{\it First principles} study of Crystalline Bundles of Single-Walled Boron Nanotubes

{\it First principles} calculations based on density functional theory are performed to study the structural and electronic properties of the crystalline bundles of (n,0) zigzag-type single-walled boron nanotubes (SWBNT). The results predict a substantial modification in the properties of SWBNT bundles relative to those of the isolated nanotubes. The predicted modification can be attributed to a significant interplay between intra- and inter-tubular bonds in determining the stability of SWBNT bundles, analogous to the role played by intra- and inter-icosahedral bonds in the boron crystalline solids. The result shows the SWBNTs exhibit polymorphism, which is likely to be the cause of the difficulty in growing SWBNTs experimentally.