Structural models for yttrium aluminium borate laser glasses: NMR and EPR studies of the system (Y2O3)0.2–(Al2O3)x–(B2O3)0.8−x

Abstract

The structure of laser glasses in the system (Y₂O₃)_0.2{(Al₂O₃)_x)(B₂O₃)_0.8−x} (0.15 ≤ x ≤ 0.40) has been investigated by means of ¹¹B, ²⁷Al, and ⁸⁹Y solid state NMR as well as electron spin echo envelope modulation (ESEEM) of Yb-doped samples. The latter technique has been applied for the first time to an aluminoborate glass system. ¹¹B magic-angle spinning (MAS)-NMR spectra reveal that, while the majority of the boron atoms are three-coordinated over the entire composition region, the fraction of three-coordinated boron atoms increases significantly with increasing x. Charge balance considerations as well as ¹¹B NMR lineshape analyses suggest that the dominant borate species are predominantly singly charged metaborate (BO_{2 /2}O⁻), doubly charged pyroborate (BO_{1 / 2}(O⁻)₂), and (at x = 0.40) triply charged orthoborate groups. As x increases along this series, the average anionic charge per trigonal borate group increases from 1.38 to 2.91. ²⁷Al MAS-NMR spectra show that the alumina species are present in the coordination states four, five and six, and the fraction of four-coordinated Al increases markedly with increasing x. All of the Al coordination states are in intimate contact with both the three- and the four-coordinate boron species and vice versa, as indicated by ¹¹B/²⁷Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, non-segregated glass structure. ⁸⁹Y solid state NMR spectra show a significant chemical shift trend, reflecting that the second coordination sphere becomes increasingly “aluminate-like” with increasing x. This conclusion is supported by electron spin echo envelope modulation (ESEEM) data of Yb-doped glasses, which indicate that both borate and aluminate species participate in the medium range structure of the rare-earth ions, consistent with a random spatial distribution of the glass components.