Structural Characterization of Rare-Earth Doped Yttrium Aluminoborate Laser Glasses Using Solid State NMR

摘要

The structure of laser glasses in the system (B2O3)_(0.6){(Al2O3)_(0.4-x)(Y2O3)_x} (0.1 ≤ x ≤ 0.25) has been investigated by means of ~(11)B, ~(27)Al, and ~(89)Y solid state NMR as well as Y-3d core-level X-ray photoelectron spectroscopy. ~(11)B magic-angle spinning (MAS) NMR spectra reveal that the majority of the boron atoms are three-coordinated, and a slight increase of four-coordinated boron content with increasing x can be noticed. ~(27)Al MAS NMR spectra show that the alumina species are present in the coordination states four, five and six. All of them are in intimate contact with both the three- and the four-coordinate boron species and vice versa, as indicated by ~(11)B/~(27)Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, nonsegregated glass structure. For the first time, ~(89)Y solid state NMR has been used to probe the local environment of Y~(3+) ions in a glass-forming system. The intrinsic sensitivity problem associated with ~(89)Y NMR has been overcome by combining the benefits of paramagnetic doping with those of signal accumulation via Carr-Purcell spin echo trains. Both the ~(89)Y chemical shifts and the Y-3d core level binding energies are found to be rather sensitive to the yttrium bonding state and reveal that the bonding properties of the yttrium atoms in these glasses are similar to those found in the model compounds YBO3 and YAls(BO3)4. Based on charge balance considerations as well as ~(11)B NMR line shape analyses, the dominant borate species are concluded to be meta- and pyroborate anions.