Long-lifetime fluorescence and crystal field calculation in Cr4+-doped Li2MSiO4, M = Mg, Zn

The diffuse reflectance spectra of Cr:Li2MSiO4, M = Mg, Zn, indicate that in these compounds, chromium ions occur in the 4+, 5+, and 6+ oxidation states simultaneously. Under selective excitation in a Cr4+ absorption band, a very long fluorescence decay time is observed for both compounds: similar to110 mus at room temperature and similar to300 mus at 30 K. These are by far the longest fluorescence lifetimes reported for Cr4+ activated materials. In Cr4+:Li2MSiO4, the E-1 excited state level lies below the lowest component of the T-3(2) level and the fluorescence decay time is dominated by the long-lifetime E-1 level for which the transition to the ground state is spin-forbidden. The reverse situation occurs for the other Cr4+ doped compounds and their fluorescence lifetimes, governed by the short-lifetime T-3(2) state, are only a few microseconds. A crystal field calculation, performed for Cr4+:Li2MSiO4, confirms the above interpretation and supports the localization of Cr4+ at the silicon site in this compound. (C) 2003 Elsevier B.V. All rights reserved.