Factors Controlling the 17O NMR Chemical Shift in Ionic Mixed Metal Oxides

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Abstract

A wide range of 17O-enriched phases ABO3 and A2BO3 (A = Li, Na, Ca, Sr, Ba, and La; B = Ti, Zr, Sn, Nb, and Al) and related compounds has been synthesized and studied using 17O magic angle spinning (MAS) NMR spectroscopy. In these highly ionic phases, the 17O 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 17O chemical shifts fall in distinct, well-defined regions (372−564 and 280−376 ppm, respectively). The ratio of isotropic 17O 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 17O 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. 119Sn 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. 27Al and 17O MAS NMR have been used to study the conversion of lanthanum and aluminum sol−gel precursors to crystalline LaAlO3 perovskite. 17O NMR proves to be more informative than 27Al NMR and shows that the formation of LaAlO3 proceeds via the reaction of separate lanthanum and aluminum oxides initially formed.

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