Structural, electronic, optical and lattice dynamic properties of the different WO3 phases: First-principle calculation

First-principle calculations were performed to investigate the structural, electronic, optical, and lattice dynamic properties of the cubic (c), hexagonal (h), tetragonal (alpha), orthorhombic (beta), monoclinic (gamma) and triclinic (delta) WO3 phases. Based on the analysis of structures and relative energies, we noted that the asymmetric distortion of WO6 octahedron was mainly responsible for the enhanced crystal stability of alpha-, beta-, gamma- and delta-WO3, while the titling of WO6 octahedron played a secondary effect. The results of the calculated dipole moment indicated that the phase transformation from beta-to gamma-WO3 induced the change of antiferroelectricity-ferroelectricity, which was contributed to the octahedral distortion along x and y directions. The band gaps calculated by PBEO hybrid function were 1.562, 1.943 and 2.244eV for c-, h- and alpha-WO3. The increasing of the band gaps were contributed to the decreasing valence band width due to the expanded octahedral volume. Importantly, the distortion of the WO6 octahedron lifted the level of 5d(xy), states at bottommost of the conduction band, which much enlarged the band gaps of beta-, gamma- and delta-WO3 to 2.974, 2.881 and 2.813eV. The calculated optical properties gave the decreasing static dielectric constants epsilon(1)(0) of 8.9, 7.6, 7.3, 5.6, 5.3 and 5.1 for c-, h-, alpha-, beta-, gamma-, and delta-WO3, respectively, which were agreement with the band gaps. The lattice dynamic properties presented unstable structures of c-, h- and alpha-WO3, and stable structures of beta-, gamma-, and delta-WO3. The instability mainly originated from the stretching vibrations of O-W-O bonds with few contributions by the blending vibrations of W-O bonds. IR and Raman spectra were discussed and compared with experimental data, which confirmed the reasonability of the calculated results. The analyzed vibrational modes with different frequencies at Gamma point noted that the grid of WO3 presented soft characteristic and the WO6 octahedron was rigid.