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

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Abstract
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)0.17−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. 45Sc chemical shifts are consistent with isolated six-coordinated scandium species. 31P{45Sc} rotational echo adiabatic passage double resonance (REAPDOR) curves show a monotonic increase in slope with increasing x, indicating that the number of 31P–45Sc 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 45Sc MAS-NMR spectra and 45Sc{31P} 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.

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