Interstitial site occupation of hydrogen atoms in intermetallic hydrides: ZrNiH_x case

The present work exhibits the possible interstitial sites for hydrogen occupation in the intermetallic lattice of ZrNi(CrB - structure type) by their Wyckoff notation and by the number of Zr and Ni atoms on the polyhedron vertices. In view of a previous model, predicting the relative interstitial occupancy in intermetallic hydrides, we propose a hydrogen location scheme for ZrNiH_x. According to this scheme the 4c₃ (4Zr) tetrahedral interstices are almost solely occupied by hydrogen atoms in ZrNiH. A small quantity of hydrogen is predicted to be located at this composition in (3Zr + 1Ni) tetrahedra. At x ≈ 2.6, 4c₃ is completely and 8f₁, 8g₁ and 8g₂ are partially occupied according to our model. The 8f₁, 8g₁ and 8g₂ tetrahedral sites are crystallographically different one from the other but they are identical with regard to the metal atoms composing them -(3Zr + 1Ni); such sites are indistinguishable in our model. Above x ≈ 2.6 we predict that (2Zr + 2Ni) tetrahedra begin to be gradually populated. The agreement between our predictions and the analysis of the existing neutron diffraction data on ZrNiD_x is only a partial one. We believe, however, that some experimental evidences from pressure-composition isotherms and inelastic neutron scattering may support our predictions. The use of a Boltzmann distribution function, previously employed in calculating the relative occupation numbers of various interstitial sites, is revised for few cases (e.g. ZrNiH_x, Y₆Mn₂₃H_x), for which a Fermi distribution function must be used.