Spectroscopic characterization of chromium (IV, V, VI) in Cr:Li2MSiO4 (M=Mg,Zn)

To understand the exceptionally long fluorescence lifetime of Cr-IV in Li2MSiO4 (M = Mg,Zn), several spectroscopic investigations are performed on Cr:Li2MSiO4 (M = Mg,Zn) crystals. X-ray absorption near-edge structure investigations attest that chromium is localized in tetrahedral sites and that Cr-VI is the major species while Cr-IV is the minor one in both compounds. Electron paramagnetic resonance studies confirm the occurrence of Cr-V in elongated tetrahedral environment (d(x)(2)-y(2) ground state), corresponding probably to the silicon site, and suggest the presence of several charge compensation schemes. Fluorescence and fluorescence dynamics of Cr-IV:Li2MSiO4 are reported. The very long Cr-IV excited state lifetime previously observed on powders samples is confirmed for the crystals (117 mus at room temperature, 305 mus at 30 K for Cr:Li2MgSiO4) and is explained by combined contributions to the emission from both E-1 level and the lowest component of the T-3(2) level, in thermal equilibrium. Contrary to the situation found in most Cr-IV activated compounds, the E-1 level lies below the T-3(2) one. (C) 2003 American Institute of Physics.