The Tetrel Bond and Tetrel Halide Perovskite Semiconductors

The ion pairs [Cs⁺•TtX₃^-] (Tt = Pb, Sn, Ge; X = I, Br, Cl) are the building blocks of all-inorganic cesium tetrel halide perovskites in 3D, CsTtX₃, that are widely regarded as blockbuster materials for optoelectronic applications such as in solar cells. The 3D structures consist of an anionic inorganic tetrel halide framework stabilized by the cesium cations (Cs⁺). We use computational methods to show that the geometrical connectivity between the inorganic monoanions, [TtX₃^-]_∞, that leads to the formation of the TtX₆^4- octahedra and the 3D inorganic perovskite architecture is the result of the joint effect of polarization and coulombic forces driven by alkali and tetrel bonds. Depending on the nature and temperature phase of these perovskite systems, the Tt···X tetrel bonds are either indistinguishable or somehow distinguishable from Tt-X coordinate bonds. The calculation of the potential on the electrostatic surface of the Tt atom in molecular [Cs⁺•TtX₃^-] provides physical insight into why the negative anions [TtX₃^-] attract each other when in close proximity, leading to the formation of the CsTtX₃ tetrel halide perovskites in the solid state. The inter-molecular (and inter-ionic) geometries, binding energies, and charge density-based topological properties of sixteen [Cs⁺•TtX₃^-] ion pairs, as well as some selected oligomers [Cs⁺•PbI₃^-] (n = 2, 3, 4), are discussed.