First-principles study of tungsten trioxide: Structural properties and polaron formation

This Ph.D. thesis centers on the theoretical investigation of ferroelectrics and related properties as well as the electron-phonon coupling mechanisms on the polaron stability in tungsten trioxide. An innovative contribution of this work is the establishment of a new perspective to the analysis of a single polaron, which probes the unexpected ferroelectric character of some perovskites with oxygen deficiencies. This perspective creates a new class of functional properties that involves electron-phonon interaction, electronic transport, electrical conductivity and internal optical transitions. Nevertheless, an appealing explanation of the crucial role of the polaron to trigger unexpected functional lattice modes, is provided to give a microscopic knowledge of the origin of several observed phenomena such as the electrochromism and thermochromism. This explanation is motivated by the curiosity to quantify these optical phenomena through a first-principal investigation of the fundamental aspects of a single polaron, which is due to the oxygen deficiency, in addition, to the important role of a polaron to generate interesting properties experimentally observed in materials with defects. We adhere to this research by developing a new concept to shed light on an important process of change of functional behavior of materials due the formation of a polaron inside. While, a study of a perfect crystal structure without defects can also reveal many new details concerning the underlying electronic instabilities, which is mirrored by a multitude of related structural instabilities. Here, We elucidated how the interaction between phonons that couple multiple lattice modes can stabilize various structural phase transitions. We present a coherent comprehensive density functional theory study to understand the dynamics behind the main features leading naturally to the notion of structural phase transition and then to the ground state of tungsten trioxide. This investigation provides also a new concept that involves the role of an artificial ferroelectric phase to make an antiferroelectric perovskite material ferroelectric through the application of an electric field. These visions to find new ways to generate interesting properties as the ferroelectricity either by means of lattice dynamics motion under a specific condition in a perfect crystal structure or by considering native defects effect that causes a polaronic material, was a big challenge for us to be developed as new concepts