Zinc-blende (α-) and NiAs-type (β-) MnAs are investigated with a combined first-principles linearized argumented plane wave and ${\mathrm{DMol}}^3$ study within both the local density approximation (LDA) and the generalized gradient approximation (GGA). First-principles calculations within the GGA predict the lattice volume for β-MnAs much better than LDA (which underestimates it by 15%) compared with experiment. The LDA calculated equilibrium lattice volume of α-MnAs is 10% smaller than that of GaAs, which is in contradiction to the well-accepted fact that the lattice volume of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ increases with x. In contrast, the GGA predicts a reasonable lattice volume for α-MnAs. The ferromagnetic α-MnAs is shown to be a metal at $a=5.7\mathrm{}\hspace{0.5em}\AA ,$ and to undergo a transition to a half-metallic phase when it expands to $a>\mathrm{}5.8\mathrm{}\hspace{0.5em}\AA$ due to the decreased bandwidth. Further, the calculated cohesive energy of β-MnAs is nearly 0.87 eV greater than that of α-MnAs, which provides theoretical support for the instability of α-MnAs.

• Received 6 April 2001

DOI:https://doi.org/10.1103/PhysRevB.65.113202