An ab initio study of the relative stabilities and equations of state of FeS polymorphs

An investigation into the relative stabilities and equations of state of st oichiometric FeS was conducted using first-principles pseudopotential calcu lations. These calculations were based on density functional theory and per formed using ultrasoft Vanderbilt pseudopotentials within the generalized g radient approximation. We have identified four stable polymorphs of FeS alo ng the 0 K isotherm as a function of pressure: troilite, an orthorhombic Mn P-type structure, a monoclinic structure, and a CsCl-type structure. The ca lculated internal energy as a function of volume for each polymorph was fit ted to 4(th) order logarithmic and 3(rd) order Birch-Murnaghan equations of state, yielding values for the bulk modulus, K, and its first and second d erivatives with respect to pressure, K' and K ". These equations of state m ay be used to characterize models of planetary cores.