Strongly intrinsic anharmonicity in the low-frequency Raman mode in nanocrystalline anatase TiO2

Temperature dependence of Raman spectra was obtained to study the unusual hardening behavior of lowest-frequency Raman mode of nanocrystalline anatase TiO2. A model was employed to analyze the frequency shifts of three Raman modes upon increasing temperature firstly. The pure-volume and pure-temperature contributions to the frequency shifts of three Raman modes have been calculated. The result indicated that the strongly intrinsic anharmonicity arising from optical-phonon couplings was the origin of hardening upon increasing temperature for the Raman mode frequencies.