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

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.