Structural and superconducting transitions in Mg1-xAlxB2 (original) (raw)

From systematic ab initio calculations for the alloy system Mg1−xAlxB2 , we find a strong tendency for the formation of a superstructure characterized by Al-rich layers. We also present a simple model, based on calculated energies and an estimate of the configurational entropy, which suggests that the alloy has two separate concentration regimes of phase separation, with critical points near x = 0.25 and x = 0.75. These results, together with calculations of electronic densities of states in several ionic arrangements, give a qualitative explanation for the observed structural instabilities, as well as the x-dependence of the superconducting Tc for x < 0.6. PACS numbers: