D.G. Westlake

Site occupancies and stoichiometries in hydrides of intermetallic compounds: Geometric considerations

Abstract Hole sizes and intersite distances were calculated at various hydrogen concentrations for the interstices in hydrides of TiCr 1.8 , ZrTi 2 , ZrV 2 , ZrCr 2 , ZrFe 2 , HfV 2 and TaV 2 with the cubic MgCu 2 C15 structure and of ZrMn 2 and ErMn 2 with the hexagonal MgZn 2 C14 structure. From a survey of other metallic hydrides, empirical criteria were chosen for the minimum hole size (0.40 A) and the minimum H-H distance (2.1 A) for hydrogen atom occupancy in stable hydrides. With these criteria, a model was developed for predicting which sites and how many of them could be occupied by hydrogen atoms in each of the intermetallic compounds listed. The resulting stoichiometries were determined by experimentally filling the interstices of model lattices constructed of Friauf polyhedra without violating either of the adopted geometric criteria. For each of the compounds rationales are presented to explain the experimental observations reported by other researchers.

Hydrides of intermetallic compounds: A review of stabilities, stoichiometries and preferred hydrogen sites

Numerous attempts have been made to explain the observed stabilities, stoichiometries and preferred hydrogen atom sites in hydrides of intermetallic compounds. The stabilities of some of these hydrides have been shown to correlate with cell volume or hole size or elastic properties or heat of formation for the intermetallic compound itself. It appears that each of these factors may influence the stability, but none plays the dominant role for all systems. The development of qualitative and quantitative models (based on the concept of imaginary binary hydrides) that have been used to rationalize observed hydrogen atom site occupancies is critically reviewed. The basis of these models is empirical and thus the successes reported for them may possibly be fortuitous. The example of LaNi5H6.5 is used to demonstrate a geometric model that uses only a minimum hole radius (0.40 A) and a minimum H-H distance (2.10 A) in the development of a rationale for the observed stoichiometries in hydrides of intermetallic compounds. This review emphasizes the need for theoretical treatment leading to fundamental understanding of such systems.

Stoichiometries and interstitial site occupation in the hydrides of zrni and other isostructural intermetallic compounds

Abstract Criteria of interstitial hole size and H-H interatomic distance were used to predict the stoichiometries and the hydrogen-occupied sites in hydrides of the intermetallic compound ZrNi and other isostructural compounds. With only these simple considerations of geometry it was possible to rationalize most of the experimental observations available for such compounds. A number of neutron diffraction experiments are recommended to test some of the predictions presented.

A geometric model for the stoichiometry and interstitial site occupancy in hydrides (deuterides) of LaNi5, LaNi4Al and LaNi4Mn

Abstract Interstitial hole radii, trigonal saddle point radii and intersite distances were calculated for the compounds LaNi 5 D 6.5 , LaNi 4 AlD 4.8 and LaNi 4 MnD 5.9 in space group P 6/ mmm and for LaNi 5 D 6 in space group P 31 m . Two criteria, a minimum hole size of 0.4 A and a minimum H-H distance of 2.1 A, were adopted to allow rationalization of the observed interstitial occupancies and stoichiometries of each of these deuterides. The rationale includes consideration of diffusion mechanisms and their ramifications for site occupancy.

Interstitial site occupation in ZrNiH

In situ X-ray diffraction measurements during stepwise hydrogenation of the intermetallic compound ZrNi confirmed the existence of a stable compound ZrNiH. Neutron diffraction measurements on ZrNiD allowed a determination of the site occupied by deuterium. This site is tetrahedrally coordinated by four zirconium atoms as predicted earlier from a model based solely on geometric considerations. Occupation of this site causes the lattice to distort from an orthorhombic to a triclinic structure. The trihydride is formed when two completely different types of sites are filled, to the exclusion of the sites occupied originally, causing reversion of the lattice to an orthorhombic structure. Other confirmed predictions of the geometric model are discussed.

Terminal solubility of hydrogen in NbTa alloys and characterization of the solid solutions

Abstract Solvus temperatures for 2, 5 and 12 at.% H were determined resistometrically in tantalum, niobium and nine NbTa alloys. The terminal solubility of hydrogen in the alloys reached a maximum for Nb 0.3 Ta 0.7 . For example, at 250 K the solvus for this pseudobinary metal-metal hydride system lies at 12 at.% H compared with 1.5 for Nb and 6 for Ta. Pressure-composition-temperature measurements were made on the Nb 0.3 Ta 0.7 -H solid solutions, but the resulting thermodynamic data exhibited no anomalous behavior. The effects of hydrogen on lattice parameters and resistivities were also determined for these systems. The apparent terminal solubilities are discussed in terms of these observations and in terms of metal atom size effects, electronic structure and the properties of the hydride phase.

Isotope effect on the solvus of the V-H system

The solvus of the V-D system was determined by a resistometric technique. The solubility of deuterium is twice that determined for hydrogen in an earlier study. In the temperature range 210 to 380K, the solubility limit increases from about 0.6 to 27.4 at. pct D. Deuter-ium causes a smaller increment in the resistivity than hydrogen.

Application of a geometric model to the hydrogenation of Ti2Ni and Ti4Fe2O

Abstract A geometric model has been used to predict which interstices should be preferred by hydrogen (deuterium) atoms in the hydrides of Ti 2 Ni and Ti 4 Fe 2 O. Crystallographically, both of these intermetallic compounds are closely related to the η carbide structure, but the model predicts significant differences in their preferred hydrogen sites. The most important tetrahedral sites in Ti 2 NiH should be those coordinated by four nickel atoms, while in Ti 4 Fe 2 OH 2.25 the analogous sites coordinated by four iron atoms should be unoccupied. In contrast with Zr 3 V 3 O, another oxygen-stabilized compound with the η carbide structure, the most important type of tetrahedral interstice in Ti 4 Fe 2 O should be the one sharing a face with an octahedral interstice containing an oxygen atom. All these predictions of the model are in agreement with published experimental determinations of site occupation.

Low temperature phase transitions in V4H3

Tetragonal V4H3 was examined by the Debye-Scherrer technique and by X-ray diffractometry at temperatures below 300 K. Two structural transitions were indicated. The observable changes in metal-metal atom distances may be manifestations of hydrogen ordering.

A resistometric determination of the diffusivity of hydrogen in niobium

Abstract The diffusivity of hydrogen in niobium was determined resistometrically between 200 and 350 K. Details of the experiment and computation allowed absolute values of D to be obtained directly. These values are in excellent agreement with those indicated by the Gorsky effect as measured in other laboratories, e.g., D ∼- 8 × 10−6cm2s−1 at room temperature. Increased precision and/or increased temperature range would be required to confirm or refute a reported change in activation energy for hydrogen diffusion. At present, it may be that neither of these increases can be achieved with this resistometric technique.