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Ytterbium and ytterbium –niobium doped sheelite single crystals have been grown by the Czochralski method in air and in protective atmospheres and subjected to additional annealing in air, in Co/CO2 atmosphere, and in forevacuum. Comparative investigations of optical absorption and photoluminescence spectra of the crystals in the visible and near-IR spectral regions upon UV excitation were performed. A consistent pattern is proposed, explaining the nature of the donor centers involved into the process of down-conversion population of 2F5/2 excited state of Yb3+ ions in the crystals.