The magnetic and structural properties of MnAs are investigated by mapping ab initio total energies onto a Heisenberg Hamiltonian. We study the dependence of the Curie temperature over the unit cell volume and an orthorhombic distortion by using the mean field approximation, and find that for orthorhombically distorted cells the Curie temperature is much smaller than for hexagonal cells. We provide an explanation for the structural changes of both the first order phase transition at 318 K and the second order phase transition at 400 K, with the cell volume driving the stability of the different structures in the paramagnetic state. The stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7 Å, above which it remains hexagonal.
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