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Cation ordering and structural changes in synthetic Mg 2 TiO 4  and Zn 2 TiO 4  spinels at temperatures across the polymorphic transition from the high-temperature cubic (Fd3m) to the low-temperature tetragonal (P4 1 22) structure are examined by  17 O magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The  17 O NMR spectra of cubic Mg 2 TiO 4  and Zn 2 TiO 4  are similar, each showing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg 2 TiO 4  and Zn 2 TiO 4  show significant narrowing because of the onset of long-range cation ordering in the tetragonal structure. The  17 O NMR spectrum of tetragonal Zn 2 TiO 4  shows two narrow peaks, at 301 and 273 ppm, corresponding to the two crystallographically distinct O sites in the tetragonally distorted spinel, showing that '70 chemical shift is sensitive to octahedral Zn- Ti substitution in Zn 2 TiO 4 . In contrast, the  17 O NMR spectrum of tetragonal Mg 2 TiO 4  shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg 2 TiO 4  and Zn 2 TiO 4  are compared. Tetragonal Zn 2 TiO 4  exhibits greater distortion than Mg 2 TiO 4 . at the M1, O1, and O2 sites. These subtle structural differences do not explain differences in the  17 O NMR spectra.

Study of the cubic to tetragonal transition in Mg 2 TiO 4 and Zn 2 TiO 4 spinels by 17 O MAS NMR and Rietveld refinement of X-ray diffraction data