Compressing arsenic⋯halogen secondary bonds : a high-pressure structural study of arsenic(III) oxide intercalates with ammonium halides

Abstract

Crystal structures of arsenic(III) oxide intercalation compounds with ammonium chloride (NH4Cl·As2O3·1/2H2O), ammonium bromide (NH4Br·2As2O3), and ammonium iodide (NH4I·2As2O3) have been determined under high pressure up to 12, 15 and 11 GPa, respectively. No phase transitions have been observed for the investigated compounds. The compression of arsenic⋯halogen secondary bonds, expressed as penetration indices of the bonds, has been shown to be a linear function of unit-cell volume ratio V/V0, where V0 is the unit-cell volume at ambient pressure, with similar slopes as the compression of arsenic⋯oxygen secondary bonds. The behavior of the arsenic coordination number, expressed as a first-order valence entropy coordination number, at high pressures and the stereoactivity of arsenic lone electron pairs in the studied intercalates are the same as in arsenic(III) oxide polymorphs – the former decreases linearly with V/V0, while the latter remains unchanged. The high-pressure study lends further support to the fact that the nature of arsenic⋯halogen and arsenic⋯oxygen secondary bonds is the same.