Structural, vibrational, and thermodynamic properties of γ-Na2TiO3: First-principles and experimental studies

Abstract

γ-Na2TiO3 is a novel sodium titanate with TiO5 trigonal bipyramids, and its properties have not been well characterized either experimentally or theoretically. In this study, γ-Na2TiO3 was prepared from the reaction of NaOH and coarse TiO2 particles, and analyzed by X-ray diffraction and vibrational spectroscopy. Meanwhile, the electronic structure, vibrational spectra, and thermodynamic properties of γ-Na2TiO3 were obtained from first-principles calculations within density functional theory. The calculated lattice parameters after geometry optimization were close to the experimental values obtained from Rietveld refinement. The calculated band gap is 3.88eV, lower than the experimental value of 4.06eV from Tauc plot, and the extensive hybridization between Ti 3d and O 2p states forms covalent Ti–O bonds. γ-Na2TiO3 has 33 active normal modes at k = 0, with 18 Raman acitive modes and 15 infrared active modes. The calculated Raman spectrum from LDA functional agrees well with the experimental one, and different vibrational modes were assigned. The strongest Raman peak at 803cm−1 was assigned to the symmetric stretching of the shortest Ti–O bond in the TiO5 unit. The calculated phonon dispersions indicate that γ-Na2TiO3 is stable. The temperature dependence of its thermodynamic properties (i.e., heat capacity, entropy, and free energy) were calculated from the phonon density of states, and compared with relevant data from the experiments and thermodynamic softwares.