Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

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 Ti_1-y Cu _y H _x and Zr_1-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. Mound is operated by the Monsanto Research Corporation for the U.S. Department of Energy under Contract No. DE-AC04-76-DP00053.     

Fermi levels of FCC and FCT hydrides of Ti and Zr

Measurements of the work functions of films of TiH_x and ZrH_x during the FCC/FCT transition have given the change of Fermi level between these two phases of the hydrides. The results agree with modern calculations of the band structures of these hydrides.     

Influence of atomic vacancy ordering in the nonmetallic sublattice on the electronic structure of titanium hydride

We present a calculation of the electronic density of states in TiH_x alloys (x=2.0, 1.7, and 1.5) with various degrees of long-range atomic order, η, in the subsystem of hydrogen and vacancies in the nonstoichiometric hydrides. In comparing the calculated results with the features of the photoelectron spectrum of TiH_1.5 we find agreement for η=0.5. We determine the temperature range for the establishment of atomic vacancy order. Siberian V. D. Kuznetsov Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–14, October, 1998.     

A pulsed NMR study of hydrogen diffusion in hydrides of group IVa transition metals

Activation energies E_A for hydrogen diffusion in hydrides of Group IVa transition metals have been determined by ¹H nuclear magnetic resonance measurements of spin lattice relaxation in both the laboratory (T₁) and rotating (T_1p) frames. For the HfHx system both activation energies obtained from T₁ data, and the value of T₁ at the minimum appear to be insensitive to hydrogen content x in the range 1.58 ? x ? 1.98. For hydrides of titanium and zirconium of approximately stoichiometric composition MH₂ (where M = metal), there is excellent agreement between activation energies obtained from T₁ and T_1p data. Mean activation energies obtained were 0.51 eV for TiH_1.98 and 0.83 eV for ZrH_1.96, consistent with a single diffusion mechanism in each case over the temperature range 260–600K and 400–800K respectively. In the case of HfH_1.98 the agreement was less good, values of 0.64 and 0.55 eV being obtained from T₁ and T_1p respectively.     

The impact of ambient gas on the magnetic properties of Ti40Zr40Ni20 powders during mechanical alloying

Ti₄₀Zr₄₀Ni₂₀ in the form of elemental powders was mechanically alloyed in a planetary ball-mill under argon and hydrogen atmospheres at an acceleration of 55 m s^?2 for different time intervals. The samples were analyzed by X-ray diffraction (XRD), vibrating-sample magnetometry (VSM) and scanning electron microscopy (SEM), which revealed the large impact of the ambient gas, i.e. argon and hydrogen, on the magnetic properties, morphology and structure. Namely, during mechanical alloying in argon the saturation magnetization decreased due to the modification of the Ni d-states upon alloying with paramagnetic Ti and Zr, whereas in hydrogen the Ni d-states remained largely unmodified, since the formation of TiH₂ and ZrH₂ was faster than the alloying with the ferromagnetic nickel. However, after 40 h we obtained a mixture of nanocrystalline Ni and ZrH₂/TiH₂ hydrides, which in equilibrium contained 1.55 mass% of hydrogen. In the case of argon we determined welding of the Ti₄₀Zr₄₀Ni₂₀ amorphous particles, whereas in hydrogen such a process was suppressed by the brittle ZrH₂/TiH₂ hydrides. In addition, we revealed that the mechanical alloying of pure Ni powder for 100 h in argon does not affect its magnetic properties.     

Observations of nanoscopic,face centered cubic Ti and TiH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Transmission electron microscopy has been used to study ball milled and H cycled NaAlH₄ with 10 mol% TiCl₃. Isolated from the main phases in this hydrogen storage system, nanocrystalline aggregates of fcc TiH _x (0≤x<0.67) were found. The value of x was determined based on the assumption of a linear increase of the TiH _x lattice parameter by increasing H content. The size of the TiH _x crystallites was in the range 10 to 20 nm, and the lattice parameter decreased from 4.22 Å in TiH_0.67 to 4.10 Å in pure fcc Ti. Non-equilibrium ball milling and subsequent H cycling in combination with a small crystallite size are believed to make the TiH _x phase stable. The present results are the first observations of fcc TiH _x with low hydrogen content, and the measured fcc lattice parameter of Ti matches first-principles calculations.     

