Cohesion in AlB2-type diborides:: A first-principles study

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 psigma-psigma, and sp-ppi (d-ppi) 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-ppi 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.