Temperature dependence of the crystal structures and phase fractions of secondary phases in a Nd-Fe-B sintered magnet

The constituent phases of a Nd14.2Fe79.4B6.3Cu0.1 sintered magnet and their crystal structures at ambient and elevated temperatures have been determined by synchrotron X-ray diffraction. At room temperature, dhcp-Nd, NdOx, hcp-Nd2O3, and Nd5Fe18B18 (Nd1.1Fe4B4) secondary phases have been identified in addition to the Nd2Fe14B main phase. The dhcp-Nd phase melts around 600 °C, corresponding to the eutectic temperature for the ternary Nd2Fe14B, Nd and Nd5Fe18B18 system. Below 600 °C, the variation of the lattice constants of the dhcp-Nd phase with temperature is strongly influenced by an internal stress from the spontaneous magnetostriction of the Nd2Fe14B phase. The difference of the lattice constants of the dhcp-Nd phase compared to those of pristine Nd metal is also significant, which is possibly associated with the inclusion of interstitial oxygen atoms at the 4f Wyckoff positions in the dhcp-Nd crystal. The validity of this scenario is confirmed by first-principles calculations of the equilibrium lattice constants of three-, four-, and nine-unit-cell supercells, in which different numbers of oxygen atoms quasi-randomly occupy some of the unoccupied 4f-sites.