Density functional theory study of structural and thermodynamical stabilities of ferromagnetic MnX (X = P, As, Sb, Bi) compounds.

Density functional theory (DFT) calculations for deriving enthalpies of formation [Formula: see text] H for ferromagnetic MnX (X [Formula: see text] P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal [Formula: see text] and orthorhombic [Formula: see text]. Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchange-correlation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFT[Formula: see text]U approach which requires [Formula: see text] as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of [Formula: see text] H for MnP and MnAs in both structures whereby the result for MnP [Formula: see text] agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding [Formula: see text] H disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFT[Formula: see text]U ansatz was employed for all compounds and elemental Mn. The values for [Formula: see text] ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of [Formula: see text] H. As a reference for pure Mn the [Formula: see text]-Mn phase was taken with [Formula: see text] eV by which choice the experimental volume is fitted. Atomic volumes and ionicities were derived applying Bader's concept resulting in ionicities of Mn less than [Formula: see text].