Electron Paramagnetic Resonance and Optical Studies of Thermoluminescence Processes in Mn-Doped YAlO3 Single Crystals

A detailed electron paramagnetic resonance (EPR), optical absorption, luminescence, and thermoluminescence (TL) study of Mn-doped YAlO3 (YAP) single crystals was performed. The crystals were grown by the Czochralski method from stoichiometric (Y/Al = 1) and yttrium-rich (Y/Al = 1.04) melts and codoped with either Si or Hf ions. The EPR measurements revealed the presence of only one type of Mn2+ center, that is, isolated Mn ions occupying Y sites (Mn-Y(2+)). It was found that only in yttrium-rich crystals, the Mn-Y(2+) ions undergo recharging to Mn-Y(3+) under ionizing irradiation, indicating that this process requires the availability of sufficiently deep electron traps. The initial charge state is fully restored only after subsequent warming above 600 K. The presented results demonstrate, moreover, that Mn-Y(3+) + e -> Mn-Y(2+) recombination is not the most efficient excitation channel of the green T-4(1) -> (6)A(1) emission of Mn-Y(2+), possibly because of the huge energy difference between the recombination (>5.39 eV) and excitation (3 eV) energies. In contrast, energy transfer to Mn-Y(2+) proves to be dominant. A general model of trapping and recombination mechanisms responsible for TL of YAP:Mn crystals above room temperature is proposed. Besides Mn-Y(2+) ions and the defect-related electron and hole traps intrinsic to the YAP lattice, the model includes also unintentional dopants such as Fe-Al(2+) acting as deep hole traps, as well as Mn-Al(4+) and Cr-Al(3+) ions acting both as deep hole and electron traps.