Atomic environment of high-field strength Nd and Al cations as dopants and major components in silicate glasses: a Nd LIII-edge and Al K-edge X-ray absorption spectroscopic study

The local atomic structure around Nd and Al ions present as dopants in SiO2 glasses and as major constituents in other silicate glasses have been studied using Nd LIII-edge EXAFS and Al K-edge XANES and EXAFS spectroscopy. The Nd EXAFS results provide the first direct evidence of clustering of Nd ions through the formation of Nd–O–Nd bonds in a SiO2 glass doped with 2400 ppm Nd2O3. The Nd–O and Nd–Nd distances in this glass are found to be 2.35 and 3.80 Å, respectively. Codoping of this glass with 7300 ppm Al2O3 is shown to result in a homogeneous dispersion of the Nd atoms. The Nd–O coordination polyhedra in this glass relax to a larger volume with respect to that in its Al-free counterpart with a significant increase in the Nd–O distance to 2.62 Å. Moreover, Nd–O–Si/Al bonds are formed at the expense of Nd–O–Nd bonds on addition of Al, indicating incorporation of Nd–O polyhedra into the silicate network; the corresponding Nd–Si distance is found to be 3.51 Å. A similar atomic environment for Nd is also observed in Nd2Al2Si3O12 glass where Nd is present as a major component rather than as a dopant. However, no such Nd–O–Si bonds are observed in a K–Nd-silicate glass indicating the fact that incorporation of Nd–O polyhedra into the silicate network without Al creates significant radial disorder in the next-nearest neighbor Si shell around Nd. The Al XANES and EXAFS spectra indicate that Al ions in all these glasses exist in four-fold coordination with Al–O distances ranging between 1.76 and 1.77 Å. A second shell of Si is observed in all compositions at distances ranging between 2.92 and 3.00 Å. Al–O–Nd bonds are also observed in the Nd2Al2Si3O12 glass. It is suggested that Nd, when incorporated into the Si, Al–O network, plays the role of charge-balancing cation for [AlO4]−1 units in aluminosilicate glasses.