Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na 2 Cl 3 and Na 4 Cl 5 and Bromide Na 4 Br 5 .

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures ( tP 10-Na 2 Cl 3 , hP 18-Na 4 Cl 5 , and hP 18-Na 4 Br 5 ), a series of isostructural cubic cP 8-AX 3 halides (NaCl 3 , KCl 3 , NaBr 3 , and KBr 3 ), and a trigonal potassium chloride ( hP 24-KCl 3 ). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl 3 - anion in hP 24-KCl 3 and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl] ∞ n - and [Br] ∞ n - , in the structures of cP 8-AX 3 compounds and in hP 18-Na 4 Cl 5 and hP 18-Na 4 Br 5 . In Na 4 Cl 5 and Na 4 Br 5 , we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides.