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.

Automated summary

This study investigates the structural changes of Dicaesium octaiodide under pressure, revealing that pressure can alter the configuration of polyiodide units. The structure is shown to be half as compressible as the similar CsI3 structure, with a higher bulk modulus and more covalent bonds.