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

Crystal structuresof arsenic(III)-oxide intercalationcompoundsRbBr & BULL;2As(2)O(3), RbI & BULL;2As(2)O(3), CsCl & BULL;2As(2)O(3), CsBr & BULL;2As(2)O(3), and CsI & BULL;2As(2)O(3) havebeen determined and crystal structures of intercalates KBr & BULL;2As(2)O(3), KI & BULL;2As(2)O(3), NH4Br & BULL;2As(2)O(3), and NH4I & BULL;2As(2)O(3) have been redetermined. All of the compoundsare isostructural, and correlations of structural parameters withionic radii of cations and anions have been analyzed. It has beenfound that unit-cell parameters, As-O bond lengths, As & BULL;& BULL;& BULL;X distances, and O-As-O and As-O-Asvalence angles correlate linearly with ionic radii of anions, where X = Cl, Br, or I. Similar correlations with ionic radiiof cations were found to be less pronounced except for the c unit-cell parameters, valence angles, and A & BULL;& BULL;& BULL;O distances, where A = K, Rb,Cs, or NH4. Structural parameters involving NH4 (+) cations deviate from linear correlations found for alkali-metalcations. 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 gaugedusing the bond-valence vector model and found to correlate with thetype of anion and the first-order valence entropy coordination number((VECN)-V-1) of arsenic. This revealed the presence of secondaryAs & BULL;& BULL;& BULL;X bonds in the intercalates andthe fact that their strength increases with increasing molar massof the halide anion. Thecomparative investigation of the crystal structuresof arsenic(III)-oxide intercalates with alkali-metal halides revealedthat As & BULL;& BULL;& BULL;X interactions are secondarybonds analogous to As & BULL;& BULL;& BULL;O secondary bonds.