A final point on the M I species is the notable difference in the position of the 0 → 1transition level of Li Ba compared to Na Ba and K Ba , occurring at a Fermi energy of around 1.0 eV rather than 0.3 eV. This is because the Li ionic radius is so small that the formation of a hole distorts the dopant off the traditional perovskite "A-site" to a square planarlike coordination, resulting in a trapped polaron (Figure ). This is in contrast to the larger Na and K species, which remain on the "A-site", resulting in a more delocalised hole and a correspondingly shallower transition level (Figure ). It is noted that the defect potential energy surfaces of Na Ba and K Ba were thoroughly explored using the ShakeNBreak methodology (via the application of systematic bond length distortions around a defect site before relaxation), but the energy-lowering, symmetry-breaking distortion exhibited by Li Ba was not reproduced. octahedron within the perovskite structure (refer to the t BaBO 3 tolerance factors in Table ).