Decomposition of W(CO)₆ at high pressures and temperatures

The decomposition of hexacarbonyltungsten, W(CO)₆, has been studied. The decomposition was induced by heating W(CO)₆ in an autoclave at 523 K and pressures up to 1.8 MPa, and by laser heating in a diamond anvil cell at pressures between 5 and 18 GPa. The products have been characterized using synchrotron X-ray diffraction, pair distribution function analysis, Raman spectroscopy and scanning electron microscopy. Decomposition in the autoclave at the lower pressures resulted in the formation of a metastable tungsten carbide, W₂C, with an average particle size of 1–2 nm, and an unidentified nanocrystalline tungsten oxide and nanocrystalline graphite with average particle sizes of 1–2 and 11 nm, respectively. The existence of nanocrystalline graphite was deduced from micro-Raman spectra and the graphite particle size was extracted from the intensities of the Raman modes. The high-pressure decomposition products obtained in the diamond anvil cell are the monoclinic tungsten oxide phase WO₂ and the high-pressure phase W₃O₈(I). The approximate average size of the graphite particles formed here was 6–8 nm. The bulk modulus of W(CO)₆ is B₀ ≃ 13 GPa.