Interplay between the spin state of manganese and the stability of the crystal structure of MnAs and MnP compounds

Starting from first principles, a study of the ground states of manganese arsenide and manganese phosphide, which have a hexagonal B81 and a distorted rhombohedral B31 crystal structure, is made by the FP–LMTO method. It is shown that in these compounds a continuous transition from a high-spin to a low-spin state occurs as the volume of the lattice is changed, and it turns out that the magnetic moment of the manganese atoms is the same at equal volumes. Starting from the results of a calculation of the total energy of these systems as a function of the parameters of the structural distortions, it is established that the high-spin state suppresses the structural distortions, while the low-spin state enhances them. It is shown that the difference of the crystal and magnetic structures of manganese arsenide and phosphide is due solely to the different lattice volumes, i.e., the substitution of arsenic by phosphorus is equivalent to the application of external hydrostatic pressure. It is conjectured that a qualitative interpretation of the mechanism of magnetostructural transitions in MnAs should proceed from the idea that the leading role is the degree of occupation of the “antibonding” state of the itinerant carriers of magnetism.