Study of the cubic to tetragonal transition in Mg2TiO4 and Zn2TiO4 spinels by 17O MAS NMR and Rietveld refinement of X-ray diffraction data

Cation ordering and structural changes in synthetic MgzTiO. and Zn2TiO. spinels at temperatures across the polymorphic transition from the high-temperature cubic (Fd3m) to the low-temperature tetragonal (P4,22) structure are examined by 170 magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The '70 NMR spectra of cubic MgzTiO. and ZnzTiO. are similar, each showing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg2TiO. and ZnzTiO. show significant narrowing because of the onset oflong-range cation ordering in the tetragonal structure. The '70 NMR spectrum of tetragonal Zn2TiO. shows two narrow peaks, at 301 and 273 ppm, corresponding to the two crystallographically distinct 0 sites in the tetragonally distorted spinel, showing that '70 chemical shift is sensitive to octahedral Zn- Ti substitution in Zn2TiO.. In contrast, the '70 NMR spectrum of tetragonal Mg2TiO. shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg2TiO. and Zn2TiO. are compared. Tetragonal Zn2TiO. exhibits greater distortion than MgzTiO. at the M I, 0 I, and 02 sites. These subtle structural differences do not explain differences in the '70 NMR spectra. The '70 NMR spectra of the cubic MgzTiO. and ZnzTiO. show no change with quench temperature above the transition to the cubic phase, suggesting that short-range ordering does not occur in cubic MgzTiO. and ZnzTiO.. A two-phase region is observed for both MgzTiO. and Zn2TiO., below 664 and 561°C, respectively, where the cubic and tetragonal phases are shown to be at equilibrium. The 170 peak position of MgTi03 is observed at 398 ppm. This chemical-shift displacement of 100 ppm to high frequency of MgzTiO. is related to increased distortion in MgTi03.