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

To understand the exceptionally long fluorescence lifetime of CrIV 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 CrVI is the major species while CrIV is the minor one in both compounds. Electron paramagnetic resonance studies confirm the occurrence of CrV in elongated tetrahedral environment (dx2−y2 ground state), corresponding probably to the silicon site, and suggest the presence of several charge compensation schemes. Fluorescence and fluorescence dynamics of CrIV:Li2MSiO4 are reported. The very long CrIV excited state lifetime previously observed on powders samples is confirmed for the crystals (117 μs at room temperature, 305 μs at 30 K for Cr:Li2MgSiO4) and is explained by combined contributions to the emission from both 1E level and the lowest component of the 3T2 level, in thermal equilibrium. Contrary to the situation found in most CrIV activated compounds, the 1E level lies below the 3T2 one.