Photoluminescence and thermoluminescence of titanium ions in sapphire crystals

Several titanium species (Ti3+, Ti4+, Ti4+-VAl clusters) were identified in sapphire samples from their optical spectra. In highly reduced samples, we observed only Ti3+ centres while oxidized samples contain Ti3+ and Ti4+ ions as well. The formation of Ti4+ ions occurs simultaneously with the formation of aluminium-vacancies which are charge compensators for Ti4+ ions and act as hole traps in charge exchange process. We could observe locally and non-locally compensated Ti4+ as well. It is shown that Ti3+ is a deep hole trap while Ti4+ is a deep electron trap. In the TL process they act as recombination centers for electrons and holes, respectively. However, in TL we observed only the emission of locally compensated Ti4+, probably because of spatial correlation