Stereochemical activity of lone-pair electrons in ABX$_{3}$ heavy-element halides

ABX$_{3}$ halides display many functionalities and properties such as ferroelectricity and superconductivity. ABX$_{3}$~(A $=$ Rb, Cs; B $=$ Sn, Pb; X $=$ F, Cl) form generally in two different structures: sheelite~(CaWO$_{4})$-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 ABX$_{3}$ halides and how they change with temperature and pressure.