  We investigated the metal-insulator transition for epitaxial thin films of
the perovskite CaFeO3, a material with a significant oxygen ligand hole
contribution to its electronic structure. We find that biaxial tensile and
compressive strain suppress the metal-insulator transition temperature. By
combining hard X-ray photoelectron spectroscopy, soft X-ray absorption
spectroscopy, and density functional calculations, we resolve the
element-specific changes to the electronic structure across the metal-insulator
transition. We demonstrate that the Fe electron valence undergoes no observable
change between the metallic and insulating states, whereas the O electronic
configuration undergoes significant changes. This strongly supports the
bond-disproportionation model of the metal-insulator transition for CaFeO3 and
highlights the importance of ligand holes in its electronic structure. By
sensitively measuring the ligand hole density, however, we find that it
increases by ~5-10% in the insulating state, which we ascribe to a further
localization of electron charge on the Fe sites. These results provide detailed
insight into the metal-insulator transition of negative charge transfer
compounds and should prove instructive for understanding metal-insulator
transitions in other late transition metal compounds such as the nickelates.
