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

Abstract The constituent phases of a Nd 14.2 Fe 79.4 B 6.3 Cu 0.1 sintered magnet and their crystal structures at ambient and elevated temperatures have been determined by synchrotron X-ray diffraction. At room temperature, dhcp-Nd, NdO x , hcp-Nd 2 O 3 , and Nd 5 Fe 18 B 18 (Nd 1.1 Fe 4 B 4 ) secondary phases have been identified in addition to the Nd 2 Fe 14 B main phase. The dhcp-Nd phase melts around 600 °C, corresponding to the eutectic temperature for the ternary Nd 2 Fe 14 B, Nd and Nd 5 Fe 18 B 18 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 Nd 2 Fe 14 B 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 4 f 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 4 f -sites.