Temperature-Dependent Vibrational Dynamics of Rutile (TiO₂) from Molecular Dynamics Simulations and Two-Dimensional Correlation Analysis Techniques

The vibrational dynamics of rutile (TiO2) as a function of temperature has been studied by combining molecular dynamics (MD) simulations in conjunction with the generalized two-dimensional correlation spectroscopy analysis (2D COS) technique. Molecular dynamics simulations within the microcanonical ensemble were performed with the self-consistent charge density functional tight binding formalism at a series of different temperatures. To validate the DFTB MD results against the experimental data, the method of atomic pair distribution functions (PDFs) was used. IR absorption spectra were calculated implementing the time correlation function formalism. Subsequently, the generalized two-dimensional correlation approach was applied to obtain further insights into the temperature-dependent vibrational dynamics. The static DFTB calculations of the vibrational force field of the rutile reproduced excellently the experimental data and allowed for more exact assignments of the corresponding experimental IR/Raman spectral bands. Through the detailed analysis of the synchronous and asynchronous 2D spectra computed on the basis of MD-generated anharmonic spectra, we have provided new insights into the couplings between the modes in the studied system, as well as into the sequential character of the temperature-induced changes in the vibrational force field. The sensitivity of IR active modes to the temperature-induced perturbation of the system decreases in the order 685 cm−1Eu mode > 370 cm−1Eu mode > 982 cm−1A2u mode. The results presented in this study clearly demonstrate the usefulness of the combination of periodic SCC DFTB MD simulations coupled to the 2D COS analysis techniques in solid-state vibrational spectroscopy.