The behavior of the active modes of the anatase phase of TiO2 at high temperatures by Raman scattering spectroscopy

In situ Raman spectroscopy is an important tool for describing the actual phase transfer conditions in metal oxides and provides important insights into the phase transition mechanism. The particle size, non-stoichiometry, and surface stress are well-known factors in the shifting and broadening of Raman modes for the TiO2 anatase phase with increasing the temperature. Also, anharmonic effects in the form of phonon-phonon coupling and optical-phonon coupling are other factors in the shifting and broadening of Raman modes at high temperatures. In this paper, TiO2 nanocrystals in the anatase phase are synthesized by the sol-gel method. Then the structure and size of the particles are investigated using XRD diffraction and SEM. Meanwhile, the contribution of phonon-phonon coupling for the third- and fourth-order has been studied for the active modes Eg, A1g, and B1g, with increasing temperature (from room temperature to 943 K). The results of the back-scattering of in situ Raman spectroscopy show that the contribution of phonon-phonon coupling for the third-order has a significant effect on the shifting and broadening of Raman active modes. Also, the 412 cm(-1) mode behavior differs from the other two modes in the contrast between anharmonic couplings contribution and pure-volume contribution.