Fluorescence spectra and homogeneous line widths of Cr3+ in glass nucleating ferroelectric Li2Ge7O15

We report optical studies of the new material – Cr3+-doped lithium–germanate glass, containing Li2Ge7O15 (LGO) nanocrystalline particles. While only broadband 4T2–4A2 fluorescence from the low-field octahedral Cr3+ sites was observed from Cr3+ ions in the glass, in LGO nanocrystals high-field Cr3+ centers emit 2E–4A2 (R–lines) fluorescence. The process of crystallization in the course of isothermal annealing of glasses was monitored spectroscopically and the nucleation of LGO crystallites was observed starting from the smallest clusters. Using the 2E–4A2 fluorescence spectra it is possible to detect the ferroelectric phase transition in LGO:Cr3+ nanocrystals, whose critical temperature was found to be similar to that of the bulk crystals. Long-lived spectral holes were burned in the inhomogeneously broadened R-lines of Cr3+ in LGO nanocrystals at low temperatures. The linear temperature dependence of hole widths shows that the homogeneous broadening of 4A2–2E transitions of Cr3+ in nanocrystals is due to interaction of Cr3+ electronic levels with the two-level systems (TLS) of the surrounding glass. The range of the Cr3+-TLS elastic dipole–dipole interaction is estimated.