Hydrogen diffusion in β-phase titanium iron hydride

NMR pulse techniques have been used to measure proton relaxation times in high purity β-TiFeH_1.03 between 94K and 413K. Recent crystal structure parameters from neutron diffraction studies of orthorhombic β-TiFeD_x have been explicitly included in the analysis of the T₂ relaxation times to obtain more accurate hydrogen diffusion parameters than were possible from previous NMR studies of β-TiFeH_x. The room temperature (300K) hydrogen diffusion constant in β-TiFeH_x is verified to be about 1.2 ± 0.8 × 10^?12cm²/s, which is 10^?4–10^?5times smaller than in many other metal hydrides such as PdH_x or LaNi₅H_x.     

Electronic structure of TiH2

The electronic structure of TiH₂ has been studied using the augmented-plane-wave method and the LCAO interpolation. The density of states and its orbital components show that the conduction band is Ti d-like and that the valence band is largely derived from the hydrogen orbitals with small Ti 3d hybridization. The electronic charges on the hydrogen atom are ～ 1.5 as compared to 1.6–1.7 of the rare-earth metal hydrides.     

The activation of FeTi for hydrogen absorption

FeTi is an interesting hydrogen storage material which has to be activated at ≈670 K for the absorption of hydrogen. We review critically the great number of previously published results and models on this activation process and emphasize the controversial points. To eliminate the controversy we analysed the variation of the surface composition of FeTi upon activation in the high-pressure cell of a photoelectron spectrometer. The initially passivating surface oxide is shown to be converted into a mixture of TiO₂ and Fe by surface segregation and chemical reduction. No evidence for the formation of Fe₂Ti₄O _x , FeTiO _x , and TiH _x is found. H₂/D₂ exchange reactions show that H₂ dissociates rapidly on Fe and FeTi, but not on TiO₂. The surface of FeTi is activated easily at 670 K. Difficulties encountered with the initial hydrogen absorption by virgin high purity FeTi are probably related to bulk (H diffusion, fracture toughness) rather than surface properties.     

X-ray spectroscopic study of the electronic structure and location of hydrogen atoms in HfTi2H x hydrides

The TiKβ₅ spectra and the electronic structure of HfTi₂H_x hydrides (x = 0, 0.5, 3.0, 4.0, 4.5, 5.95) are investigated. The location of hydrogen atoms in the structure of the HfTi₂H _x hydrides is determined, and the hydrogen occupancies of the positions e and g are calculated. The inference is made that the hydrogen atoms occupy only the e positions at a low hydrogen content up to x = 3.0 (this is confirmed by the experimental crystallographic data) and the positions e and g at a higher hydrogen content.     

Electronic structure and stability relations of CaF2 and NaCl phases in a Ti-H system

Calculations are made of the electronic structures of TiH_0.73 and TiH hydrides having a NaCl crystal structure and TiH_1.5, TiH_0.73, and TiH having a CaF₂ structure. Estimates of their superconductivity temperature indicate that this should be significantly higher for the stoichiometric NaCl phase compared to nonstoichiometric phases. The results are compared with the characteristics of the χ phase of titanium hydride TiH_0.73 which undergoes a superconducting transition at 4.3 K. Fiz. Tverd. Tela (St. Petersburg) 40, 195–197 (February 1998)     

Influence of hydrogen on structural and magnetic properties of the hexagonal Laves phase HoMn2

The HoMn₂ compound crystallizes in the cubic C15 or hexagonal C14 Laves phases depending on preparation. The effect of hydrogen absorption on structural and magnetic properties of HoMn₂H_x hydrides for the C14 phase has been investigated by XRD and AC/DC magnetometry in the temperature ranges of 75-380 K and 4-390 K, respectively. In addition to general features revealed by RMn₂H_x compounds (R=rare earth or Yttrium), unusual behavior of these hydrides was found. In particular, a transformation from the hexagonal to the monoclinic structure was detected, the same as that observed for cubic HoMn₂H_x compounds. The structural transformations are correlated to the magnetic behavior. The presented results are compared mainly with the properties of the cubic HoMn₂H_x hydrides as well as with those of other RMn₂H_x hydrides. Tentative magnetic and structural phase diagrams are proposed.     

