PbCl2 and SnCl2 at high-pressures as analogs for SiO2 metallization

Abstract:

PbCl₂ and SnCl₂ crystallize in the orthorhombic cotunnite structure, a high-pressure crystal structure of silica, making these salts important analogs for understanding the bonding properties of silicates at conditions of deep planetary interiors. Using infrared absorption (FTIR) spectroscopy to peak pressures of 50-70 GPa at room temperature, we document closure of the electronic energy gaps for both salts as they transition from ionic toward metallic states under compression. The gaps likely reflect the separation between occupied-states primarily associated with the anion, and unoccupied metal-ion states. Room-temperature x-ray diffraction to 80 GPa and first-principles calculations (density functional theory) reveal a continuous displacive transition in PbCl₂ (orthorhombic I-II transition), which is expected to become metallic around 100 GPa; metallization of SnCl₂ is anticipated near 80 GPa.

High-pressure shock experiments show that fluid SiO₂ is metallic at the high temperatures achieved in giant impacts (> 1-2 eV). Thus, the transition to metallic states of several crystalline AX₂ analogs suggests that rocky matter may more generally become metallic inside large terrestrial planets. These studies imply a blurring between the traditional concepts of mantle and core, with metallic silicates potentially being present in crystalline form in the mantles and in liquid form in the cores of super-Earths.