On the occupation of interstitial sites by hydrogen atoms in A₆B₂₃-type intermetallic compounds (A = Y, Th;B = Mn)

Neutron diffraction data on A₆Mn₂₃D_x (A = Y, Th) are compared with the results of a recently proposed model predicting the relative hydrogen occupancy of various interstitial series in intermetallic hydrides. There is a good agreement between the experimental results and the model predictions for Y₆Mn₂₃D_8.3, in which the octahedral (6Y)4a and the tetrahedral (3Y + 1Mn)32f{hook}₃ interstitial sites are almost fully occupied. The experiments indicate that the tetrahedral (2A + 2Mn)96k void is unoccupied till x {reversed tilde} 23 and this fact is interpreted to be a result of a strong electrostatic repulsive potential of the occupied 32f{hook}₃ site, situated only ∼ 1.5 Å apart from 96k. Instead 96k, the next interstitial site to be occupied by hydrogen atoms is identified in this work as a 48i octahedral site (2A + 4Mn). It is shown that this site is identical with a corresponding occupied site reported by the neutron diffraction investigators. In view of the very short 32f{hook}₃-96k distances, a qualitative explanation of the different Y₆Mn₂₃ and Th₆Mn₂₃ hydrogen absorption capacities has been proposed.