**19. NMR**

Y3Al5O12 generally adopts a cubic garnet structure with lattice parameter of 12 Å (space group Ia¯3d). Its structure consists of a network where aluminum atoms reside both in octahedral and tetrahedral interstices whereas yttrium atoms occupy dodecahedral sites. Since the chemical shift of 27Al NMR is sensitive to the local coordination, this technique is largely applied for checking the different phases and coordination states of Al centers in aluminates. As a result, MAS NMR 27Al study has been undertaken on YAG samples in order to apprehend what kind of interatomic movements occur during sol-gel process. octahedral AlO6 sites resonate between 15 and 30 ppm, the much less common AlO5 sites between 40 and 25 ppm and tetrahedral AlO4 between 80 and 50 ppm. On this account, the 2.6 ppm major resonance in the spectrum of the uncalcined xerogel corresponds to sixcoordinate aluminum. Besides, two other distinct peaks located at about 35.4 and 61 ppm can be assigned to five and four-fold coordinated aluminum atoms. It can be noticed that the resonance at 35.4 ppm can also be consistent with a tetrahedral site distorted due to the presence of oxygen defects. 27Al NMR spectral features of the sintered YAG powder reveal three signals at 0.426, 23.6 and 62.7 ppm, which can be imputed to the three types of coordination. The octahedral band has shrinked into the sharp signal at 0.426 ppm. Furthermore, since it has been demonstrated that Al atoms can only occupy tetrahedral and octahedral sites in crystallized YAG, we can deduce that the 35.4 and 23.6 ppm signals respectively in xerogel and crystallized powder correspond to distorted tetrahedral sites and not to five-fold coordinated one. Besides, by integrating peak areas, the 27Al tetrahedral/octahedral ratio has been determined. Contrary to other studies, it remains the same after calcination, even if the resonance relative to "distorted tetrahedral" sites has weakened in favour of tetrahedral one **[20].** 
