Lone-Pair Origins of Polymorphism: Sn Monochalcogenides as a Case Study

The multiple structures presented by the group IV monochalcogenide family of compounds beyond the symmetric rock salt structure are commonly ascribed to their stereochemically active lone pairs. Recently, several new, stoichiometrically equivalent, ambient phases of SnS and SnSe have been synthesized in addition to the ground state orthorhombic α-Pnma phase, thus raising the question of the role of bonding and, specifically, of lone pairs in determining the structure. To examine this role, the possible, stable structures of SnS and SnSe are mapped by density functional theory calculations, guided by an evolutionary algorithm, to uncover known and yet-to-be synthesized phases. The stability of the metastable phases is evaluated by phonon spectrum calculations, and their electronic properties are determined. An analysis of the lone pairs in these structures identifies several possible hybridization schemes between the anion p-states and metal s-states that correlate with the crystal geometries. In contrast, the related SnO and SnTe compounds that exhibit only a single phase have either stronger or weaker hybridizations, respectively. Therefore, we propose that the varying p-state contributions to the lone pairs in SnS and SnSe, reflecting different hybridization schemes made possible by the intermediate hybridization, render the rich phase diagram of group IV monochalcogenides.