Thermal Iodine Loss Cascade of W₅I₁₆

Tungsten iodide compounds feature a surprising diversity of binary compounds. Their formation conditions and phase equilibria depend very much on the surrounding iodine partial pressure and temperature. Herein we focus on square-pyramidal tungsten iodide cluster compounds with their iodine loss, structural rearrangements, cross-linking behavior, and final transformation into the octahedral cluster compound β-W₆I₁₂ at elevated temperatures. Reactions depart from W₅I₁₆ at different iodine pressures and temperatures. The thermal decomposition of W₅I₁₆ at low iodine pressure passes through a number of cluster compounds (W₅I₁₅, W₅I₁₃·xI₂, β-W₅I₁₂, and W₅I₁₁) under release of iodine. At higher iodine pressures, only one intermediate compound (α-W₅I₁₂) was found. Thermal decomposition of W₅I₁₆ was examined by differential thermal/thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction (XRD) measurements. New compounds (W₅I₁₅, W₅I₁₃·xI₂, and W₅I₁₁) were structurally characterized by means of XRD techniques. The crystal structure of W₅I₁₁ reveals a nice relationship to its transformation product W₆I₁₂.