Lead-halide perovskites have been at the forefront of photovoltaic (PV) research in the last decade due to their rapidly increasing efficiencies. 1,2 Much of their excellent optoelectronic performance has been associated with the ns 2 electronic configuration of the lead cation in combination with the halide anion leading to materials that are highly defect tolerant. This characteristic feature can guide the search for 'perovskite-inspired materials' which reproduce the optoelectronic performance while avoiding the use of toxic elements. On the other hand, metal chalcogenides have not managed to achieve the same efficiencies as halide perovskites, predominantly due to intrinsic defects limiting their performance. However, they have shown improved water and air stability, compared to their halide conterparts, which is essential in the development of PV modules that can compete with the device lifespans of silicon-based technologies. As a result, metal chalcohalides offer a promising alternative to both classes of materials, combining the ambient stability of metal chalcogenides with the desirable cation-anion bonding of halide perovskites. Initial solar cells containing these materials have shown promise, achieving power-conversion efficiencies (PCE) > 4%. In this work we focus on Sn 2 SbS 2 I 3 , a promising mixed-metal chalcohalide from the A II 2 B III Ch 2 X 3 class of materials consisting of ns 2 lone pair cations from group 14 (A = Sn, Pb) and 15 (B = Sb, Bi). The first device was fabricated by Nie et al. and obtained a PCE of 4.04% using a single step, solution-based chemical deposition process. Although this material family has been known since the 1980s, it has seen renewed interest in recent years, primarily due to its potential for optoelectronic applications. Sn 2 SbS 2 I 3 was first synthesized by Olivier-Fourcade et al. and characterized using X-ray diffraction (XRD). They determined that Sn 2 SbS 2 I 3 crystallizes into the Cmcm centrosymmetric space group and that the structure is made up of infinite Sn 2 S 2 chains directed along the a lattice vector connected by distorted trigonal prismatic SbI 6 polyhedra along c, as shown in Figure (a). The MX 6 polyhedra are trigonal face sharing, and bi-capped by the chalcogen atoms from the neighbouring M 2 Ch 2 chains. Ibanez et al. also assigned the Cmcm space group, but report a reduction in the R-factor from 0.105 to 0.066 when Sb is shifted from the 4c Wyckoff site to the 8f site (a displacement of the Sb atom along the c lattice vector) with a 50% partial occupancy, Figure Without knowledge of the underlying atomic structure of these materials, accurate predictions of the crystal properties and optoelectronic performance are impossible, hampering the development of this emerging material class. In this work, we investigate the various proposed crystal structures for Sn 2 SbS 2 I 3 using a suite of computational techniques (including density functional theory, cluster expansion, and molecular dynamics) in combination with X-ray diffraction measurements to determine the true ground-state crystal structure of the A 2 BCh 2 X 3 family. We extend our model to investigate temperature-dependent disorder in these materials, resolving the room temperature structure and corresponding optoelectronic properties. Finally, we elucidate the impact of static and dynamic disorder on the electronic structure, highlighting implications for future studies of this and related mixed-metal chalcohalides.