  Emergent superconductivity is strongly correlated with the symmetry of local
atomic configuration in the parent compounds of iron-based superconductors.
While chemical doping or hydrostatic pressure can change the local geometry,
these conventional approaches do not provide a clear pathway in tuning the
detailed atomic arrangement predictably, due to the parent compounds
complicated structural deformation in the presence of the
tetragonal-to-orthorhombic phase transition. Here, we demonstrate a systematic
approach to manipulate the local structural configurations in BaFe2As2
epitaxial thin films by controlling two independent structural factors
orthorhombicity (in-plane anisotropy) and tetragonality (out-of-plane/in-plane
balance) from lattice parameters. We tune superconductivity without chemical
doping utilizing both structural factors separately, controlling local
tetrahedral coordination in designed thin film heterostructures with substrate
clamping and bi-axial strain. We further show that this allows quantitative
control of both the structural phase transition, associated magnetism, and
superconductivity in the parent material BaFe2As2. This approach will advance
the development of tunable thin film superconductors in reduced dimension.
