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

Abstract

Density functional theory (DFT) calculations for deriving enthalpies of formation δH for ferromagnetic MnX (X = P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal B81 and orthorhombic B31. Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchangecorrelation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFT+U approach which requires Ueff = (U-J) as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of δH for MnP and MnAs in both structures whereby the result for MnP B31 agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding δH disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFT+U ansatz was employed for all compounds and elemental Mn. The values for Ueff ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of δH. As a reference for pure Mn the --Mn phase was taken with Ueff = 1.3 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 +0.7.