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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
Source Normalized Impact per Paper (SNIP) 2017: 1.059

Online
ISSN
1945-3027
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Volume 80, Issue 9-10

Issues

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

Roberta L. Millard / Ronald C. Peterson / Brian K. Hunter
Published Online: 2015-11-13 | DOI: https://doi.org/10.2138/am-1995-9-1003

Abstract

Cation ordering and structural changes in synthetic Mg2TiO4 and Zn2TiO4 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 Mg2TiO4 and Zn2TiO4 are similar, each showing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg2TiO4 and Zn2TiO4 show significant narrowing because of the onset of long-range cation ordering in the tetragonal structure. The 17O NMR spectrum of tetragonal Zn2TiO4 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 Zn2TiO4. In contrast, the 17O NMR spectrum of tetragonal Mg2TiO4 shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg2TiO4 and Zn2TiO4 are compared. Tetragonal Zn2TiO4 exhibits greater distortion than Mg2TiO4. at the M1, O1, and O2 sites. These subtle structural differences do not explain differences in the 17O NMR spectra.

The 17O NMR spectra of the cubic Mg2TiO4 and Zn2TiO4 show no change with quench temperature above the transition 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°C, respectively, where the cubic and tetragonal phases are shown to be at equilibrium.

The 17O peak position of MgTiO3 is observed at 398 ppm. This chemical-shift displacement of 100 ppm to high frequency of Mg2TiO4 is related to increased distortion in MgTiO3.

About the article

Received: 1994-11-04

Accepted: 1995-06-08

Published Online: 2015-11-13

Published in Print: 1995-09-01


Citation Information: American Mineralogist, Volume 80, Issue 9-10, Pages 885–896, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-1995-9-1003.

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© 2015 by Walter de Gruyter Berlin/Boston.

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