Local environment of scandium in aluminophosphate laser glasses: structural studies by solid state NMR spectroscopy

The first NMR study of a scandium oxide containing glass system is reported. The influence of scandium substitution for aluminium in the sodium aluminophosphate glass system (NaPO3)(0.83-)( Al2O3)(0.172-x)(Sc2O3)(x) (0 <= x <= 0.11) has been studied in detail by multinuclear high-resolution NMR experiments. Introduction of scandium into this glass system results in a systematic change in the aluminium speciation: as the substitutional parameter x is increased, aluminium is successively converted from six-to four-coordination. This result suggests that the scandium ions require more non-bridging oxygen atoms for coordination than aluminium in these glasses. Sc-45 chemical shifts are consistent with isolated six-coordinated scandium species. 31 P{45Sc} rotational echo adiabatic passage double resonance ( REAPDOR) curves show a monotonic increase in slope with increasing x, indicating that the number of P-31 - Sc-45 dipole - dipole couplings increases, consistent with a random spatial distribution of scandium. These results rule out the possibility of rare-earth clustering in these glasses. Both the Sc-45 MAS-NMR spectra and Sc-45{P-31} rotational echo double resonance ( REDOR) data indicate that the scandium species adopt a constant, compositionally independent local environment dominated by phosphorus in the second coordination sphere.