  Control of order-disorder phase transitions is a fundamental materials
science challenge, underpinning the development of energy storage technologies
such as solid oxide fuel cells and batteries, ultra-high temperature ceramics,
and durable nuclear waste forms. At present, the development of promising
complex oxides for these applications is hindered by a poor understanding of
how interfaces affect lattice disordering processes and defect transport. Here
we explore the evolution of local disorder in ion-irradiated La$_2$Ti$_2$O$_7$
/ SrTiO$_3$ thin film heterostructures using a combination of high-resolution
scanning transmission electron microscopy (STEM), position-averaged convergent
beam electron diffraction (PACBED), electron energy loss spectroscopy
(STEM-EELS), and \textit{ab initio} theory calculations. We observe highly
non-uniform lattice disordering driven by asymmetric oxygen vacancy formation
across the interface. Our calculations indicate that this asymmetry results
from differences in the polyhedral connectivity and vacancy formation energies
of the two interface components, suggesting ways to manipulate lattice disorder
in functional oxide heterostructures.
