A multi-nuclear multiple-field nuclear magnetic resonance study of the Y2O3-Al2O3 phase diagram

We have been investigating several compounds belonging to the Y2O3-Al2O3 phase diagram by means of aluminum-27, oxygen-17, and yttrium-89 high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy. All aluminum, oxygen and yttrium sites of the five crystalline compounds (C-Y2O3, Y4Al2O9, YAlO3, Y3Al5O12, and alpha -Al2O3) have been resolved and the NMR parameters deduced. Al-VI sites exhibit isotropic chemical shift delta (iso) between 0 and 10 ppm and small quadrupolar coupling constant C-Q, whereas Al-IV are located between 70 and 80 ppm and show large C-Q. O-17 resonances are characterized by small Co and delta (iso) ranging from 72 ppm (Al2O3) to 358 ppm (Y2O3) and showing a strong sensitivity to the nature of the second coordination sphere of oxygen as well as its coordination number. Y-89 delta (iso) are found between 184 and 314 ppm and are not strongly correlated to the coordination state of yttrium, the chemical shift anisotropy being found moderate (similar to 100 ppm). On the basis of the results obtained with the crystalline phases, we have characterized the oxygen environment in a glassy sample of composition Y3Al5O12 and described the variation of the local structure during the crystallization process of YAlO3 from the sol-gel raw product. In the vitreous state, oxygen's environments can be described in terms of OYk Al4-k, sites, with respective populations not distributed in a purely random fashion, and two types of 5-fold-like aluminum environments can be evidenced. A quantitative description of the heat treatments of the YAlO3 precursor is given by O-17 NMR experiments in terms of Y4Al2O9, Y3Al5O12, and amorphous phases formed. New oxygen environments are also evidenced and may be attributed to the hexagonal metastable form of YAlO3.