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.