  Recent experimental measurements of Ag impurity diffusion in the {\Sigma}5
(310) grain boundary (GB) in Cu revealed an unusual non-Arrhenius behavior
suggestive of a possible structural transformation [Divinski et al., Phys. Rev.
B 85, 144104 (2012)]. On the other hand, atomistic computer simulations have
recently discovered phase transformations in high-angle GBs in metals [Frolov
et al., arXiv:1211.1756v2 (2013)]. In this paper we report on atomistic
simulations of Ag diffusion and segregation in two different structural phases
of the Cu {\Sigma}5 (310) GB which transform to each other with temperature.
The obtained excellent agreement with the experimental data validates the
hypothesis that the unusual diffusion behavior seen in the experiment was
caused by a phase transformation. The simulations also predict that the
low-temperature GB phase exhibits a monolayer segregation pattern while the
high-temperature phase features a bilayer segregation. Together, the
simulations and experiment provide the first convincing evidence for the
existence of structural phase transformations in high- angle metallic GBs and
demonstrate the possibility of their detection by GB diffusion measurements and
atomistic simulations.
