Study of tie cubic to tetragonal transition in Mg_2TiO_4 and Zn_2TiO_4 spinels by ~(17)O MAS NMR and Rietveld refinement of X-ray diffraction data

摘要

Cation ordering and structural changes in synthetic Mg_2TiO_4 and Zn_2TiO_4 spinels at temperatures across the polymorphic transition from the high-temperature cubic (Fd3m) to the low-temperature tetragonal (p4122) structure are examined by 17O magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The 17O NMR spectra of cubic Mg_2TiO_4 and Zn_2TiO_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_2TiO_4 and Zn_2TiO_4 show significant narrowing because of the onset of long-range cation ordering in the tetragonal structure. The 17O NMR spectrum of letragonal Zn_2TiO_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 O chemical shift is sensitive to octahedral Zn-Ti substitution in Zn_2TiO_4. In contrast, the 17O NMR spectrum of tetragonal Mg_2TiO_4 shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg_2TiO_4 and Zn_2TiO_4 are compared. Tetragonal Zn_2TiO_4 exhibits greater distortion than Mg_2TiO_4 at the Ml, 01, and O_2 sites. These subtle structural differences do not explain differences in the 17O NMR spectra. The 17O NMR spectra of the cubic Mg_2TiO_4 and Zn_2TiO_4 show no change with quench temperature above the transition to the cubic phase, suggesting that short-range ordering does not occur in cubic Zn_2TiO_4 and Zn_2TiO_4. A two-phase region is observed for both Zn_2TiO_4 and Zn_2TiO_4, below 664 and 561 deg C, respectively, where the cubic and tetragonal phases are shown to be at equilibrium. The ~(17)O peak position of MgTiO_3 is observed at 398 ppm. This chemical-shift displacement of 100 ppm to high frequency of Zn_2TiO_4 is related to increased distortion in MgTiO_3.