Site selective spectroscopy and molecular dynamics simulation of Eu(III) ZBLAN glasses

The fluorescence line narrowed emission spectrum of europium(III)-doped ZBLANP glasses, at 77 K, has been investigated as a function of excitation frequency in the region of the F-7(0) --> D-5(0) transition near 17 300 cm(-1). Three well-resolved components are found in the region of the D-5(0) --> F-7(1) transition whose separation is an approximately linear function of the excitation frequency. Additional spectral features in this region are due to non-selective excitation caused by phonon side-band absorption. The emission in the D-5(0) --> F-7(2) region contains six resolvable features, but their splitting is less strongly correlated with the excitation frequency. The same optical properties were found in a 10 m fibre sample at 77 K. The optical spectrum is related to the glass structure by a molecular dynamics simulation, using a cubic cell containing 393 atoms, of which two are europium(III) ions, and potential parameters optimised with respect to crystalline analogues. In all 24 europium sites were simulated. They have a mean fluoride coordination number of 7.9. Within a 3.0 Angstrom range, 3 europium sites are 9 coordinate, 6 are 7 coordinate and the 15 are 8 coordinate. Using a standard choice of f-electron radial integrals, and a basis containing 164 states from the f(6) configuration, good agreement is found between the simulated and observed fluorescence line-narrowed emission. In the D-5(0) --> F-7(1) region there is no relationship between the splitting and the fluoride coordination number, but the splitting pattern can be predicted by a random-charge shell model. In D-5(0) --> F-7(2) region the dominant short range part of the potential leads to distinct features for ions of different coordination number.