A study of rare earth doped silicate and phosphate glasses
The complementary techniques of X-ray diffraction and EXAFS have been applied to silicate and phosphate glass systems containing varying quantities of rare earth elements. The silicate systems that have been studied are rare earth doped fibre optic preforms of interest to the optoelectronics and telecommunications industry. Techniques were developed to allow spatially resolved diffraction and EXAFS data to be taken from the small ~1mm diameter core region of the preforms. Absorption maps were made displaying the distribution of the rare earth ions as a function of radial position, and similarly for the germanium codopant incorporated in these systems. The diffraction results show as expected, that silica dominates the structure evident in the preforms whilst EXAFS measurements taken at the germanium K-edge, suggest that this codopant occupies an eightfold coordinated site surrounded by oxygen atoms at 1.7A. The phosphate glasses studied are rare earth metaphosphates of the composition R(PO3)3. X-ray diffraction measurements were taken showing that the network structure of these systems is essentially constructed from PO4 tetrahedra. The EXAFS experiments, performed on a range of glasses over the rare earth LIII edges, show a trend in the first shell distance rare earth-oxygen, consistent with the Lanthanide contraction, the rare earth ions occupy-ing sites with between six and eight-fold coordination of oxygen atoms about the rare earth ions.