  Engineering structure of grain boundaries (GBs) by solute segregation is a
promising strategy to tailor the properties of polycrystalline materials.
Theoretically it has been suggested that solute segregation can trigger phase
transitions at GBs offering novel pathways to design interfaces. However, an
understanding of their intrinsic atomistic nature is missing. Here, we combine
atomic resolution electron microscopy atomistic simulations to discover that
iron segregation to GBs in titanium stabilizes icosahedral units (cages) that
form robust building blocks of distinct GB phases. Due to their five-fold
symmetry, the Fe cages cluster and assemble into hierarchical GB phases
characterised by a different number and arrangement of the constituent
icosahedral units. Our advanced GB structure prediction algorithms and
atomistic simulations validate the stability of these observed phases and the
high excess of Fe at the GB that is accommodated by the phase transitions.
