Electronic and optical properties of lead titanate from beyond-DFT methods

Solving the Bethe-Salpeter equation (BSE) on top of a GW calculation is the state of the art approach to determine the optical properties of a material from first principles, while keeping into account the effects of electron-hole interaction. This thesis focuses on applying the aforementioned methods to investigate the electronic and optical properties, the latter of which have not been thoroughly studied in the literature, of a ferroelectric perovskite, PbTiO3. Particular attention is paid to proper convergence in parameter space, and a well-known approximation, modelBSE, is employed to reduce the computational cost. The electronic properties of ferroelectric PbTiO3 obtained through GW, specifically the band-gap and band structure, are found to be in relatively good agreement with the literature, while the comparison of optical properties and spectra proves more problematic. In addition to the low-temperature, ferroelectric phase of the material, which has received most of the attention in previous works, the high-temperature, paraelectric phase is also studied, together with a series of structures resulting from an interpolation between the two. This allows for a better characterization of how the optical absorption spectra change while moving from one phase to the other. Some of the excitons found for the paraelectric and ferroelectric phases are also analyzed more deeply, in order to better understand the nature of the transitions. Lastly, an interpolated structure with cell parameters close to those validated experimentally is identified, and its optical gap found to be 3.403 eV, which is relatively near to a previously reported value (3.00 eV). The double-peak shape of its absorption spectrum is then noticed to bear some resemblance to experimental observations.