Vibrational and electronic properties of Na2Ti6O13

Sodium hexatitanate, Na2Ti6O13, in the form of elongated microcrystals, were synthesized by solid-state reaction of titanium dioxide and sodium carbonate. The monoclinic structure was confirmed by X-ray powder diffraction and selected area electron diffraction. Electron diffraction was used to determined zone axis and the crystal-grown direction, which allowed to perform polarized Raman measurements. Energy band gap and the electronic transition were studied by UV-Vis diffuse reflectance and using Tauc relation. Density functional theory (DFT) calculations also revealed an indirect allowed bandgap transition and a good agreement between theoretical and experimental energy band gaps were demonstrated. For the first time, the assignment of the vibrational modes was carried out in the samples and thoroughly compared with theoretical data obtained from DFT calculations. The most relevant of the 60 predicted optical phonon modes of Na2Ti6O13 were experimentally observed and assigned to their symmetries.