Quaternary mixed-metal ${M\left(\text{II}\right)}_{2}M\left(\text{III}\right){Ch}_2{X}_3$ chalcohalides are an emerging material class for photovoltaic absorbers that combines the beneficial optoelectronic properties of lead-based halide perovskites with the stability of metal chalcogenides. Inspired by the recent discovery of lead-free mixed-metal chalcohalides (MMCHs) materials, we utilized a combination of density functional theory and machine learning to determine compositional trends and chemical design rules in the lead-free and lead-based materials spaces. We explored a total of 54 ${M\left(\text{II}\right)}_{2}M\left(\text{III}\right){Ch}_2{X}_3$ materials with $M$(II) = Sn, Pb; $M$(III) = In, Sb, Bi; Ch = S, Se, Te; and $X$ = Cl, Br, I per phase ($Cmcm$, $Cmc{2}_1$, and $P{2}_1/c$). The $P{2}_1/c$ phase is the equilibrium phase at low temperatures, followed by $Cmc{2}_1$ and $Cmcm$. The fundamental band gaps in $Cmcm$ and $Cmc{2}_1$ are smaller than those in $P{2}_1/c$, but direct band gaps are more common in $Cmcm$ and $Cmc{2}_1$. The effective electron masses in $P{2}_1/c$ are significantly larger than in $Cmcm$ and $Cmc{2}_1$, while the effective hole masses are nearly the same across all three phases. Using random forest regression, we found that the two electron acceptor sites [Ch and $X$] are crucial in shaping the properties of MMCH compounds. Furthermore, the electron donor sites [$M$(II) and $M$(III)] can be used to fine-tune the material properties to desired applications. These design rules enable precise tailoring of MMCH compounds for a variety of applications.

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• Band gap
• Composition
• Density of states
• Electronic structure
• Optoelectronics
• Spin-orbit coupling
• Structural properties
• Crystal structures
• Metal halides
• Solar cells
• Density functional theory
• Machine learning
• Random forests