Structure development in sol-gel derived yttrium aluminium oxide, yttrium gallium oxide and their solid solutions

The structural study of the garnet system Y3Al5O12 (YAG) prepared by different sample preparations (solid-state sintering and sol-gel) is reported. Different sample preparations were investigated to find a suitable method for obtaining pure YAG. 27Al and 89Y MAS NMR were used to investigate the short range structure and results are compared to long-range order information from x-ray diffraction (XRD). These two techniques are comlementary in detecting phase composition and disorder in samples. There is a challenge in preparing pure Y3Al5O12 since intermediate phases easily grow when sintered at high temperature, i.e. YAlO3 (YAP), Y4Al2O9 (YAM), Y2O3 (Yttria) and Al2O3 (Corundum). Therefore structural information on these impurity phases was obtained to compare with samples under study. Different sample preparations were also used to try to obtain pure YAP and YAM. Since it is difficult to obtain pure YAG by high temperature solid-state sintering, the sol-gel technique was used to produce pure YAG at low temperature. In this work, pure YAG has been obtained at temperatures as low as 800oC using a sol-gel citrate-nitrate with combustion process (SGCNCT). Other sol-gel syntheses used are: sol-gel glycolate (SGG), citrate-nitrate (SGCN) and modification of the citrate-nitrate ratio with combustion process (SGCNCT). The modification of the citrate to nitrate ratio and controlling the mixing temperature as low as 50- 60oC is a novel route to produce pure YAG at low as 800oC. This technique was then used to prepare different garnet systems, i.e. Y3Ga5O12 (YGG) and also garnet solid solution system, Y3Al5-xGaxO12. Again, information from NMR was compared with results from XRD to determine different phase distribution and disorder. The 27Al chemical shift range for different structural units (e.g. AlO4, AlO5 and AlO6) in YAG allowed the aluminium distribution in these materials to be refined. This is important when dealing with low temperature amorphous phases. For example, the AlO5 site cannot be traced using XRD. The work was extended to study different nuclei, i.e. 89Y and 71Ga NMR. 27Al and 71Ga is a quadrupolar nuclei with spin, I=5/2 and 3/2, respectively. Extensive broadening of the NMR signal was observed at some sites due to the quadrupolar interaction and the effect of this on the quantitation of 27Al and 71Ga NMR is discussed. The substitution of Al and Ga in the garnet system was investigated. The different distribution of next nearest neighbour atom (nnn) produces a peak shift in 27Al, 71Ga NMR and also XRD.