The energetic preference of the heavier chalcogenides to adopt the R3 ̅ m structure can be rationalized by the stereochemical activity of the Bi 6s lone pair. Bi 3+ experiences an inert-pair effect due to the relativistic contraction of the 6s orbital. Interactions between the Bi 6s and anion p orbitals produces a high-energy antibonding orbitals that can interact with the Bi 6p, thus indirectly mediating Bi 6s/6p hybridization. Since this s/p mixing is forbidden by symmetry in an octahedral bonding environment, a structural distortion is required that results in the lone pair occupying a coordination site. The distortion is only favored when the stabilization due to the anion-mediated s/p mixing offsets the loss of electrostatic stabilization due to the reduced coordination number. The orbital interaction between the Bi 6s and S 3p orbitals is the strongest among the three bismuth chalcogenides, as the relative energy difference is the smallest. Bi2S3 therefore preferentially adopts the low-symmetry orthorhombic Pnma phase with an active lone pair, and remains stable even under pressure. Table Optimized lattice parameters, relative lattice energies ∆𝑈 latt and dynamical stabilities of the eight bismuth chalcogenide phases considered in this work. shown in parentheses for comparison. b Lattice energies reported relative to the most stable phase of each chalcogenide.