrogen diffusion inLaNi4BH1.5

The ${\mathrm{CeCo}}_{4}$B-type ternary compound ${\mathrm{LaNi}}_{4}$B reacts with hydrogen gas at room temperature to form the hydride ${\mathrm{LaNi}}_{4}$${\mathrm{BH}}_{1.5}$ in the pressure range 1-100 atm. Rigid-lattice proton second moments indicate that hydrogen only occupies sites within the La-Ni basal planes in this hydride. The La-B planes, which do not accommodate hydrogen, appear to act as barriers to long-range hydrogen diffusion along the [001] direction. We have studied the diffusion behavior of hydrogen in ${\mathrm{LaNi}}_{4}$${\mathrm{BH}}_{1.4}$ by NMR measurements of the proton relaxation times ${T}_{1}$ and ${T}_{1\ensuremath{\rho}}$ and find evidence for at least two activated-jump processes. The long-range diffusion process is characterized by lower mobilities and larger activation energies than observed in ${\mathrm{LaNi}}_{5}$${\mathrm{H}}_{6}$; comparison of the results with those previously obtained for ${\mathrm{LaNi}}_{4}$${\mathrm{AlH}}_{4.3}$ suggests that the activated step for diffusion in these hydrides is passage of hydrogen through the $12o$ sites, or through saddle points joining these sites to other sites occupied by hydrogen. Evidence for a low-activation-energy process is also observed; this process is interpreted as a localized hopping within clusters of $3f\ensuremath{-}12n$ sites that does not result in hydrogen transport. These observations are consistent with the five-site model [$\frac{P6}{\mathrm{mmm}} ({D}_{6h}^{1})$ space group] for hydrogen site occupancy in ${\mathrm{LaNi}}_{5}$${\mathrm{H}}_{6}$, but they are less easily related to site occupancies for the $P31m ({C}_{3v}^{2})$ model. Additionally, the presence of several anomalies in the ${\mathrm{LaNi}}_{4}$B x-ray powder pattern prompted us to conduct single-crystal investigations which indicate that the ${\mathrm{CeCo}}_{4}$B structure represents only a subcell in a larger hexagonal antiphase domain structure extending along the directions of the basal-plane axes.

Temperature dependence of hydrogen site occupancy in Yttrium dihydride

Neutron inelastic scattering and diffraction techniques were used to determine the temperature dependence of the octahedral site occupancy of hydrogen in YH 2.0 between 15 and 420 K. The results show an approximately constant fraction of the hydrogen atoms on octahedral sites up to 200 K followed by a monotonic decrease contrary to intuition and simple theory. This decrease in site disorder with increasing temperature is explained qualitatively by including the different effects of hydrogen at tetrahedral and octahedral sites on the metal lattice entropy and quantitatively by including hydrogen-hydrogen interactions.

Deuterium site occupation and magnetism in Ho 6Fe 23D x, compounds

High resolution neutron diffraction techniques were used to establish the hydrogen (deuterium) site occupation and sublattice magnetization of a series of Ho 6Fe 23D x compounds ( x = 1.5, 8.2, 12.1 and 15.7). The f.c.c. structure of the parent compound was maintained except for the D 12.1 sample which exhibited a tetragonal distortion. The f.c.c. lattice was reestablished for the higher D 15.7 composition. Diffraction and inelastic hydrogen vibration spectra data demonstrate conclusively that the deuterium atoms in the D 1.5 material fully occupy the octahedral a site with the residual in the tetrahedral f site. At the D 8.2 and higher concentration level, the a site is empty and the deuterium atoms partially occupy only the tetrahedral sites. The 4 K holmium magnetic moment was near the free-ion value of 10 μ B for all compounds and was oriented antiparallel to all four iron site moments (b, d, f 1 and f 2). For the cubic materials, the magnetization on the iron sites varied with deuteration in the range 0.8–2.4 μ B , with the D 15.7 compound showing the highest iron moment for all sites. A spin reorientation is observed in the tetragonal material between room temperature and 4 K with the holmium and iron b and n 1 moments rotating out of the basal plane.

Preferred hydrogen sites and hydride stoichiometry for A6B23 compounds

The variety of interstices available for potential occupation by hydrogen in A6B23 intermetallic compounds makes them important tests of a geometric model recently developed for rationalizing observed hydride stoichiometries and preferred hydrogen sites. The model was applied to Y6Mn23D18 and Th6Mn23D16, for which structure determinations can be found in the literature. Both compounds have cubic structures of space groupFmSm, and they have 116 metal atoms per unit cell. Nine different kinds of sites (708 sites per unit cell) have been considered for potential occupancy. Hole radii and inter-site distances were calculated for all these types. Concepts of minimum hole radius (0.040 nm) and minimum H-H distance (0.210 nm) were used to predict hydrogen-site occupancy. The model correctly predicts that, in addition to a and f3 sites, j1 sites (Wyckoff notation) should be involved in both of these compounds at these H(D) concentrations. The location actually observed for the deuterium atom is in the hexahedral i site that has its centre in the trigonal face shared by two j1 sites. For Y6Mn23D23, the model correctly predicts the partial occupation of h sites, which are coordinated by two yttrium and three manganese atoms. Other predictions have been made, including one regarding partial occupation of I sites when there are more than 25 hydrogens per formula unit. The latter predictions have not been tested experimentally. With this crystal structure, both compounds appear to have a maximum absorption capacity of 33 hydrogens per formula unit.

