Cohesion in AlB₂-Type Diborides: A First-Principles Study

Abstract

Systematic trend in lattice constants and heats of formation is studied for AlB₂-type diborides from first principles. Observed trend in the lattice constants and the c/a ratio can be interpreted by anisotropic bonding of pσ-pσ and sp-pπ (d-pπ) in the sp (d) metal diborides. Calculated heats of formation are in good agreement with experimental data available and their chemical trend in the d metal diborides is governed mostly by filling of d-pπ bonding and anti-bonding bands. Obtained electronic bonding mechanism may explain several structural aspects including reasons why MgB₂ has so large c/a ratio among the diborides and why there exist only few diboride phases with AlB₂ type.