Excited states of chromium(III) in translucent glass-ceramics as prospective laser materials

Abstract

The first excited quartet state 4T 2 of chromium(III) provides broad-band luminescence in the near infrared. If the first doublet state 2E is below the 4T 2 potential minimum, a narrow band may be emitted with high yield, as known from ruby. Glass-ceramics containing Cr(III) may show high quantum efficiency of 4T 2 at room temperature, contrary to most other compounds. When the heat treatment is such that micro-crystallites of spinel, gahnite, mullite, virgilite,.., are obtained having dimensions smaller than the wavelength of violet light, transparent glassy-looking materials are obtained which may be suitable for tunable lasers similar to alexandrite, and luminescent solar concentrators. The absorption and emission spectra of Cr(III) are treated here in the angular overlap model. The sub-shell energy difference A (= 10 Dq) of Cr(III) in glasses is around 0.86 and in glass-ceramics 0.96 to 1.05 times its value of the aqua ion. Evidence is presented that the band widths and Stokes shift cannot be fully explained by effects of totally symmetric stretching. The multi-dimensional potential surfaces, combined with the Tanabe-Kamimura theorem and the Jahn-Teller effect, provide a more complete explanation of the spectroscopic behaviour. Anti-fer-romagnetic and spin-orbit coupling are shortly reviewed. Comparison between chromium(III) luminescence and the exceptional Stokes shift of hexacyanocobaltate(III) is made.