Clustering of Aluminum and Phosphorus in a Silica Host.

Of the many possible combinations of dopants that may be incorporated into silica, the combination of aluminum and phosphorus is of special interest because of the consequences for processing and index control in optical fibers. Information regarding the spatial proximity (clustering) of aluminum and phosphorus dopants in silica optical-fiber preforms is obtained from magnetic double-resonance experiments. Separation of Al-P magnetic dipole-dipole interactions from corresponding Al-Al and P-P interactions in the presence of the large electric quadrupole interaction of the Al is obtained. The data give direct confirmation that the aluminum and phosphorus do form a spatial association. However, the association does not correspond to single ALPO sub 4 chemical species, as in a solution, but is probably a heterogeneous distribution of silicoaluminophosphate regions. Unlike phosphorus, which enters isolated sites when in excess, all aluminum spins that are visible in the NMR spectra appear to be coupled to phosphorus nuclei.