Effect of glass composition on Judd–Ofelt parameters and radiative decay rates of Er 3+ in fluoride phosphate and phosphate glasses

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For fluoride phosphate and phosphate glasses, Judd–Ofelt Ω t parameters, spontaneous emission probabilities, radiative lifetimes and branching ratios of several Er 3+ transitions were determined from the absorption spectra by the Judd–Ofelt treatment. The compositional changes of Ω 2 and Ω 6 are attributed to changes in the bonding between Er 3+ and surrounding ligand groups. The phosphate groups play a special role. They show electron donation from the 2p orbitals of the oxygen ions, as well π-electron donation from the resonating P–O π-bond. Larger electron donation from the ligand anions leads to larger Ω 2, but smaller Ω 6. By contrast, larger π-electron donation from the phosphate groups increases Ω 6. The changes of Ω 2 with increasing phosphate content and with variation of modifier ions are ascribed to changes in the content and polarizability of the oxygen ions, leading to changes in the electron donation from the ligands at the rare earth (RE) sites. In the case of Pb 2+, changes in the degree of asymmetry at the RE sites could also be important. The smaller Ω 6 parameter of phosphate glasses compared with fluoride phosphate glasses is caused by larger σ-electron donation due to larger oxygen content and by smaller π-electron donation due to smaller delocalization of the π-bonds. The changes of Ω 6 with phosphate content, in the case of fluoride phosphate glasses and with variation of modifier ions, are dominated by changes in the π-electron donation.