  Though the electrical responses of the various polymorphs found in
ferroelectric polycrystalline thin film HfO$_2$ are now well characterized,
little is currently understood of this novel material's grain sub-structure. In
particular, the formation of domain and phase boundaries requires investigation
to better understand phase stabilization, switching, and interconversion. Here,
we apply scanning transmission electron microscopy to investigate the atomic
structure of boundaries in these materials. In particular, we find
orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclinic
interphase boundaries formed throughout individual grains. The results inform
how interphase boundaries can impose strain conditions that may be key to phase
stabilization. Moreover, the atomic structure near interphase boundary walls
suggests potential for their mobility under bias, which has been speculated to
occur in perovskite morphotropic phase boundary systems by mechanisms similar
to domain boundary motion.
