Optical Spectroscopy study of Type 1 natural and synthetic sapphires

Low iron, pale blue natural and synthetic sapphire samples were studied by low temperature absorption and luminescence spectroscopy. For comparison, a bicolour pink and blue corundum from Vietnam was also considered. From radioluminescence and photoluminescence excitation spectra of both Cr 3+ and Ti3+, experimental evidence was obtained for attributing an absorption band at 17 500cm-1, currently interpreted as intervalence charge transfer, to overlapping crystal field transitions of Cr3+ and Ti3+. An important role was also proposed for Cr2+; indeed, it is possible to propose that the colour of pale blue sapphires is mainly determined by chromium in its two valence states while Ti3+ and Fe3+ have a minor role. © IOP Publishing Ltd.