HYDROGEN ABSORPTION PROPERTIES AND MAGNETIC BEHAVIOR OF RARE EARTH - TRANSITION METAL BORIDES

The hydrogen absorption properties of the hexagonal CeCo(,4)B-type ternary borides R Co(,4)B (R = La, Pr, Sm) and RNi(,4)B (R= Ca, La, Pr), as well as the related ternary La(,3)Ni(,13)B(,2) (LaNi(,4.33)B(,0.67)), have been investigated. Maximum hydrogen uptakes are given by RCo(,4)BH(,4.5), CaNi(,4)BH(,3.2), RNi(,4)BH(,1.5) (R = La, Pr), and La(,3)Ni(,13)B(,2)H(,13.7)(LaNi(,4.33)B(,0.67)H(,4.6)). Phenomenological chemical rules are used to describe the observed hydrogen capacities. These rules suggest that the R-B planes do not accommodate hydrogen. In the RCo(,4)B-H(,2) system, three distinct hydride phases exist at ambient temperature in the pressure range 0-100 atm. Models for hydrogen site occupancy in these phases are proposed and tested via configurational entropy calculations.

Emphasis is given to the hydride LaNi(,4)BH(,1.5) which is used as a model to further understand hydrogen diffusion in CaCu(,5)-type materials such as LaNi(,5-x)Al(,x)H(,z). NMR measurements of the proton rigid-lattice second moment confirm that hydrogen does not occupy sites in the vicinity of the La-B planes. These planes consequently act as barriers to hydrogen diffusion parallel to the {001} direction and limit hydrogen mobility. Reduced hydrogen diffusion is not responsible for the slow LaNi(,4)B-H(,2) reaction kinetics, however; measurements suggest that the rate-limiting step is a surface process.

In related studies, a single crystal investigation of the LaNI(,4)B structure indicates that the CeCo(,4)B atomic arrangement is only a subcell in a larger superstructure extending along the {100} direction. The expressions a(,0) = 6a(,0)('') and c(,0) = c(,0)('') describe the relationship between the lattice parameters of the CeCo(,4)B-type subcell and those of the superstructure.

Magnetic behavior of the CeCo(,4)B-type materials R(M(,1-x)M(,x)(''))(,4)B with R = rare earth and M,M' = Fe, Co, Ni is also described. Ternary and pseudoternary materials with R = Sm exhibit giant intrinsic magnetic hardness. Coercivity in SmCo(,4)B is dominated by nucleation, whereas pseudoternaries show hardness due to pinning mechanisms. Pseudoternaries containing Fe exhibit properties amendable to permanent magnet applications.