Charge transfer transitions in optical spectra of $\mathrm{Ni_cMg_{1- c}O}$ oxides

Radiative recombination with charge transfer was observed in Ni$_c$ Mg$_{1- c}$O (c = 0.008) oxides over the 8–300 K temperature range. This recombination occurs as a result of strong hybridization of the Ni$^{2+}$ ion 3d-states and the band states. The charge transfer radiation excitation spectrum shows vibrational LO repeats of two exciton lines having charge transfer energy intervals of about 35 meV. The NiO nanocrystal absorption spectrum shows two weak peaks with energies of 3.510 and 3.543 eV, which are highly dependent on temperature. They are interpreted as charge transfer excitons at the edge of NiO fundamental absorption. The distance between the charge transfer exciton lines in the Ni$_c$ Mg$_{1- c}$ oxide spectra are caused by spin-orbit splitting of the valence band peak that was formed by the $p$-states of the oxygen ion