Heat of Formation in (Mg, X)B2 (X = Li, Na, Ca, Al)

The recent discovery of superconductivity in MgB2 with Tc ∼ 39 K caused exciting in the solid state physics community. A semi-empirical Miedema method and the Toop's model were applied to calculate the heat of formation (HT) for the ternary systems of B-Mg-X with X = Li, Na, Ca, Al. HT for (Mg, X)B2 was determined from the projection of HT B-Mg-X ternary system into the level with the equivalent boron composition at xB = 2/3. The calculated results showed that a meta-stable or an unstable phase forms with X take the hole dopants of Li and Na, while a stable phase of (Mg, X)B2 forms only with X take di-valence such as Ca or the electronic dopants, such as Al.