A neutron diffraction study of the deuterium site occupancy in Nd 5Fe17D15.5

The intermetallic compound Nd₅Fe₁₇ has potentially important magnetic properties and is being investigated as a possible permanent magnetic material. As with many other rare-earth transition metal compounds, this phase can absorb significant quantities of W(D) and lesser amounts of N. Gravimetric measurements indicate that ~14-16 H(D) atoms can be absorbed per formula unit leading to roughly a 20% increase in saturation magnetization and Curie temperature. We have carried out powder neutron diffraction studies to determine the location of D atoms in a sample of Nd₅Fe₁₇ containing approximately 15.5D/f.u. The large, complex unit cell of Nd₅Fe₁₇, with 21 crystallographically distinct sites per unit cell makes this a daunting task, but the diffraction diagram shows dramatic changes with D uptake, suggesting that such a determination is possible. We have identified about 15 sites fully or partially occupied by D. The majority of these are tetrahedral sites, as expected. Among remaining sites, one is an octahedral site and others show unusual environments with 5 metal neighbors. A few D atoms show anomalously short bonds to neighboring metal atoms suggesting that their positions may not be entirely correct. However, elimination of these atoms from the model, one by one, leads to relaxation of other positions, and in some cases, other bonds become unacceptably short. In addition, some D atoms show very short bonds to neighboring D sites. In these cases the sites are found to be only partially occupied, suggesting that the adjacent sites cannot be simultaneously occupied