The stability of carbon nitride materials

Interest in solid materials made of carbon and nitrogen has surged in the past few years following the prediction that dense and crystalline C{dollar}sb3{dollar}N{dollar}sb4{dollar} compounds should have extreme hardness and other interesting properties. However, the products of experimental syntheses to date have been amorphous, low-density materials of varying stoichiometry. In addition, the stability of nitrogen within these materials has been poor.; The decomposition rates of paracyanogen (pCN), a solid carbon nitride, into carbon and molecular nitrogen are presented herein as a function of pressure and temperature and delineate the metastability of carbon nitride materials. The thermal stability is shown to increases dramatically with pressure. Kinetic analysis of the decomposition process demonstrates that the mechanism is unimolecular at high nitrogen-content and bimolecular at low nitrogen-content. The former is a rapid process above 600{dollar}spcirc{dollar}C, while the latter is a very slow, diffusion-limited process below 1000{dollar}spcirc{dollar}C. In addition, bulk amorphous carbon nitrides are demonstrated to be unstable (explosive) at elevated temperatures because of a large decomposition enthalpy.; The observed increase in kinetic stability with pressure is a result of a large activation volume. However, there is also an observable decrease in the decomposition enthalpy that is consistent with simple thermodynamic estimates. As a result, carbon nitrides are predicted to become thermodynamically stable between 50 and 100 GPa. A detailed analysis of the expected temperature- and pressure-dependence of the free-energy and enthalpy of formation is presented.; Finally, experimental evidence is presented which suggests that pressure- and temperature-induced structural changes in carbon nitrides are consistent with a slow 'graphitization' process at pressures under 20 GPa.