  We report the production of covalently bonded selenium double-helices within
the narrow cavity inside double-wall carbon nanotubes. The double-helix
structure, characterized by high-resolution transmission electron microscopy
and X-ray diffraction, is completely different from the bulk atomic arrangement
and may be considered a new structural phase of Se. Supporting ab initio
calculations indicate that the observed encapsulated Se double-helices are
radially compressed and have formed from free Se atoms or short chains
contained inside carbon nanotubes. The calculated electronic structure of Se
double-helices is very different from the bulk system, indicating the
possibility to develop a new branch of Se chemistry.
