Optical absorption of the d4ion Fe4+in pleochroic amethyst quartz

Publisher Website
Google Scholar
Abstract
Optical absorption spectra have been taken at 300K for samples of amethyst quartz that show anomalous pleochroism instead of dichroism. The transition moment of the 945 nm band is parallel to a lattice C2axis, called the 'optical C2axis'. The strong visible band which produces the purple colour can be decomposed into three overlapping components B0, B1, B2at 527, 538.5 and 599.5 nm respectively. B0is polarised parallel to the optical C2axis whereas B1, B2are polarised perpendicular to this axis at theta =+53 degrees , -57 degrees to C3respectively. The absorption coefficient in the visible was proportional to the concentration of a recently discovered C2symmetry S=2 centre through a series of annealing and re-irradiation experiments. This work establishes that the S=2 centre produces the optical absorption and that it is an Fe4+ion on a silicon site, verifying Lehmann and Moore's model (1966) of the colour centre.