Optical spectroscopy analysis of the Eu3+ ions local structure in calcium diborate glasses

A detailed analysis of the optical properties of Eu3+ ions in calcium diborate glasses has been performed in order to correlate them with the local environment of the lanthanide ions in these glasses. From the excitation and emission spectra under broadband excitation the energy level diagram of the Eu3+ ion and the atomic parameters have been obtained. Moreover, it is found that the Eu3+ ions are predominantly bonded with the bridging and non-bridging oxygens of different borate groups containing the BO3 units. Under site selective excitation in the 7F0→5D0 inhomogeneously broadened peak, valuable information about the splitting of the 7F1 and 7F2 levels and the lifetime of the 5D0 level has been obtained as a function of the excitation wavelength. The linewidth of the low-energy components of the 5D0→7F1 emission shows an increase with the splitting of this level that can be well explained considering non-radiative de-excitation processes towards lower 7F1 Stark levels. From the splitting of the 7F1 and 7F2 levels, the second and fourth rank crystal-field parameters have been calculated as a function of the excitation wavelength. The observed behaviour suggests the existence of an unique class of site for the Eu3+ ions, with a large range of values for the crystal-field strength parameter NV(B2q), from 1000 to 3200 cm−1, that includes weak, medium and strong crystal-field environments for the Eu3+ ions in the calcium diborate glass. The J-mixing is shown to play a relevant role in these calculations for medium and strong crystal-field environments, i.e., for values of the crystal-field strength parameter NV(B2q) over 1400 cm−1.