First-principles study of polarons in WO3

Polarons are quasiparticles emerging in materials from the interaction of extra
charge carriers with the surrounding atomic lattice. They appear in a wide va-
riety of compounds and can have a profound impact on their properties, making
the concept of a polaron a central and ubiquitous topic in material science. Al-
though the concept is known for about 75 years, the origin of polarons is not yet
fully elucidated. This thesis focuses on WO 3 as a well-known prototypical system
for studying polarons, which inherent polaronic nature is linked to its remark-
able electrical and chromic properties. The primary objective of this research is
to provide a comprehensive atomistic description and understanding of polaron
formation in WO 3 using first-principles density functional theory (DFT) calcula-
tions. Additionally, the investigation explores the interactions between polarons
and the possibility of bipolaron formation. Following a systematic strategy, we
first extensively analyze the dielectric and lattice dynamical properties of WO 3 in
both the room-temperature P 2 1 /n and ground-state P 2 1 /c phases. Our specific
focus is on characterizing the zone-center phonons, which serve as the founda-
tion for identifying the phonon modes involved in the polaron formation and
charge localization process. Subsequently, we examine the impact of structural
distortions on the electronic structure of WO 3 to elucidate the interplay between
structural distortions and electronic properties, thereby laying the groundwork
for understanding electron-phonon couplings. By incorporating these critical fac-
tors, we address our primary research goals. The most common explanation for
the polaron formation is associated with the electrostatic screening of the extra
charge by the polarizable lattice. Here, we show that, even in ionic crystals, this
is not necessarily the case. We demonstrate that polarons in this compound arise
primarily from non-polar atomic distortions. We then unveil that this unexpected
behavior originates from the undoing of distortive atomic motions, which lowers
the bandgap. As such, we coin the name of anti-distortive polaron and validate
its appearance through a simple quantum-dot model, in which charge localization
is the result of balancing structural, electronic, and confinement energy costs.
Then, we also study the polaron-polaron interaction and present the formation
of the antiferromagnetic W 4+ bipolaronic state with relatively large formation
energy. Our analysis of the W 4+ bipolaronic distortions on the global structure
reveals the same behavior as in experiments where the highly distorted monoclinic
phase transforms into a tetragonal phase as a function of doping. Additionally,
leveraging our previous findings on asymmetric polaronic distortion and examin-
ing different merging orientations, we stabilize the antiferromagnetic W 5+ -W 5+
bipolaronic state with an energy lower than the W 4+ state. This thesis clari-
fies the formation of unusual medium-size 2D polarons and bipolarons in WO 3 ,which might be relevant to the whole family of ABO 3 perovskites, to which WO 3
is closely related. The simplicity of the concept provides also obvious guidelines
for tracking similar behavior in other families of compounds.