A neutron diffraction study of the deuterium site occupancy in Nd5Fe17D15.5

The intermetallic compound Nd5Fe17 has potentially important magnetic prope rties and is being investigated as a possible permanent magnetic material. As with many other rare-earth transition metal compounds, this phase can ab sorb significant quantities of H(D) and lesser amounts of N. Gravimetric me asurement indicate that similar to 14-16H(D) atoms can be absorbed per form ula unit leading to roughly a 20% increase in saturation magnetization and Curie temperature. We have carried out powder neutron diffraction studies t o determine the location of D atoms in a sample of Nd5Fe17 containing appro ximately 15.5D/f.u. The large, complex unit cell of Nd5Fe17, with 21 crysta llographically distinct sites per unit cell makes this a daunting task, but the diffraction diagram shows dramatic changes with D uptake, suggesting t hat such a determination is possible. We have identified about 15 sites ful ly or partially occupied by D. The majority of these are tetrahedral sites, as expected. Among remaining sites, one is octahedral site and others show unusual environments with 5 metal neighbors. A few D atoms show anomalousl y short bonds to neighboring metal atoms suggesting that their positions ma y not be entirely correct. However, elimination of these atoms from the mod el, 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 b e only partially occupied, suggesting that the adjacent sites cannot be sim ultaneously occupied.