Theoretical predictions of novel potassium chloride phases under pressure

Evolutionary structure searches predict two hitherto unknown phases of KCl that are the most stable in the pressure regime of 200-600 GPa. I4₁/amd-KCl, which has the lowest enthalpy between ∼200-350 GPa, can be thought of as being composed of two three-connected nets. This structure can be compared with that of the Cs-IV electride (Cs⁺e^-): the potassium ions assume the positions of the cesium ions, and the chloride ions are found roughly in the regions of the valence electrons. Above ∼350 GPa a Pnma phase, which is isotypic with phases of CsH and CsI that are stable under pressure, becomes preferred. Just as in Pnma-CsI, the atoms in Pnma-KCl assume an hcp-like lattice; these alkali halides resemble the high-pressure structures of the isoelectronic noble gas solids Xe and Ar, respectively. The equation of state of KCl is extended to 600 GPa, enabling the use of this alkali halide as a pressure guage in ultra-high pressure static compression experiments. KCl is predicted to remain insulating to at least 420 GPa.