  Despite recent successes in the synthesis of boron nanotubes (BNTs), the
atomic arrangement of their walls has not yet been determined and many
questions about their basic properties do remain. Here, we unveil the dynamical
stability of BNTs by means of first-principles molecular dynamics simulations.
We find that free-standing, single-wall BNTs with diameters larger than 0.6 nm
are thermally stable at the experimentally reported synthesis temperature of
870$^\circ$C and higher. The walls of thermally stable BNTs are found to have a
variety of different mixed triangular-hexagonal morphologies. Our results
substantiate the importance of mixed triangular-hexagonal morphologies as a
structural paradigm for atomically thin boron.
