Role of typical elements in Nd2Fe14X ( X=B , C, N, O, F)

The magnetic properties and structural stability of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}X$ ($X=\mathrm{B}$, C, N, O, F) are theoretically studied by first-principles calculations focusing on the role of $X$. We find that B reduces the magnetic moment (per formula unit) and magnetization (per volume) in ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$. The crystal-field parameter ${A}_{2}^{0}\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}$ of Nd is not enhanced either, suggesting that B has minor roles in the uniaxial magnetocrystalline anisotropy of Nd. These findings are in contrast to the long-held belief that B works positively for the magnetic properties of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$. As $X$ changes from B to C, N, O, and F, both the magnetic properties and stability vary significantly. The formation energies of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}X$ and $\ensuremath{\alpha}$-Fe relative to that of ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{17}X$ are negative for $X=\mathrm{B}$ and C, whereas they are positive when $X$ = N, O, and F. This indicates that B plays an important role in stabilizing the ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ phase.