Abstract
Low iron, pale blue natural and synthetic sapphire samples were studied bylow 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 bothCr3+ and Ti3+, experimental evidence was obtained for attributing an absorption band at17 500 cm−1, currently interpreted as intervalence charge transfer, to overlapping crystal field transitions ofCr3+ andTi3+. An important rolewas also proposed for Cr2+; indeed, it is possible to propose that the colour of pale blue sapphiresis mainly determined by chromium in its two valence states whileTi3+ and Fe3+ have a minor role.
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