Structural instabilities of double perovskite Pb₂CoTeO₆ from first principles

The B-site ordered double perovskite Pb₂CoTeO₆ is an attractive multiferroic candidate due to the presence of the lone pair cation Pb and the magnetic cation Co. However, experimental work so far has not reported a polar phase, instead detecting a structural phase transition sequence with increasing temperature between the non-polar structures P2₁/n (a⁺ b^- b^-)→I2/m (a⁰ b^- b^-)→R-3 (a^- a^- a^-)→Fm-3m (a⁰ a⁰ a⁰). In order to investigate potential strategies to stabilize Pb₂CoTeO₆ in a polar structure, we use group-theoretic analysis to enumerate possible space groups involving octahedral rotations, polar cation displacements, and combinations of these distortions. We then utilize density functional theory calculations to explore the energetics and crystal structure of  Pb₂CoTeO₆ when confined to each of these symmetries. We also explore how epitaxial strain modifies the relative energies and structural distortion amplitudes of the various structural phases of Pb₂CoTeO₆.