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

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.

Oxygen-17 nuclear magnetic resonance measurements on apatite-type lanthanum silicate (La9.33(SiO4)6O2)

“…The intensity ratio of peak #4 at 600 ppm is 7.7% The intensity ratio of #4 to the sum of #1-#4 is about 8.1%, which is in good agreement with the calculated value of 7.6% for the O4 site that is based on the stoichiometry of the apatite. The oxide ion in lanthanum oxide-related materials was found in the range 450-600 ppm [26]. Therefore, the peak of #4 at 600 ppm could be assigned to the O4 site, as shown in Fig.…”

Solid State NMR Interaction Parameters of Oxygens Linking Titanium and Silicon in Crystalline Cyclic Titanodiphenylsiloxanes

“…Recently a much more complete study that also included 3Q MAS NMR of 17 O showed, in nanocrystalline TiO 2 for most of the sites with oxygen only coordinated to titanium, that chemical shift dispersion was the dominant broadening mechanism [31]. It has also been shown in other 17 O NMR studies of crystalline materials with ionic M-O bonds that the central transition can show very significant chemical shift anisotropy [32][33][34]. For the crystalline materials with Ti-O-Si linkages the 17 O NMR shows second-order quadrupolar lineshapes (at least at lower applied magnetic fields).…”

Solid-state NMR calculations for metal oxides and gallates: Shielding and quadrupolar parameters for perovskites and related phases

“…Experimental values, where available, are also shown, though it should be noted that the determination of such quantities by fitting of the spectra is often hindered by difficulties in separating features due to chemical shift anisotropy (CSA) and quadrupolar interactions [40]. Discussing the group II monoxide data first, a lack of experimental CSA determinations is evident, which, for MgO, SrO and BaO at least, is in keeping with the cubic symmetry at the O sites.…”