Structural, electronic, and magnetic properties of α- and β-MnAs: LDA and GGA investigations

Zinc-blende ({α}-) and NiAs-type ({β}-) MnAs are investigated with a combined first-principles linearized argumented plane wave and DMol³ 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 Ga_1-xMnₓAs 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 , and to undergo a transition to a half-metallic phase when it expands to a>5.8 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.