A brief historical survey is given of how the study of coercitivity mechanisms in SmCo5 permanent-magnet materials eventually led to the discovery of the favourable hydrogen sorption properties of the compound LaNi5. It is shown how continued research by many investigators dealing with a variety of different physical and chemical properties has resulted in an advanced understanding of some of the principles that govern hydrogen absorption and which are responsible for the changes in physical properties that accompany it. The problems associated with various applications of LaNi5-based hydrogen-storage materials are also briefly discussed. A large part of this paper is devoted to the applicability of LaNi5-type materials in batteries. Research in this area has resulted in the development of a new type of rechargeable battery: the nickel-hydride cell. This battery can be charged and discharged at high rates and is relatively insensitive to overcharging and overdischarging. Special attention is given to the nature of the electrode degradation process and the effect of composition variations in LaNi5-related materials on the lifetime of the corresponding hydride electrodes when subjected to severe electrochemical charge-discharge cycles.

From permanent magnets to rechargeable hydride electrodes

The authors review available experimental information on the existence, thermodynamic stability and physical properties of hydrides formed by the absorption of hydrogen gas in intermetallic compounds of two transition metals. The emphasis is on stability. It is shown that empirical models for the stability of ternary hydrides can be reconciled with ideas based on the results of band structure calculations of (binary) metallic hydrides. It is concluded that metallic hydrides can be looked upon as alloys of metallic hydrogen. In addition to the thermodynamic properties of ternary metallic hydrides the authors discuss experimental information on electronic properties (magnetic, super-conductivity, band structure features) and on crystallographic and metallurgical properties (neutron scattering, nuclear and electron spin resonance, Mossbauer spectroscopy, diffusion). Applications are briefly reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/45/9/001/pdf

Hydrides formed from intermetallic compounds of two transition metals: a special class of ternary alloys

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.

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

Metal hydrides, as energy storage media, are receiving considerable attention. The amount of literature concerning the properties of these materials has increased markedly over the past few years. In this paper, we conduct a review of the alloy groups which absorb large quantities of hydrogen. These alloy classes are designated as AB5, AB, AB2, AB3 and A2B7 and Mg-based compounds. These materials are discussed with emphasis placed on thermodynamic and kinetic properties, activation and deactivation, poisoning effects and storage capacity.

Storing energy in metal hydrides: a review of the physical metallurgy

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.

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

A correlation has been established between the stabilities of metal hydrides (measured in terms of the free energies of formation of the hydride phase per mole H 2 ) and the radii of tetrahedral holes in both hexagonal AB 5 (D2 d ) and cubic AB (B 2 ) intermetallic compounds. The correlation demonstrates that as the tetrahedral hole size increases, the stability increases. The results of this correlation show conclusively that hole size can be employed effectively in determining the stabilities of intermetallic compound-hydrogen phases. A model, which depends solely upon lattice parameters, was developed to compute the radii of tetrahedral holes in hexagonal AB 5 intermetallic compounds. A similar model, which requires a ratio of metallic radii in addition to the lattice parameter, has been employed to compute the radii of tetrahedral holes in the cubic AB intermetallic compounds. The thermodynamic and structural properties of hexagonal AB 5 -hydrogen systems and cubic AB-hydrogen systems have been compiled and are presented. The quantitative correlations are excellent. A change in valence of the A atom in the hexagonal AB 5 intermetallic compounds may have a significant effect on the stabilities of the hydride phases formed. In those cases where the B atoms were partially substituted in the hexagonal AB 6 and the cubic AB intermetallic compounds, a good correlation of stability with hole size was found. Gross deviations of the Ce-base AB 5 hydride data from the correlation line established by all the other AB 5 hydrides were observed. These were rationalized on the basis of well-documented stress-induced lattice contractions of Ce intermetallic compounds. The resulting contracted hole sizes of the CeNi 5 and CeCo 5 compounds modified the stability of their hydrides. These compound hydrides demonstrate a displaced but similar correlation to the other AB 5 hydrides, i.e . the smaller the hole size, the less stable is the system.

A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds

The structures of four families of intermetallic compounds are analyzed in terms of the number, sizes, positions and symmetry properties of tetrahedral interstitial holes. The structure types considered are hexagonal AB5 (D2d), cubic AB2 (C15), hexagonal AB2 (C14) and orthorhombic AB (Bf). Holes and clusters of holes are considered in terms of their suitability for hydrogen occupancy in the formation of intermetallic compound hydrides.

