Spontaneous non-stoichiometry and ordering of metal vacancies in degenerate transparent conductive oxides

The existence of non-stoichiometry in oxides is often thought to be a growth effect rather than a specific electronic instability. We show via DFT calculations that the presence of electrons in the conduction band of insulators can destabilize materials with respect to the spontaneous formation of cation vacancy acceptors. These tendencies result ultimately in the formation of ordered vacancy compounds (OVCs). For Ca-Al-O and Ba-Nb-O, we find that OVCs with different stoichiometries l:m:n are stable under different ranges of elemental chemical potentials, controllable during synthesis. Since each l:m:n OVC depletes the conduction band of electrons by a different amount, the formation of OVCs can be used to reduce plasma absorption, modify interband absorption, and enhance materials stability. This clarifies how non-stoichiometry often seen in oxides is an electronic effect – a high Fermi energy induces the formation of electron-killer acceptors such as cation vacancies. This also provides insights into the stability of the transparent conductive states while pointing out that controllable formation of non-stoichiometric degenerate insulators can be used to design next-generation transparent conductive oxides.