Hole Trapping Energies as Evidence for the Existence of Free Small Polarons in Oxide Crystals
The energy needed to liberate a hole trapped by an Mg2+ ion in Al2O3 (sapphire) was deduced from the temperature dependence (T = 220−300 K) of the recombination kinetics following UV excitation of compensated crystals. The result obtained, 0.7 eV, is much smaller than the value deduced from the activation energy of the p-type conductivity observed for uncompensated Mg:Al2O3 crystals at 1600–1900 K by Wang and Kröger. A possible interpretation is that free holes form small polarons in Al2O3, with site-to-site jumping energy about 2.4 eV at high T. Theoretical calculations by Mackrodt et al support this hypothesis. It is suggested that free holes may also form small polarons in MgO and CaO.
KeywordsCation Vacancy Solid State Phys Small Polaron Free Hole Large Polaron
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