Study of the mechanism inducing ferroelectricity in anti-Ruddelsden-Popper structures

In this master thesis, we investigate the ferroelectric properties of Ba4Sb2O and compare it to BaTiO3. Our analysis includes the study of the ferroelectric properties such as the polarization and the Born effective charges, the mechanism inducing ferroelectricity, a comparison with Ca4Sb2O and Sr4Sb2O and the behavior of Ba4Sb2O when subjected to stress. The calculation of the polarization using the Born effective charges and the displacements gives us a value of 11.18 µC/cm2 while the Berry phase approach leads to a value of 7.33 µC/cm2. These values are comparable to the one found in BaTiO3, where the polarization is around 27 and 36 µC/cm2 depending on the phase considered. We found that it is the hybridization of the O 2p states with the Ba 5d states that leads to the off-center displacement of the O and to the stabilization of the ferroelectric state. This situation is analogue to the one found in BaTiO3 where it is the hybridization of the Ti 3d states with the O 2p states that induces the ferroelectric distortion. Furthermore, we showed that the different behaviors of paraelectric Ca4Sb2O, antiferroelectric Sr4Sb2O and ferroelectric Ba4Sb2O are associated to the widening of the gap between the valence and the conductive bands. From the strain analysis of Ba4Sb2O, we found that a compressive strain applied in the a, b plane could enhance the stability of the ferroelectric phase over the paraelectric phase while increasing the polarization.