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 ph ase diagram by means of aluminum-27, oxygen-17, and yttrium-89 high-resolut ion solid-state nuclear magnetic resonance (NMR) spectroscopy. All aluminum , oxygen and yttrium sites of the five crystalline compounds (C-Y2O3, Y4Al2 O9, YAlO3, Y3Al5O12, and alpha -Al2O3) have been resolved and the NMR param eters deduced. Al-VI sites exhibit isotropic chemical shift delta (iso) bet ween 0 and 10 ppm and small quadrupolar coupling constant C-Q, whereas Al-I V 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 3 58 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 delt a (iso) are found between 184 and 314 ppm and are not strongly correlated t o the coordination state of yttrium, the chemical shift anisotropy being fo und moderate (similar to 100 ppm). On the basis of the results obtained wit h the crystalline phases, we have characterized the oxygen environment in a glassy sample of composition Y3Al5O12 and described the variation of the l ocal 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 environme nts 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, Y 3Al5O12, and amorphous phases formed. New oxygen environments are also evid enced and may be attributed to the hexagonal metastable form of YAlO3.