Evidence for independent motional processes on the two interstitial sublattices of a layer-structured metal hydride: Hydrogen spin-lattice relaxation and motional narrowing in zirconium monohalide hemihydrides,ZrXH0.5

We report the results of a proton-magnetic-resonance investigation of hydrogen location and motion in the hemihydrides $\mathrm{Zr}X{\mathrm{H}}_{0.5}$ of the metallic layer-structured monohalides $\mathrm{Zr}X$ of zirconium ($X=\mathrm{B}\mathrm{r},\mathrm{C}\mathrm{l}$). Wide-line and pulsed NMR methods were employed to measure the temperature dependence of the linewidth and second moment and of the spin-lattice relaxation time in the laboratory and rotating frames. The results indicate that hydrogen forms an ordered structure on the tetrahedral ($T$) interstitial sublattice within the Zr metal bilayers, with some (small) random occupancy of octahedral ($O$) sites. Two stages of motional narrowing observed in the wide-line measurements and double minima found in the relaxation times are consistent with the occurrence of essentially independent hydrogen motional processes on the $T$ and $O$ interstitial sublattices. Hydrogen site occupancy probabilities, jump frequencies, activation energies for hydrogen diffusion, and conduction-electron contributions to the proton spin-lattice relaxation rate are deduced from the measurements.

Proton NMR studies of hydrogen diffusion and electronic structure in crystalline and amorphous titanium-copper hydrides

The proton line shapes, spin-lattice and rotating-frame relaxation times, and Knight shifts have been measured in crystalline TiCu${\mathrm{H}}_{0.94}$, ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{1.9}$, and ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{2.63}$, and in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$. The second moments of the rigid-lattice line shapes indicate that protons occupy ${\mathrm{Ti}}_{4}$ interstitial sites in crystalline TiCu${\mathrm{H}}_{0.94}$ and ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{1.9}$ while both ${\mathrm{Ti}}_{4}$ and ${\mathrm{Ti}}_{4}$${\mathrm{Cu}}_{2}$ sites are occupied in crystalline ${\mathrm{Ti}}_{2}$Cu${\mathrm{H}}_{2.63}$. Although a detailed determination of hydrogen site occupancy in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$ was not possible, the proton second moment implies octahedral site occupancy in addition to tetrahedral site occupancy. The host-metal structure and hydrogen site occupancies strongly influence both the hydrogen-diffusion behavior and electronic properties. The proton hyperfine interactions appear to be dominated by the transferred core-polarization terms from the Ti $d$ states near the Fermi level as previously found in $\ensuremath{\gamma}$-phase $\mathrm{Ti}{\mathrm{H}}_{x}$ and other Ti-based ternary hydrides. The density of electron states at the Fermi level as monitored at the proton sites is apparently reduced in amorphous $a$-TiCu${\mathrm{H}}_{1.4}$ when compared with densities for the crystalline samples. It is suggested that this reduction may reflect a smearing of the energy bands with the elimination of long-range order in the amorphous phase. The greatly enhanced hydrogen diffusion (relative to crystalline TiCu${\mathrm{H}}_{0.94}$ and $\ensuremath{\gamma}\ensuremath{-}\mathrm{Ti}{\mathrm{H}}_{x}$) has been confirmed for amorphous $a$-TiCu${\mathrm{H}}_{1.3\ifmmode\pm\else\textpm\fi{}0.1}$. The diffusion behavior in crystalline ${\mathrm{Ti}}_{2}\mathrm{Cu}{\mathrm{H}}_{x}$ supports the view that H-atom diffusion jumps through octahedral sites have lower activation energies than the direct tetrahedral-to-tetrahedral jump paths.

Determination of the hydrogen site occupation in the α phase of zirconium hydride and in the α and β phases of titanium hydride by inelastic neutron scattering

Inelastic neutron scattering measurements have been used to determine the site occupied by hydrogen in alpha Zr-H and alpha and beta Ti-H. The measured optical peak frequencies were only consistent with hydrogen being located on the tetrahedral sites in all cases. The significance of the energy shift between the hydride phase and the solid-solution phase is discussed.

Hydrogen absorption mechanism and location in intermetallic compounds

The purpose of this review is to acquaint the reader with the new field of ternary hydrides and to expose him to its relevance to modern energy technology. Studying their fundamental properties is one fascinating aspect in view of their novel and unusual nature. The methods of hyperfine interactions have an important role to play in their microscopic understanding, as we have illustrated through a few examples. The absorption mechanism of hydrogen has been shown to be closely related to the nature of surface overlayers and their catalytic activity. Similar measurements will be of importance also in many of the other catalytic reactions used in industry. The microscopic nature of hyperfine interaction tools also helps in determining the hydrogen site occupancy. In addition, such methods can in principle yield dynamic information which are useful in investigating relevant properties such as diffusion rates of hydrogen.

166 Er Mössbauer and X-ray diffraction study of ErMn 2 hydrides

The pressure-composition isotherm at 22 °C for ErMn 2H x between 1 and 10 4 Torr shows the formation of two hydride phases with compositions x = 4.6 and x = 4.0 and with equilibrium hydrogen pressures of 15 Torr and less than 1 Torr respectively. Both hydrides are expanded versions of the C14 hexagonal Laves structure of ErMn 2 with volume expansions of 23% and 37% for the lower and higher order phases respectively. A slight preferential expansion parallel to the c 0 axis is observed. The 166 Er Mössbauer results show that the magnetic moment of erbium in both hydride phases is below the free-ion value measured for ErMn 2. The ferromagnetic ordering present in ErMn 2 and ErMn 2H 4 is absent in ErMn 2H 4.6 down to 1.5 K. Two erbium sites are detected in ErMn 2H 4 instead of the one expected for the C14 structure. The origin of these sites is discussed in terms of possible configurations of hydrogen site occupancy.