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, W ₂C, 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 WO ₂ and the high-pressure phase W ₃O ₈(I). The approximate average size of the graphite particles formed here was 6-8 nm. The bulk modulus of W(CO) ₆ is B ₀ ≈ 13 GPa.