Equation of state, phase transitions, and band-gap closure in PbCl2 and SnCl2

The equations of state and band-gap closures for PbCl2 and SnCl2 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 Co2Si structure, in contrast to the postcotunnite structure as previously suggested. First-principles calculations confirm this finding and predict a second phase transition to a Co2Si-like structure between 75– 110 GPa in PbCl2 and 60–75 GPa in SnCl2. Band gaps were measured under compression to ∼70 GPa for PbCl2 and ∼66 GPa for SnCl2 and calculated up to 200 GPa for PbCl2 and 120 GPa for SnCl2. 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 AX2 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.