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We have studied the temperature dependencies of the photoluminescence (PL)intensity of 4.1 eV in microcrystalline powder of hexagonal boron nitride inthe range of 7-1100 K. The results obtained have been analyzed within the bandmodel of energy levels of associated donor-acceptor pairs based on impurity(ONCN) complexes. Luminescence enhancement processes at T<200 K and twoindependent channels of external thermal activation quenching are typical ofthe observable luminescence mechanisms under direct (4.26 eV) excitations ofthe samples. It has been shown that, at T>220 K, when directly excited, thesamples diminish the PL intensity because of the processes of thermalionization of the donor level of the ON-center (122 meV) and the deep acceptorlevel of the CN-center (1420 meV) as parts of the (ONCN)-complex. Thetemperature enhancement region with an activation energy of 15 meV is due tothe decay of a bound Wannier-Mott exciton followed by transfer of excitation tothe associated donor-acceptor pair.

Temperature effects in luminescence of associated oxygen-carbon pairs in  hexagonal boron nitride under direct optical excitation within 7-1100 K range