A study of phosphorus in silicate glass with 31P nuclear magnetic resonance spectroscopy

In this study, 31P nuclear magnetic resonance spectra of phosphorus-doped silica have been analyzed to determine the concentration, chemical bonding and spatial distribution of phosphorus. The silica samples were prepared as performs for optical waveguide fibers. The two samples examined contained 1.12±0.02 and 0.06±0.02 wt. % phosphorus. Comparison with the profile of the refractive index shows that a nominal 1.0 wt. % phosphorus increases the refractive index 0.055% relative to pure fused silica. A single, well-defined 31P chemical shielding powder pattern is observed which implies that one species of phosphorus is dominant. Based on the 31P isotropic chemical shift and the magnitude and orientation of the shielding anisotropy, the phosphorus species is interpreted to be P(O) (O−)3. That is, the phosphorus is pentavalent and forms one terminal P=O bond and three bridging P-O− bonds. The distribution of phosphorus-phosphorus internuclear distances, determined from the 31P spin-echo data, indicates that phosphorus is randomly substituted into the silica network: there is no evidence of phosphate clusters.