  Topological insulator (TI) materials are exciting candidates for integration
into next-generation memory and logic devices because of their potential for
efficient, low-energy-consumption switching of magnetization. Specifically, the
family of bismuth chalcogenides offers efficient spin-to-charge conversion
because of its large spin-orbit coupling and spin-momentum locking of surface
states. However, a major obstacle to realizing the promise of TIs is the
thin-film materials' quality, which lags behind that of epitaxially grown
semiconductors. In contrast to the latter systems, the Bi-chalcogenides form by
van der Waals epitaxy, which allows them to successfully grow onto substrates
with various degrees of lattice mismatch. This flexibility enables the
integration of TIs into heterostructures with emerging materials, including
two-dimensional materials. However, understanding and controlling local
features and defects within the TI films is critical to achieving breakthrough
device performance. Here, we report observations and modeling of large-scale
structural defects in (Bi,Sb)$_2$Te$_3$ films grown onto hexagonal BN,
highlighting unexpected symmetry-breaking rotations within the films and the
coexistence of a second phase along grain boundaries. Using first-principles
calculations, we show that these defects could have consequential impacts on
the devices that rely on these TI films, and therefore they cannot be ignored.
