  We analyze the dislocation content of grain boundary (GB) phase junctions,
i.e., line defects separating two different GB phases coexisting on the same GB
plane. While regular GB disconnections have been characterized for a variety of
interfaces, GB phase junctions formed by GBs with different structures and
different numbers of excess atoms have not been previously studied. We apply a
general Burgers circuit analysis to calculate the Burgers vectors b of
junctions in two {\Sigma}5 Cu boundaries previously simulated with molecular
dynamics. The Burgers vectors of these junctions cannot be described by the
displacement shift complete (DSC) lattice alone. We show that, in general, the
normal component of b is not equal to the difference in the GB excess volumes,
but contains another contribution from the numbers of GB atoms per unit area
{\Delta}N required to transform one GB phase into another. In the boundaries
studied, the latter component dominates and even changes the sign of b. We
derive expressions for the normal and tangential components of b in terms of
the DSC lattice vectors and the non-DSC part due to {\Delta}N and additional GB
excess properties, including excess volume and shears. These expressions
provide a connection between GB phase transformations driven by the GB free
energy difference and the motion of GB junctions under applied normal and shear
stresses. The proposed analysis quantifies b and therefore makes it possible to
calculate the elastic part of the energy of these defects, evaluate their
contribution to the nucleation barrier during GB phase transformations, and
treat elastic interactions with other defects.
