  Topological materials are derived from the interplay between symmetry and
topology. Advances in topological band theories have led to the prediction that
the antiperovskite oxide Sr$_3$SnO is a topological crystalline insulator, a
new electronic phase of matter where the conductivity in its (001)
crystallographic planes is protected by crystallographic point group
symmetries. Realization of this material, however, is challenging. Guided by
thermodynamic calculations we design and implement a deposition approach to
achieve the adsorption-controlled growth of epitaxial Sr$_3$SnO single-crystal
films by molecular-beam epitaxy (MBE). In-situ transport and angle-resolved
photoemission spectroscopy measurements reveal the metallic and non-trivial
topological nature of the as-grown samples. Compared with conventional MBE, the
synthesis route used results in superior sample quality and is readily adapted
to other topological systems with antiperovskite structures. The successful
realization of thin films of topological crystalline insulators opens
opportunities to manipulate topological states by tuning symmetries via
epitaxial strain and heterostructuring.
