Luminescence mechanisms for borate glasses: The role of local structural units

The increasing knowledge of local order and detailed structural information on the neighboring atoms of impurity ions or radiation-induced defect states in borate glasses is a powerful argument for more detailed models leading to a better understanding of luminescence mechanisms, recombination kinetics, and other related phenomena. Examples of some previous models for the thermoluminescence quenching mechanisms for gamma-irradiated aluminoborate glasses doped with Fe are briefly revised. The Racah parameters and the ligand field intensity for a transition element ion such as Cr3+ are found to be useful parameters, sensitive to the nature of local symmetry and distortions, which can be controlled by an adequate glass composition design. The Fano antiresonances observed in the optical absorption spectra appear at the positions expected from the Tanabe-Sugano diagram. The optical absorption spectrum of barium aluminoborate glasses doped with Cr3+ and Nd3+ is merely a superposition of the respective independent absorptions. However, the fluorescence spectrum appears to be shifted, thus indicating the occurrence of an energy transfer process from Cr3+ to Nd3+ excited states, which is observed even at room temperature. Both the fluorescence and excitation spectra of glasses doped with chromium and neodymium show the Fano antiresonance effect but exhibit a Lamb shift of the valleys associated with neodymium over the emission bands of Cr3+.