Anhydrous Hexagonal As₂O₃ Intercalates with Alkali-Metal Halides: A Comparative Study of Crystal Structures and Arsenic(III) Lone-Pair Stereoactivity Present Therein

Abstract

Crystal structures of arsenic­(III)-oxide intercalation compounds RbBr·2As2O3, RbI·2As2O3, CsCl·2As2O3, CsBr·2As2O3, and CsI·2As2O3 have been determined and crystal structures of intercalates KBr·2As2O3, KI·2As2O3, NH4Br·2As2O3, and NH4I·2As2O3 have been redetermined. All of the compounds are isostructural, and correlations of structural parameters with ionic radii of cations and anions have been analyzed. It has been found that unit-cell parameters, As–O bond lengths, As···X distances, and O–As–O and As–O–As valence angles correlate linearly with ionic radii of anions, where X = Cl, Br, or I. Similar correlations with ionic radii of cations were found to be less pronounced except for the c unit-cell parameters, valence angles, and A···O distances, where A = K, Rb, Cs, or NH4. Structural parameters involving NH4 + cations deviate from linear correlations found for alkali-metal cations. Linear correlations have also been found between intercalates’ unit-cell volumes and the molar volumes of the constituting salts. The stereoactivity of arsenic lone electron pairs has been gauged using the bond-valence vector model and found to correlate with the type of anion and the first-order valence entropy coordination number (1VECN) of arsenic. This revealed the presence of secondary As···X bonds in the intercalates and the fact that their strength increases with increasing molar mass of the halide anion.