In the cases of the hydrides LaNi5H6 and LaCo5H4, the parent hexagonal structure P6/mmm is changed on hydriding to trigonal P31m in the former case and to orthorhombic Cmmm in the latter case. The symmetry changes that correspond to these transformations are shown to be directly related to the amount of hydrogen absorbed; i.e. some potential sites in the parent compound are retained and others are not, depending on the structural change.

In those cases where structural data for the hydride phases are not available or where the parent structure does not change on hydriding, it is shown that reasonable estimates of the extent of hydrogen absorption can be made based on the number and the sizes of available holes or hole clusters in the parent compound.

Relationships between intermetallic compound structure and hydride formation

The CaF2-type dihydrides of the group IIIa elements represent a relatively large family of similar metallic hydrides for which relationships between interatomic distances and stabilities can be derived. Available data for the thermodynamic properties of formation of these dihydrides were assembled and the enthalpies and free energies of formation are compared with the metal-hydrogen distances in the hydrides. The data show that for these hydrides there is a maximum in the stability-distance curve; the most stable dihydrides are HoH2 and DyH2 with metal-hydrogen distances of 2.237A and 2.252 A respectively. A modified Born-Mayer ionic model is applied to this family of dihydrides. In addition to the usual Coulomb (attraction) and overlap (repulsion) terms, an additional attraction term is introduced to take into account residual metallic cohesion in the hydride lattice. The results obtained from this model are compared with the thermochemical data using the BornHaber cycle. In making this comparison a parameter β (which has units of length) is introduced which accounts for the influence of the 4f electronic structure on the repulsive potential for the lanthanide dihydrides. It is shown that the relation between stability and the sum of the metallic radius and the tetrahedral hole radius (in the metal) is similar to that between stability and the actual metal-hydrogen distance in these dihydrides. This similarity is used to rationalize observed correlations between stability and tetrahedral hole sizes in intermetallic compound hydrides.

Structures and stabilities of the group IIIa dihydrides

The thermionic work functions of five different beryllium samples were studied as a function of the degree of surface oxidation. The state of oxidation of each sample was determined by visual observation and by analysis of x-ray diffraction patterns. The data were analyzed by the Schottky plot-Richardson plot method. One wire was completely oxidized to BeO and its Richardson work function was found to be 3.80±0.06 e V and the thermionic constant was 31 amps/cm2-∮K2. From visual and x-ray analyses of the various samples, it is concluded that the lowest work function determined for any of the beryllium samples, 3.22±0.08 eV, is the closest to the work function of pure polycrystalline beryllium metal.

Effect of surface oxidation on the thermionic work function of beryllium.

The compounds and phase relationships in the lithium-rhodium-hydrogen system have been characterized. In addition to the well-known saline hydride LiH, the compounds that exist in this system are the intermetallic compound LiRh and the two ternary hydrides Li 4 RhH 4 and Li 4 RhH 5 . No evidence was found, at hydrogen pressures up to 1 atm and temperatures up to 600°C, for reaction between hydrogen and rhodium. The intermetallic compound LiRh is formed by direct combination of the elements in closed iron crucibles at a temperature of 700°C. The crystal structure of LiRh is hexagonal. The ternary hydride Li 4 RhH 4 can be prepared by heating a 4:1 molar ratio mixture of lithium hydride and rhodium powder in an open iron crucible, either in vacuum or in an inert atmosphere. When the experiment is carried out in a hydrogen atmosphere, hydrogen is taken up by the condensed phase and the compound Li 4 RhH 5 is formed. At room temperature both compounds are brittle, black crystalline solids. The structure of Li 4 -RhH 4 is tetragonal; that of Li 4 RhH 5 is orthorhombic. Pressure-composition isotherms were determined over the temperature range 600°C–800°C and at pressures up to 1 atm. The gram atom fraction of hydrogen in the condensed phase ranged from 0 to 0.5; the latter limit corresponds to the ternary hydride Li 4 RhH 5 . No plateau (invariant) pressure ranges were observed at these temperatures. The integral heat of solution to form a liquid of composition Li 4 RhH 4 was determined as −12.5 kcal (g atom H) −1 . Pressure-composition isotherms were also determined by hydriding and dehydriding the intermetallic compound LiRh over the temperature range from room temperature to 700°C and at pressures up to 1 atm. The isotherms determined in these experiments are presented and discussed in the text.

Charles B. Magee

Papers

A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds

Relationships between intermetallic compound structure and hydride formation

Structures and stabilities of the group IIIa dihydrides

Effect of surface oxidation on the thermionic work function of beryllium.

Compounds and phase relationships in the lithium-rhodium-hydrogen system