Structure and nonlinear optical properties of sodium–niobium phosphate glasses

The structural description of xNb2O5–(1 − x)(NaPO3) (0 ⩽ x ⩽ 0.40) glass network is reported through high-field (18.8 T) 17O and 93Nb nuclear magnetic resonance, O1s X-ray photoelectron spectroscopy and Raman spectroscopy. The different oxygen sites (POP, PONa, NbOP and NbONb) are resolved on the spectra, their relative proportion determined and compared to indirect calculations obtained from the decomposition of 31P NMR spectra. A good agreement is obtained, thus confirming the formation of the Nb(OP)6−y(ONb)y units with increasing y leading to the clusterization of NbO6 octahedra, mainly for x > 0.2, which is related to the incomplete dissociation of Nb2O5 in the NaPO3 melt. The formation of these clusters is responsible for the large increase of the nonlinear optical properties, mainly due to the high polarizability of the Nb–O–Nb bond.