Stability of numerous novel potassium chlorides at high pressure

K-Cl is a simple system displaying all four main types of bonding, as it contains (i) metallic potassium, (ii) elemental chlorine made of covalently bonded Cl₂ molecules held together by van der Waals forces and (iii) an archetypal ionic compound KCl. The charge balance rule, assigning classical charges of “+1” to K and “−1” to Cl, predicts that no compounds other than KCl are possible. However, our quantum-mechanical variable-composition evolutionary simulations predict an extremely complex phase diagram, with new thermodynamically stable compounds K₃Cl, K₂Cl, K₃Cl₂, K₄Cl₃, K₅Cl₄, K₃Cl₅, KCl₃ and KCl₇. Of particular interest are 2D-metallic homologs K_n+1Cl_n, the presence of positively charged Cl atoms in KCl₇ and the predicted stability of KCl₃ already at nearly ambient pressures at zero Kelvin. We have synthesized cubic "Equation missing" -KCl₃ at 40–70 GPa and trigonal "Equation missing" -KCl₃ at 20–40 GPa in a laser-heated diamond anvil cell (DAC) at temperature exceeding 2000 K from KCl and Cl₂. These phases were identified using in situ synchrotron X-ray diffraction and Raman spectroscopy. Upon unloading to 10 GPa, "Equation missing" -KCl₃ transforms to a yet unknown structure before final decomposition to KCl and Cl₂ at near-ambient conditions.