Structure and stability of Mg-intercalated boron nanotubes and crystalline bundles.

First principles calculations based on density functional theory predict a highly selective adsorption site for Mg atoms and negligible preference for the growth of Mg islands on the tubular surface of Mg-intercalated (small diameter) boron nanotubes, thereby establishing the criterion for understanding the growth mechanism of single-walled boron nanotubes (SWBNTs) supported by magnesium. On the other hand, the Mg-SWBNT bundles can be considered as an 'electrostatic' bound system consisting of partially ionized Mg and partially ionized tubules. The metallic character of the tubular Mg-B bundles is then attributed to boron atoms forming a metallic wire, while the role of Mg atoms is limited in enhancing the stability of the crystalline bundles.