STUDY OF THE CUBIC TO TETRAGONAL TRANSITION IN MG2TIO4 AND ZN2TIO4 SPINELS BY O-17 MAS NMR AND RIETVELD REFINEMENT OF X-RAY-DIFFRACTION DATA

Cation ordering and structural changes in synthetic Mg2TiO4 and Zn2TiO 4 spinels at temperatures across the polymorphic transition from the h igh-temperature cubic (Fd3m) to the low-temperature tetragonal (P4(1)2 2) structure are examined by O-17 magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The O-17 NMR spectra of cubic Mg2TiO4 and Zn2TiO4 are similar, each s howing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg2TiO4 and Z n2TiO4 show significant narrowing because of the onset of long-range c ation ordering in the tetragonal structure. The O-17 NMR spectrum of t etragonal Zn2TiO4 shows two narrow peaks, at 301 and 273 ppm, correspo nding to the two crystallographically distinct O sites in the tetragon ally distorted spinel, showing that O-17 chemical shift is sensitive t o octahedral Zn-Ti substitution in Zn2TiO4. In contrast, the O-17 NMR spectrum of tetragonal Mg2TiO4 shows only one peak, at 298 ppm. The st ructures of cubic and tetragonal Mg2TiO4 and Zn2TiO4 are compared. Tet ragonal Zn2TiO4 exhibits greater distortion than Mg2TiO4 at the M1, O1 , and O2 sites. These subtle structural differences do not explain dif ferences in the O-17 NMR spectra. The O-17 NMR spectra of the cubic Mg 2TiO4 and Zn2TiO4 show no change with quench temperature above the tra nsition to the cubic phase, suggesting that short-range ordering does not occur in cubic Mg2TiO4 and Zn2TiO4. A two-phase region is observed for both Mg2TiO4 and Zn2TiO4, below 664 and 561 degrees C, respective ly, where the cubic and tetragonal phases are shown to be at equilibri um. The O-17 peak position of MgTiO3 is observed at 398 ppm. This chem ical-shift displacement of 100 ppm to high frequency of Mg2TiO4 is rel ated to increased distortion in MgTiO3.