Photoluminescence and thermoluminescence of titanium ions in sapphire crystals

Several titanium species (Ti(3+), Ti(4+), Ti(4+)-V(Al) clusters) were identified in sapphire samples from their optical spectra. In highly reduced samples, we observed only Ti(3+) centres while oxidized samples contain Ti(3+) and Ti(4+) ions as well. The formation of Ti(4+) ions occurs simultaneously with the formation of aluminium-vacancies which are charge compensators for Ti(4+) ions and act as hole traps in charge exchange process. We could observe locally and non-locally compensated Ti(4+) as well. It is shown that Ti(3+) is a deep hole trap while Ti(4+) 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 Ti(4+), probably because of spatial correlation. (C) 2001 Elsevier Science Ltd. All rights reserved.