The equations of state and band-gap closures for $\mathrm{Pb}{\mathrm{Cl}}_2$ and $\mathrm{Sn}{\mathrm{Cl}}_2$ were studied using both experimental and theoretical methods. We measured the volume of both materials to a maximum pressure of 70 GPa using synchrotron-based angle-dispersive powder x-ray diffraction. The lattice parameters for both compounds showed anomalous changes between 16–32 GPa, providing evidence of a phase transition from the cotunnite structure to the related ${\mathrm{Co}}_2\mathrm{Si}$ structure, in contrast to the postcotunnite structure as previously suggested. First-principles calculations confirm this finding and predict a second phase transition to a ${\mathrm{Co}}_2\text{Si-like}$ structure between 75– 110 GPa in $\mathrm{Pb}{\mathrm{Cl}}_2$ and 60–75 GPa in $\mathrm{Sn}{\mathrm{Cl}}_2$. Band gaps were measured under compression to ∼70 GPa for $\mathrm{Pb}{\mathrm{Cl}}_2$ and ∼66 GPa for $\mathrm{Sn}{\mathrm{Cl}}_2$ and calculated up to 200 GPa for $\mathrm{Pb}{\mathrm{Cl}}_2$ and 120 GPa for $\mathrm{Sn}{\mathrm{Cl}}_2$. We find an excellent agreement between our experimental and theoretical results when using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, which suggests that this functional could reliably be used to calculate the band gap of similar $A{X}_2$ compounds. Experimental and calculated band-gap results show discontinuous decreases in the band gap corresponding to phase changes to higher-coordinated crystal structures, giving insight into the relationship between interatomic geometry and metallicity.

• Received 8 September 2020
• Revised 20 April 2022
• Accepted 29 November 2022

DOI:https://doi.org/10.1103/PhysRevB.107.134113