The development of advanced Nd-Fe-B permanent magnet materials with high coercivity draws much attention to the relation between coercivity and microstructure at the grain boundaries of the magnets. A disordered face-centered cubic (fcc)-NdO x phase formed at the interface of Nd/Nd-Fe-B is observed, and it is believed to take an important role in coercivity generation. To have a thorough understanding of the formation mechanism of this particular oxide and its relation to the surface coercivity, a ground state analysis for whole oxygen concentration in Nd-O has been performed by combining the LSDA + U and the cluster expansion method. Systematic calculations revealed that a sequent fcc-based structure formed by introducing oxygen vacancies into NdO is stable in almost all the 0-50% oxygen concentration range, whereas in a series of hexagonal close-packed (hcp)-based structures developed from hP5-Nd2O3 no stable structure is observed, which coincides with the experimental measurement very well. A further analysis of formation energies and relevant changes in electronic structures of single oxygen vacancy in various structures revealed the insight of such fcc-based phase formation and further explained the relation between the phase stability and coercivity.
ASJC Scopus subject areas
- Materials Science(all)