LUMINESCENCE OF CR-3+ IN KYANITE

In this paper we report the luminescence studies of octahedrally coordinated Cr3+ ions in a natural sample of kyanite (Al2O3:SiO2). The chromium ion can substitute for any one of the four nearly octahedral Al sites with the average metal-ligand distance of about 1.91 angstrom. Using absorption, excitation and emission spectra (steady state and time resolved) as well as decay measurements in the temperature range of 26 to 600 K we were able to clearly identify three different emissions, originating in three different sites. Using spectroscopic data and a one-configurational coordinate model we were able to calculate 10 Dq for each of the active sites. It appears that the range of 10 Dq spanned by the sites is much larger than expected on the basis of the average bond lengths and the electrostatic theory of 10 Dq. The fourth non-emitting site parameters were deduced and the likely reasons for the lack of luminescence from it are discussed. The strongly changing doublet-quartet coupling in different sites of kyanite manifests itself in doublet radiative lifetimes spanning the range of 100-3000-mu-s.