Electronic and structural properties of superconducting diborides and calcium disilicide in the AlB₂ structure

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We report a detailed study of the electronic and structural properties of the 39K superconductor and of several related systems of the same family, namely,, and. 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 σ-bonding bands contribute significantly at the Fermi level. Similarities and differences between and (whose superconducting properties have not been yet investigated) %it is not yet know if it is a superconductor) are analyzed in detail. Our calculations for show that metal substitution cannot be fully described in a rigid band model. 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.