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 Cl(2) 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 K(3)Cl, K(2)Cl, K(3)Cl(2), K(4)Cl(3), K(5)Cl(4), K(3)Cl(5), KCl(3) and KCl(7). Of particular interest are 2D-metallic homologs K(n+1)Cl(n), the presence of positively charged Cl atoms in KCl(7), and the predicted stability of KCl(3) already at nearly ambient pressures at zero Kelvin. We have synthesized cubic [Image: see text] -KCl(3) at 40–70 GPa and trigonal [Image: see text] -KCl(3) at 20–40 GPa in a laser-heated diamond anvil cell (DAC) at temperature exceeding 2000 K from KCl and Cl(2). These phases were identified using in situ synchrotron X-ray diffraction and Raman spectroscopy. Upon unloading to 10 GPa, [Image: see text] -KCl(3) transforms to a yet unknown structure before final decomposition to KCl and Cl(2) at near-ambient conditions.