Interplay between breathing and polar instabilities in transition metal perovskites with active A-sites

Abstract

Active A-site cations like Bi or Pb in ABO_{3} perovskites are known for their valence-skipping nature as well as for the stereochemical activities connected to lone pairs. While the former gives rise to breathing distortions in compounds like BiNiO_{3} or PbCrO_{3}, the latter has been held responsible for ferroelectric, polar distortions in compounds like BiFeO_{3} or PbVO_{3}. The microscopic origins of both distortions have been argued to stem from the hybridization between Bi(Pb) and O. Employing first-principles calculations, together with a variational solution of the first-principles-inspired model Hamiltonian, we investigate the interplay of two types of distortion instabilities in Bi/Pb-based transition metal perovskites. Our study reveals that in the absence of orbital degeneracy of the B site, the preference of one over the other is dictated by the relative positioning of O 2p level with respect to the A-site 6p level. Closeness of the two levels favors polar distortion over the breathing and vice versa, level positioning of O 2p being dictated by the strength of B-O hybridization.