  A Gaussian Approximation Potential (GAP) was trained using density-functional
theory data to enable a global geometry optimization of low-index rutile IrO2
facets through simulated annealing. Ab initio thermodynamics identifies (101)
and (111) (1x1)-terminations competitive with (110) in reducing environments.
Experiments on single crystals find that (101) facets dominate, and exhibit the
theoretically predicted (1x1) periodicity and X-ray photoelectron spectroscopy
(XPS) core level shifts. The obtained structures are analogous to the
complexions discussed in the context of ceramic battery materials.
