Trace hydrogen in Verneuil-grown corundum and its colour varieties — an IR spectroscopic study

IR spectra of Verneuil-grown colourless corundum, rose to deep red ruby, blue, «amethyst»- and «alexandrite»-coloured sapphire show a distinct variability in the region of the OH stretching fundamental. Hydrogen, bonded to oxygen, varies from approximately 1 to 3.5 ppm. Extremely pleochroic and narrow OH bands are centered at 3310, 3230 and 3185 cm −1 . The OH dipoles are oriented perpendicular to the c-axis. Assuming vacancies in the cation lattice, OH is coordinated to two Al atoms forming «groups» of face-sharing Al 2 (OH)O 8 -double octahedra