C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

1 1 6 H I G H L I G H T S 2 0 2 3 I

X-ray diffraction unravels the bonding complexity of polyhalogen anions

High-pressure experiments using X-ray diffraction to study the alkali halides systems Na-Cl, Na-Br, K-Cl, and K-Br have revealed the existence of two different types of infinite linear halogen chains [Cl]∞

n- and [Br]∞ n- proof that they can be stabilised

in two different forms.

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. High pressure dramatically changes the chemistry of materials. Quantum-chemical calculations predict unusual but stable stoichiometries of alkali halides with different types of polyhalogen anions, such as, for example, NaCl3 and NaCl7, KBr3 and KBr5, and CsFm (m = 2-6), and suggest uncommon properties. Despite the numerous predictions, very few of them have been experimentally confirmed. So far, the syntheses of the compounds with the stoichiometries AX3, A3X, and AX5 (A is an alkali metal, X is a halogen) have been reported at high pressures and characterised using powder X-ray diffraction (PXRD). Although the synergy of PXRD and ab-initio structure predictions is very helpful to obtain a structure solution, the interpretation of some PXRD remains ambiguous.

The focus of this study was the synthesis of novel alkali halides with polyhalogen anions. Liquid carbon tetrachloride (CCl4) or solid carbon tetrabromide (CBr4) together with the corresponding alkali halide salt (NaCl, NaBr, and KBr) were chosen as starting materials to provide a halogen-rich environment. Using a diamond- anvil cell (DAC), the samples were compressed to the target pressures (41 to 80 GPa) and laser-heated to around 2000 K. The reaction products were studied using single- crystal XRD at beamlines ID11 and ID27. Single-crystal XRD provides both precise structure determination and

Fig. 94: The structure of the novel (a) isostructural sodium polychloride hP18-Na4Cl5 and sodium polybromide hP18-Na4Br5, and (b) sodium sesquichloride tP10-Na2Cl3 synthesised in this work.