The temperature dependence of the Raman spectra of chromium-doped titanite (CaTiOSiO4)

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. The antiferroelectric–paraelectric phase transition in Cr4+‐doped titanite was studied with resonance Raman spectroscopy by varying the sample temperature. The Cr–O Raman bands are shown between 700 and 1300 cm−1 including three Si–O bands indicated with green arrows. In the insert of the figure, a plot of the intensity of the Cr–O symmetric stretching mode against the monoclinic angle β at various temperatures illustrates the occurrence of the phase transition at ~500 K as well as an anomaly existing between ~450 and 500 K.