Effects of preheating on diaspore: Modifications in colour centres, structure and light emission

The thermal properties and thermal stability of diaspore-corundum (AIOOH-Al2O3), from Goian (Pontevedra, Spain) were studied by means of differential thermal analysis (DTA), thermogravimetry (TG), high temperature X-ray diffraction (HTXRD) and thermally stimulated luminescence (TL) techniques. The samples were annealed to link the combined effect of (i) dehydroxylation, (ii) oxidation-reduction of chromophores (Mn 0.5%, Fe2O3 0.12%, TiO2 0.021 % and Cr 80 ppm) determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and (iii) phase transitions whilst sample heating (i.e. alpha-AIOOH -> alpha'-Al2P3 -> alpha-Al2O3). The blue colour of diaspore, attributed to the Ti4+-Fe2+ intervalence-charge-transfer mechanism, turns to white (circa 500 degrees C) in good agreement with the DTA endothermic peak (dehydroxylation). The coexistence of alpha-AIOOH and alpha'-Al2P3 phases has been detected by in situ HTXRD and could be correlated to the thermoluminescence tests performed on preheated aliquots (up to 500 degrees C). (c) 2005 Elsevier Ltd. All rights reserved.