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Keywords:

  • phase transition;
  • chromium (IV);
  • titanite;
  • doping

Summary

The temperature dependence of the Raman bands of Cr4+ modes which show enhanced intensities due to pre-resonance effects is reported from 293 to 673 K in chromium-doped titanite (CaTiOSiO4). Some aspects of the temperature dependence of Raman bands in pure, synthetic titanite which have not been previously published are also included in this study. Two Raman-active components of Ag and Bg symmetry, respectively, of the symmetric Si–O mode in titanite are predicted under P21/a symmetry and also been identified in this phase for the first time. The one component of Bg symmetry disappears just above the antiferroelectric–paraelectric transition at ~500 K in accordance with the predictions under A2/a symmetry for the high-temperature phase. Two resonance-enhanced components of the Cr4+–O stretch are also evident in the P21/a phase and only one could be identified in the A2/a phase, again in accordance with group-theoretical predictions. These observations can be used to characterize the P21/a and A2/a phases of pure synthetic and chromium-doped titanite. The temperature dependence of the Cr4+–O modes can be approximated by two-dimensional Ising behavior with the critical exponent β ≈ 1/8 below 450 K. Between 450 and 498 K, anomalous behavior is observed and this could be due to the appearance of mobile anti-phase boundaries (APBs). Anomalous behavior also persists to temperatures above 500 K. The half-width of the Ti–O stretching mode reflects the influence of the order parameter (Ti–O displacements) as well as mobile anti-phase boundaries. No evidence could be found of the existence of other ions such as Cr4+-ions in Ti-sites and/or Cr3+-ions also in Ti-positions in Cr-doped titanite in the Raman spectra using different laser lines to excite the spectra. Copyright © 2013 John Wiley & Sons, Ltd.