| The synthesis of new solids with interesting properties is one of the most important areas of solid state chemistry. Many of the most interesting and/or useful materials known are only thermodynamically metastable yet remain kinetically stable indefinitely. Although diamond is the hardest known substance, diamond coated tools cannot be used for the machining of ferrous metals as they catalyze the conversion of diamond to graphite. As a result, there are opportunities for the synthesis of new hard materials with properties that are complementary to those of diamond. Our research focus is the high pressure synthesis of three dimensional carbon networks and carbon nitrides.; In one set of experiments, we examine the cryogenic quenching of a transparent phase of carbon obtained by room temperature compression of graphite. This phase becomes transparent at high pressure, however upon release of pressure it immediately reverts to graphite. For the first time, we have quenched this phase at liquid helium temperatures and probed back transformation to graphite using microfocus Raman spectroscopy as a function of temperature at ambient pressure.; In another series of experiments, we used high pressure and high temperature conditions to synthesize amorphous carbon nitrides containing up to 38 percent nitrogen, and a novel crystalline carbon phase which may represent a new sp{dollar}sp2{dollar} bonded allotrope. In the synthesis, the organic precursor tetracyanoethylene (C{dollar}rmsb6Nsb4{dollar}) is compressed to 42 GPa (1 GPa {dollar}approx{dollar} 10,000 atm.) followed by laser heating to {dollar}sim{dollar}1500 K to effect a reconstructive transformation to carbon nitride and a novel crystalline phase of carbon. Using analytical electron microscopy, TEM and STEM, in conjunction with diffraction, electron energy loss spectroscopy and energy dispersive x-ray analysis the composition structure and bonding of these materials is determined. We found that the nitrogen content increases with increasing pressure. This finding in addition to theoretical calculations prompted us to carry out experiments at pressures exceeding 1 Mbar. And perform characterization in situ using synchrotron x-ray diffraction. |
