First-principles studies of lone-pair-induced distortions in epitaxial phases of perovskite SnTiO3 and PbTiO3.

In this project, a computational investigation utilizing density functional theory methods is carried out to elucidate the differences in stereochemical lone-pair activity of Pb2+ and Sn2+ A-site ions in epitaxial polar ATiO3 perovskites. The contrasting tendencies for the lead- and tin-based compounds to form different phases--Amm2 for the former versus Cm for the latter-under biaxial tension are connected to the amount of charge concentrated within the lone pair lobes. Specifically, phases are energetically more preferable when as much charge as possible is dissipated out of the lobe, thus lowering the cost of Coulomb repulsions between the lone pair and the surrounding oxygen cage. Although a strong band-gap tuning was predicted in (fictitious) SnTiO3 during the tensile P4mm → Cm phase transformation [see W. D. Parker, J. M. Rondinelli, and S. M. Nakhmanson, Phys. Rev. B 84,245126 (2011)], we find the same effect to be considerably weaker in PbTiO3. The insights gained about the electronic-level underpinnings of transitional behavior in such lone-pair-active epitaxial ferroelectrics may be used in the design of a new generation of more efficient electromechanical and electrooptical devices. [ABSTRACT FROM AUTHOR]