Factors controlling the O-17 NMR chemical shift in ionic mixed metal oxides

A wide range of O-17-enriched phases ABO(3) and A(2)BO(3) (A = Li, Na, Ca, Sr, Ba, and La; B = Ti, Zr, Sn, Nb, and Al) and related compounds has been synthesized and studied using O-17 magic angle spinning (MAS) NMR spectroscopy. In these highly ionic phases, the O-17 electric field gradients are small, and as a result highly resolved NMR spectra that reveal subtle structural inequivalences are observed. For titanates and zirconates the O-17 chemical shifts fall in distinct, well-defined regions (372-564 and 280-376 ppm, respectively), The ratio of isotropic O-17 chemical shifts from isostructural titanates and zirconates with the same A cation is constant, and this ratio is close to the ratio of the polarizing powers of titanium and zirconium. The B cation appears to be the dominant influence in determining the O-17 chemical shift in these compounds. Additionally the number of oxygen resonances and the shift difference between them increases as the symmetry of the structure decreases. Sn-119 MAS NMR has been applied to a variety of stannates and shows a large shift difference (68.2 ppm) between CaSnO3 phases with the ilmenite and GdFeO3 perovskite type crystal structures. Al-27 and O-17 MAS NMR have been used to study the conversion of lanthanum and aluminum sol-gel precursors to crystalline LaAlO3 perovskite. O-17 NMR proves to be more informative than Al-27 NMR and shows that the formation of LaAlO3 proceeds via the reaction of separate lanthanum and aluminum oxides initially formed.