Doped ferroelectrics and polar metals from DFT first-principles study

Abstract

Since the recent confirmation of the existence of polar metals, subsequent to the experimental observation of the ferroelectric behaviour in LiOsO 3 , the search of such materials has known a dramatic regain of interest. Their possible applications would, among other things, allow to increase substantially the memory storage capacity in devices or enable to explore fascinating quantum phenomena.

Even though such a type of materials was already suspected to exist half a century ago, it was nonetheless commonly thought that polar atomic distortions are restricted to insulators while the charge carriers in conductors would screen out the long-range Coulomb interaction that tends to favour the creation of polar motions. The aim of this Master thesis is to rely on atomistic first principles calculations based on density functional theory (DFT) in order to achieve better microscopic understanding of the interplay between polar distortion and free carriers.

Focusing on the important family of ABO 3 perovskites, we first reinvestigate the microscopic origin of a polar distortion in prototypical ferroelectric insulators like BaTiO 3 or PbTiO 3 . Then we explore how the ferroelectric distortion is affected when progressively doping these materials. A special emphasis is given to the study of the evolution under electron doping of the zone-center ferroelectric phonon instability in their reference cubic phase and of its A- or B-type character.

Doing so, we also question the first-principle method that is commonly used to study doped ferroelectric in recent literature and that simply consists in adding extra electrons compensated by a positive background. On the one hand, we demonstrate that cell relaxation previously reported using this method is erroneous since they rely on ill-defined deformation potentials. On the other hand, we highlight that conventional methods to access absolute deformation potentials from DFT and to correct that problem cannot be easily applied to such complex materials : they do not provide a unique value and even fail to predict whether an additional charge leads to an expansion or a contraction of the volume, so calling for further methodological developments.