Electronic and structural properties of superconducting MgB2, CaSi2, and related compounds

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 family, namely, Mg0.5Al0.5B2, BeB2, CaSi2, and CaBeSi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, 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-dimensional c-bonding bands contribute significantly at the Fermi level. Similarities and differences between MgB2 and BeB2 (whose superconducting properties have not been yet investigated) are analyzed in detail. Our calculations 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 substitution in the Si planes leads to a stable compound similar in many aspects to diborides.