Taken together, these results indicate a strong energetic preference for particular configurations. This can be understood by the fact that there are two distinct chalcogen environments in the R3 ̅ m structure, which we refer to as Chal and Chal . The single Chal position is in the interior of the layers, where the chalcogen atom forms three short and three long bonds to six Bi atoms in a distorted octahedral environment. The two Chal positions are at the edges of the quintuple layers, where the chalcogens form three covalent bonds to Bi atoms and project a lone pair of electrons into the interlayer spacing. It is thus more favorable for smaller and more electronegative anions to occupy the Chal sites, which benefit from stronger electrostatic interactions with the Bi cations, and for larger and more polarizable anions to occupy the Chal sites, which benefit from more space to accommodate sterically-depending lone pairs.