Deformation of polyiodides in Cs2I8 crystals at high pressure

Dicaesium octaiodide is composed of layers of zigzag polyiodide units (I-8(2-)) intercalated with caesium cations. Each I-8(2)- unit is built of two triiodides bridged with one diiodine molecules. This system was subjected to compression up to 5.9 GPa under hydrostatic conditions. Pressure alters the supramolecular architecture around I-8(2-), leading to bending of the triiodide units away from their energetically preferred geometry (D-infinity h). Short I-2 center dot center dot center dot I-3(-) contacts compress significantly, reaching lengths typical for the covalently bonded polyiodides. Unlike in reported structures at ambient conditions, pressure-induced catenation proceeds without symmetrization of the polyiodides, pointing to a different electron-transfer mechanism. The structure is shown to be half as compressible [B-0 = 12.9 (4) GPa] than the similar CsI3 structure. The high bulk modulus is associated with higher I-I connectivity and a more compact cationic net, than in CsI3. The small discontinuity in the compressibility trend around 3 GPa suggests formation of more covalent I-I bonds. The potential sources of this discontinuity and its implication on the electronic properties of Cs2I8 are discussed.