Thermal variation of the optical absorption of UO2: determination of the small polaron self-energy

The temperature variation of UV–VIS–NIR optical spectra of UO2 have been investigated from room temperature up to 1173 K with careful in situ oxygen partial pressure control. The deduced optical absorption edge exhibits a strong temperature dependence. Its value decreases from ∼2 eV at room temperature to ∼0.8 eV at 1173 K. Such thermal behaviour is interpreted as the consequence of the existence of a strong electron–phonon coupling (small polaron). In the temperature range 300–1173 K, the model yields a hopping radius of ∼2 Å and a polaron self-energy of Ep=−0.38 eV.