Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Smith, S. J. Articles by Woodfield, B. F. Search for Related Content GeoRef GeoRef Citation

Heat capacities and thermodynamic functions of TiO₂ anatase and rutile: Analysis of phase stability

¹ Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, U.S.A.
² State Key Structural Chemistry Lab, Fujian Institute of Research on the Structure of Matter, Fuzhou 350002, P.R. China
³ Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, California 95616, U.S.A.

Correspondence: ^* E-mail: Brian_Woodfield{at}byu.edu

At high temperature, coarse-grained (bulk) rutile is well established as the stable phase of TiO₂, and nanophase anatase, thermodynamically stable relative to nanophase rutile, transforms irreversibly to rutile as it coarsens. The lack of experimental heat-capacity data for bulk anatase below 52 K lends uncertainty to its standard entropy and leaves open a slight possibility that anatase may have a thermodynamic stability field at low temperature, as suggested by some theoretical calculations. In the present study, the molar heat capacities of rutile and anatase were measured from 0.5 K to about 380 K. These data were combined with previously measured high-temperature heat capacities, and fits of the resulting data set were used to generate C_P,m°, ₀^TS_m°, ₀^TH_m°, and ₀^TG_m ° values at smoothed temperatures between 0.5 and 1300 K for anatase and 0.5 and 1800 K for rutile. Using these new data and the enthalpy of transformation between anatase and rutile at 298 K, the change in Gibbs free energy for the transition between anatase and rutile from 0 to 1300 K was calculated. These calculations reveal that the transformation from bulk anatase to bulk rutile is thermodynamically favorable at all temperatures between 0 and 1300 K, confirming that bulk anatase does not have a thermodynamic stability field. Implications for the natural occurrence of these two minerals in terrestrial, lunar, and planetary settings are discussed. In particular, anatase requires low-temperature aqueous conditions for its formation and may be a reliable indicator of such conditions in both terrestrial and extraterrestrial settings.

Key Words: Rutile • anatase • TiO₂ • heat capacity • thermodynamics • phase transformation