Behavior of BaBr2, BaI2, and BaCl2 at high pressure as analogs to SiO2.

Abstract:

Motivated by the expectation that giant and supergiant planets likely contain rocky components in their cores, we study analogs of the archetypal rock constituent, SiO₂. The crystal structures of SiO₂ under ultra-high pressures, greater than feasible experimental conditions, are believed to be documented by the high pressure structural sequence of AX₂ compounds. Experimental and theoretical work agree on a high pressure transition to the cotunnite (orthorhombic) phase, with first-principles theory predicting that SiO₂ transforms to the cotunnite structure at 750 GPa. However, the existence of a postcotunnite (monoclinic) phase as the final high pressure polymorph, suggested by X-ray diffraction (XRD) experiments on ambient pressure cotunnite-structured AX₂ compounds (e.g. PbCl₂, SnCl₂), has been challenged by density-functional theory (DFT) computations. This disagreement could perhaps be due to sensitivity in diamond-anvil cell (DAC) experiments to pressure gradients; conversely, it could perhaps arise from limitations of DFT.

This study further explores both the experimental and theoretical sides of this debate, with an aim to resolve this discrepancy. We present synchrotron XRD data on the AX₂ compounds BaCl₂, BaBr₂, and BaI₂, compressed up to 70 GPa at room temperature in a DAC. Here we compare our experimentally observed crystallography and equations of state with results from our DFT simulations.