Stereochemical activity of lone-pair electrons in ABX3 heavy-element halides

ABX₃ halides display many functionalities and properties such as ferroelectricity and superconductivity. ABX₃ (A = Rb, Cs; B = Sn, Pb; X = F, Cl) form generally in two different structures: sheelite (CaWO₄) -like and perovskite. The interplay of the stereochemically active lone pair (i.e., second-order Jahn-Teller) B-cation and other structural distortion is the key to determining the stable structure and subsequently the physical properties. It turns out that in these heavy p-block element compounds relativistic effects influence the activity of the lone pair in a nontrivial way. In this talk we will present our first-principles study of structural properties of this family of ABX₃ halides and how they change with temperature and pressure.