Role of hydrogen in the absorption of ionizing radiation energy by a metal-hydrogen system

Ab initio calculations of the electronic structure of pure Pd, pure Ti, and PdH_x and TiH_x (x = 1, 2, 3) systems are performed within the local density approximation. It is found that the electronic subsystem of metals containing dissolved hydrogen increases their capacity to absorb the energy of electromagnetic radiation and accumulate it for a longer time than pure metals. These two factors promote the nonequilibrium migration of hydrogen atoms and their release from metals upon exposure to ionizing radiation.     

Magnetic properties of titanium vanadium hydrides

Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti_1?yV_yH_x in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x^?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x^?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration.     

Nuclear magnetic resonance study of HfV2Hx and ZrV2Hx (0 ⩽ x ⩽ 4.2) hydrides

A proton NMR spin echo study of the system HfV₂H_x and ZrV₂H_x (0 ? x ? 4.5) hydrides are reported. The T₁ spin lattice relaxation rate enables us to extract the activation energy for diffusion, E_a, as well as the attempt frequency v₀; both depend strongly on the hydrogen concentration. The existence of a correlation between E_a and v₀ in these and other hydride systems yield the temperature dependence of the activation energy.     

Hydrogen mobility in crystalline and amorphous Zr2PdHx

NMR measurements of proton rotating-frame relaxation times were made to deduce hydrogen diffusion parameters for crystalline and amorphous Zr₂PdH_x. Enhanced proton mobility was observed in glassy a-Zr₂PdH_2.9 relative to diffusion behavior in any crystalline Zr₂PdH_x sample. Both host crystal structure and proton site occupancy make significant contributions to the diffusion process where smaller activation energies correspond to jumps involving octahedral sites.     

On the thermodynamics of phase transitions in metal hydrides

Metal hydrides are solutions of hydrogen in a metal, where phase transitions may occur depending on temperature, pressure etc. We apply Le Chatelier’s principle of thermodynamics to a particular phase transition in TiH _x , which can approximately be described as a second-order phase transition. We show that the fluctuations of the order parameter correspond to fluctuations both of the density of H⁺ ions and of the distance between adjacent H⁺ ions. Moreover, as the system approaches the transition and the correlation radius increases, we show -with the help of statistical mechanics-that the statistical weight of modes involving a large number of H⁺ ions (‘collective modes’) increases sharply, in spite of the fact that the Boltzmann factor of each collective mode is exponentially small. As a result, the interaction of the H⁺ ions with collective modes makes a tiny suprathermal fraction of the H⁺ population appear. Our results hold for similar transitions in metal deuterides, too. A violation of an -insofar undisputed-upper bound on hydrogen loading follows.     

A 1H NMR study of the effects of Ni additions on the behavior of hydrogen in β-vanadium hydride

The behavior of hydrogen in the hydrides of vanadium metal containing small amounts of nickel was studied by ¹H NMR spectroscopy. FT spectra and spin-spin and spin-lattice relaxation times for VH_0.77, V_0.95Ni_0.05H_0.73, and V_0.90Ni_0.10H_0.65 were measured at temperatures between 77 and 400 K. On the addition of nickel the number of hydrogen atoms on O₂₂ sites decreases and the superstructure of hydrogen is altered. Different effects of nickel on hydrogen diffusion are observed above and below about 200 K, and, therefore, the mechanism of hydrogen diffusion is assumed to change at this temperature.     

Étude par rmn de la localisation et des mouvements des protons dans les hydrures MgH2 et Mg2NiH4

Wide line and pulsed NMR studies are reported for MgH₂; and Mg₂NiH₄ hydrides at 79 and 30 MHz in the temperature range between 100 and 500 K. Line shape data confirm a rutile type structure for MgH₂, and lead to the evaluation that 97% of the hydrogen is present in this phase, the remainder being in solid solution MgH_x, (x<0.04).Mg₂NiH₄ shows a Gaussian line, whose peak-to-peak width decreases from 6.16 to 4.28 G in the range 320–365 K. From 365 to 480 K the spectrum shows a second, narrower, line (0.85 G), implying that approximately 7% of the protons have migrated from their initial position to energetically less stable sites. The thermal behaviour of the T₁ and T₂ relaxation times shows a dramatic variation in the 320–370 K temperature range in connection with the change of the proton localization. Relaxation mechanisms can be attributed mainly to conduction electron-nucleus interactions. With rising temperature, diffusion mechanisms are also involved. A diffusion activation energy of about 0.35 eV has been determined, with a diffusion coefficient 3.45 × 10^?8 < D < 4.6 × 10^?8 cm²/s.