Interstitials in tetrahedrally close-packed phases: C, N, O, and F in ?-tungsten from first principles

Several tetrahedrally close-packed (tcp) phases, such as ?-W, ?-Ta, and ?-U, are believed to be stabilized by impurities. Here we analyze the various ways in which impurities can be dissolved in tcp structures paying special attention to interstitial configurations in the ?-W (A15) structure. We find that at most there are only seven interstitial positions possible. Through ab initio calculations we show that common impurities such as C, N, O, and F, dissolve interstitially in ?-W and that N, O, and F interstitials prefer the same position in the ?-W (A15) structure