Phonon anharmonicity of rutile SnO2studied by Raman spectrometry and first principles calculations of the kinematics of phonon-phonon interactions

Raman spectra of rutile tin dioxide (SnO₂) were measured at temperatures from 83 to 873 K. The pure anharmonicity from phonon-phonon interactions was found to be large and comparable to the quasiharmonicity. First-principles calculations of phonon dispersions were used to assess the kinematics of three-phonon and four-phonon processes. These kinematics were used to generate Raman peak widths and shifts, which were fit to measured data to obtain the cubic and quartic components of the anharmonicity for each Raman mode. The B₂₆ mode had a large quartic component, consistent with the symmetry of its atom displacements. The broadening of the B₂₆ mode with temperature showed an unusual concave-downwards curvature. This curvature is caused by a change with temperature in the number of down-conversion decay channels, originating with the wide band gap in the phonon dispersions.