Concerning atomic sites and capacities for hydrogen absorption in the AB 2 Friauf-Laves phases

Abstract

There is considerable confusion in the literature concerning the maximum number of hydrogen atoms per AB 2 that can be absorbed by the C15 and C14 Friauf-Laves phases (variously reported as 5.125 and 7). If all the tetrahedral sites are independently available to hydrogen, the number could approach 17; since, in practice, it rarely exceeds five, we assume that one or more limiting principles operate. The one that we discuss here is assumed to be mainly electrostatic and to be due to electron transfer resulting from electronegativity differences between the hydrogen and metal atoms. Assuming the operation of an exclusion rule which states that two tetrahedra with a face in common may not both contain hydrogen atoms at their centers, after an exhaustive model study we find a maximum occupancy of six hydrogen atoms per AB 2 in C15 and of 6 1 3 in C14. Two attractive models accommodating six hydrogen atoms (one for C15, the other for C14) are examined in some detail and the possible implications of these and related models with respect to the step-function absorption model of Jacob, Shaltiel and coworkers is discussed. The possible occupation of triangle sites is discussed briefly.