Self-trapping of the <Emphasis Type="Italic">d</Emphasis>-<Emphasis Type="Italic">d</Emphasis> charge transfer exciton in bulk NiO evidenced by X-ray excited luminescence

Soft X-ray (XUV) excitation did make it possible to avoid the predominant role of the surface effects in luminescence of NiO and revealed a bulk luminescence with a puzzling well isolated doublet of very narrow lines with close energies near 3.3 eV which is assigned to recombination transitions in self-trapped d-d charge transfer (CT) excitons formed by coupled Jahn-Teller Ni⁺ and Ni³⁺ centers. The conclusion is supported both by a comparative analysis of the CT luminescence spectra for NiO and solid solutions Ni _x Zn_{1 − x} O, and by a comprehensive cluster model assignment of different p-d and d-d CT transitions, their relaxation channels. To the best of our knowledge, it is the first observation of the luminescence due to self-trapped d-d CT excitons.