  BaCe$_{0.25}$Mn$_{0.75}$O$_{3-\delta}$ (BCM), a non-stoichiometric oxide
closely resembling a perovskite crystal structure, has recently emerged as a
prospective contender for application in renewable energy harvesting by solar
thermochemical hydrogen generation. Using solar energy, oxygen-vacancies can be
created in BCM and the reduced crystal so obtained can, in turn, produce H2 by
stripping oxygen from H2O. Therefore, a first step toward understanding the
working mechanism and optimizing the performance of BCM, is a thorough and
comparative analysis of the electronic structure of the pristine and the
reduced material. In this paper, we probe the electronic structure of BCM using
the combined effort of first-principles calculations and experimental O K-edge
x-ray absorption spectroscopy (XAS). The computed projected density-of-states
(PDOS) and orbital-plots are used to propose a simplified model for
orbital-mixing between the oxygen and the ligand atoms. With the help of
state-of-the-art simulations, we are able to find the origins of the XAS peaks
and to categorize them on the basis of contribution from Ce and Mn. For the
reduced crystal, the calculations show that, as a consequence of dielectric
screening, the change in electron-density resulting from the reduction is
strongly localized around the oxygen vacancy. Our experimental studies reveal a
marked lowering of the first O K-edge peak in the reduced crystal which is
shown to result from a diminished O-2p contribution to the frontier unoccupied
orbitals, in accordance with the tight-binding scheme. Our study paves the way
for investigation of the working-mechanism of BCM and for computational and
experimental efforts aimed at design and discovery of efficient water-splitting
oxides.
