Self-trapping of the $d$-$d$ 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 Ni$^{3+}$ 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 assignement 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.