  For the first time, we report the formation of pentagonal atomic chains
during tensile deformation of ultra thin BCC Fe nanowires. Extensive molecular
dynamics simulations have been performed on $<$100$>$/{110} BCC Fe nanowires
with different cross section width varying from 0.404 to 3.634 nm at
temperatures ranging from 10 to 900 K. The results indicate that above certain
temperature, long and stable pentagonal atomic chains form in BCC Fe nanowires
with cross section width less than 2.83 nm. The temperature, above which the
pentagonal chains form, increases with increase in nanowire size. The
pentagonal chains have been observed to be highly stable over large plastic
strains and contribute to high ductility in Fe nanowires.
