  We present a systematic trend study of the symmetric tilt grain boundaries
about the <110> axis in molybdenum. Our results show that multiple structural
phases, some incorporating vacancies, compete for the boundary ground state. We
find that at low external stress vacancies prefer to bind to the boundaries in
high concentrations, and moreover, that external stress drives structural phase
transitions which correspond to switching the boundaries on and off as
pipe-diffusion pathways for vacancies. Finally, we present physical arguments
which indicate these phenomena are likely to occur in the other bcc transition
metals as well.
