Electronic and structural properties of superconducting MgB2, CaSi2, and related compounds - art. no. 104507

We report a detailed study of the electronic and structural proper-ties of the 39 K superconductor MgB2 and of several related systems of the same fam ily, namely, Mg0.5Al0.5B2, BeB2, CaSi2, and CaBeSi. Our calculations, which include zone-center phonon frequencies and transport properties, are perfo rmed within the local density approximation to the density functional theor y, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dim ensional c-bonding bands contribute significantly at the Fermi level. Simil arities and differences between MgB2 and BeB2 (whose superconducting proper ties have not been yet investigated) are analyzed in detail. Our calculatio ns for Mg0.5Al0.5B2 show that metal substitution cannot be fully described in a rigid band model. CaSi2 is studied as a function of pressure, and Be s ubstitution in the Si planes leads to a stable compound similar in many asp ects to diborides.