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

摘要

Density functional theory (DFT) calculations for deriving enthalpies of formation Delta H for ferromagnetic MnX (X = P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal B8(1) and orthorhombic B3(1). 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 U-eff = (U - J) as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of Delta H for MnP and MnAs in both structures whereby the result for MnP B3(1) agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding Delta H 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 U-eff ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of Delta H. As a reference for pure Mn the gamma-Mn phase was taken with U-eff = 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.