Diboride bifullerenes and binanotubes

The structures and energy characteristics of a new class of nanotubes and fullerenes formed by binary layers of a trigonal network of metal atoms M and a graphite-like network of boron atoms are considered by the example of magnesium and zirconium diborides. It is shown that, contrary to the familiar carbon monolayer nanotubes, the dependence of the strain energy of diboride bitubulene on its diameter $D$ deviates from the $1/D^2$ law because of the «crustlike» shape of a free fragment of the MB$_2$ bilayer, while bitubulenes with axis parallel to the M-M bonds have the optimum shape at a given diameter. Such bitubulenes are expected to be metallic because of the specific features of the band structure of bilayered diborides.