A Multi-nuclear Multiple-Field Nuclear Magnetic Resonance Study of the Y2O3−Al2O3 Phase Diagram

We have been investigating several compounds belonging to the Y 2 O 3 −Al 2 O 3 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-Y 2 O 3, Y 4 Al 2 O 9, YAlO 3, Y 3 Al 5 O 12, and α-Al 2 O 3 ) have been resolved and the NMR parameters deduced. Al VI sites exhibit isotropic chemical shift δ 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 . 17 O resonances are characterized by small C Q and δ iso ranging from 72 ppm (Al 2 O 3 ) to 358 ppm (Y 2 O 3 ) and showing a strong sensitivity to the nature of the second coordination sphere of oxygen as well as its coordination number. 89 Y δ 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 (∼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 Y 3 Al 5 O 12 and described the variation of the local structure during the crystallization process of YAlO 3 from the sol−gel raw product. In the vitreous state, oxygen's environments can be described in terms of OY k Al 4 - 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 YAlO 3 precursor is given by 17 O NMR experiments in terms of Y 4 Al 2 O 9, Y 3 Al 5 O 12, and amorphous phases formed. New oxygen environments are also evidenced and may be attributed to the hexagonal metastable form of YAlO 3 .