Exotic compounds of monovalent calcium synthesized at high pressure

It is well known that atoms of the same element in different valence states show very different chemical behaviors. Calcium is a typical divalent metal, sharing or losing both of its valence electrons when forming compounds. Attempts have been made to synthesize compounds of monovalent calcium ions for decades, but with very little success (e.g., in clusters). Pressure can result in substantial changes in the properties of atoms and chemical bonding, creating an extensive variety of unique materials with special valence states. In this study, using the ab initio evolutionary algorithm USPEX, we search for stable calcium–chlorine (Ca–Cl) system compounds at pressures up to 100 GPa. Besides the expected compound CaCl2, we predict three new compounds with monovalent Ca to be stable at high pressures, namely, CaCl, Ca5Cl6, and Ca3Cl4. According to our calculations, CaCl is stable at pressures above 18 GPa and is predicted to undergo a transition from nonmagnetic Fm-3m-CaCl to ferromagnetic Pm-3m-CaCl at 40 GPa. Ca5Cl6 and Ca3Cl4 are stable at pressures above 37 and 73 GPa, with space groups P-1 and R-3, respectively. Following these predictions, we successfully synthesized Pm-3m-CaCl in laser-heated diamond anvil cell experiments. The emergence of the unusual valence state at high pressures reveals exciting opportunities for creating entirely new materials in sufficiently large quantities for a variety of potential applications.