Results are given of a study of the change in hydrogen sorption properties caused by partly replacing La or Ni in LaNi5 by other metals. Desorption isotherms at 40 deg C were measured for or Cu, and for La0.8R0.2Ni5, where represents Nd, Gd, Y, and Er, also Th and Zr. Equilibrium hydrogen pressures in the two-phase region can be explained qualitatively by assuming that the more stable the original RNi5 compound, the weaker is the tendency to form a RNi5--hydride, i.e., the higher the equilibrium pressure obtained for the hydride. This assumption which connects the stability of ternary hydrides with the heat of formation of the original compounds can also be formulated in a more quantitative way. This leads to a fair description of the experimental observations for the compounds having the CaCu5 type of structure.

Hydrogen absorption in LaNi5 and related compounds: Experimental observations and their explanation

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

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 LaNi system has been reinvestigated in the range 50–100 at. % Ni. The following compounds were observed: LaNi, LaNi 1.4 , LaNi 2 , LaNi 3 , La 2 Ni 7 and LaNi 5 . For these compounds the lattice constants have been determined. The phase LaNi 5 has a relatively large homogeneity region at elevated temperatures. For the compounds LaNi x within this region the equilibrium hydrogen pressure, p , of the 40 °C isotherms was found to change from 2.75 atm for LaNi 4.9 to about 9.2 atm for LaNi 5.4 according to In p = Cx + q with C = 2.4, q = − 10.75.

Phase relations and hydrogen absorption in the lanthanum-nickel system

The pressure-composition isotherms of the SmCo5-H and LaNi5-H systems have been measured for absorption as well as desorption. In the two-phase region of these systems, hysteresis is found to occur. It is argued that the magnitude of the hysteresis is related to the rather large volume expansion which accompanies the formation of the hydride.

Sorption hysteresis in the LaNi5-H and SmCo5-H systems

The lattice constants, the Curie temperature and the saturation magnetization of the pseudobinary system LaCo 5x Ni 5−5x have been determined as a function of composition. The variation of the lattice constants suggests an anisotropic magneto-volume effect accompanying the paramagnetic-ferromagnetic transition. The hydrogen absorption capacity decreases markedly with x in the series LaCo 5x Ni 5−5x . The nature of the hydride phases corresponding to the various plateau pressures of the absorption isotherms has been studied by X-ray diffraction. The hydrogen equilibrium pressure at the first hydride is strongly reduced when x increases. Dependent on x one can discern two regions where this equilibrium pressure varies as In p = Cx + q , the absolute value for C being largest for small x values.

Hydrogen absorption and magnetic properties of LaCo5xNi5−5x compounds

Pressure-composition isotherms and results of X-ray and magnetization measurements are presented for the pseudobinary systems CeCo5H and LaCo5-H In the latter system two orthorhombic hydrides are formed with hydrogen contents of ≈335 (β hydride) and ≈43 H (γ hydride) From ln p vs 1/T plots the heat of formation of the hydrides was derived: −108 and −77 kcal/mol H2, respectively In the CeCo5H system one orthorhombic (β) hydride is observed with a hydrogen content not exceeding 3 H CeCo 5 , At room temperature the primary hydrogen solubility in the hexagonal CeCo5 phase is relatively large Magnetization measurements (σ∥ − T) were performed on aligned powder specimens The magnetization of CeCo5 decreases only moderately (≈ 20%) due to the absorption of 22 CeCo 5  The σ∥ − T curves of the two LaCo5 hydrides are quite different, the β hydride being much more strongly dependent on temperature than the γ hydride (in the temperature range 4–320 K) From an extrapolation of the σ∥ curves the Co moments at 0 K are determined: 114 μB in the β hydride and 034 μB in the γ hydride (144 μB in LaCo5)

F.A. Kuijpers

Papers

Sorption hysteresis in the LaNi5-H and SmCo5-H systems

Hydrogen absorption and magnetic properties of LaCo5xNi5−5x compounds

Investigations on the LaCo5H and CeCo5H systems

Hydrogen in rare earth intermetallics