Comparison of optical spectra and computer-simulated structure of rare-earth-doped fluoroberyllate glasses

The atomic structure at rare earth sites in fluoroberyllate glasses is simulated using Monte Carlo and molecular dynamics techniques. Both simple BeF 2 glass and fluoroberyllate glasses with alkali and alkaline earth modifiers are treated. Changes in glass composition affect the local geometry and coordination at the rare earth. To correlate these structural changes with the inhomogeneously broadened linewidths and intesities observed in optical spectra, the Stark splittings at each simulated site due to second-order terms in the crystal-field potential are calculated using a point-charge model. Structural variations and their changes with composition are also analyzed in terms of a principal axis system of the local charge distribution. The predicted site-to-site variations in the rare-earth energy levels agree with the range and distribution of energy level splittings observed in laser-induced fluorescence line-narrowed and broadband optical spectra of Eu 3+ and Nd 3+. The origin of the residual inhomogeneous broadening in nonresonant fluorescence line-narrowing experiments caused by accidental coincidences of excitation levels is also evident from the predicted energy levels.