Modeling WO₃ beyond the Harmonic Approximation: Theoretical Characterization and Thermal Expansion

The structural, thermal, electronic, and optical properties of different WO3 polymorphs are calculated from first principles. Based on density functional theory and the quasi-harmonic approximation, the thermal expansion coefficient and the specific heat capacity as a function of temperature are modeled. Similar trends are observed for all phases, with only a slight dependence on the crystal symmetry. The dielectric function, refractive index, and absorption coefficient are calculated using an efficient combination of G0W0 and time-dependent density functional perturbation theory in the long-range correction approximation. An excellent agreement to experiment can be observed. Finally, Raman spectra for all phases are calculated and discussed based on their symmetry and phononic eigenmodes. The hypothetic cubic phase often employed in theoretical investigations fails to reproduce the aforementioned properties, limiting its application as a simple model for the real polymorphs.