Temperature dependence of Raman scattering in pentasodium trioxoundecafluoro-trititanate

Experimental results from temperature dependent Raman spectroscopy measurements which were obtained at room temperature up to 325 °C from pentasodium trioxoundecafluoro-trititanate [Na₅(Ti₃O₃F₁₁)] powder are reported in the present study. The following Raman lines A_1g, B_1g, B_2g and B_3g show significant broadening and shifting. Two models are used to approximate the temperature-dependent Raman shift and the Full Width at Half Maximum (FWHM) of the observed Raman modes: linear and Klemens models. The Klemens model provides a much better approximation than the linear approximation. Consequently, anharmonic phonon-phonon interactions are considered to be responsible for the observed temperature dependence of the Raman spectra of pentasodium trioxoundecafluoro-trititanate. In addition, it is understood that three- and four-phonon processes combined together contribute to the damping process. Observed redshifts and blueshifts are explicitly attributed to the change in vibrational frequencies due to thermal expansion or to the change in volume resulting from phonon softening induced by the increase in annealing temperature. In more details, the observed blue shifts are attributed to a change in mean vibrational energy at constant volume. Furthermore, it is found that Na₅(Ti₃O₃F₁₁) undergoes another phase transition at ~ 300 °C.