Pressure-induced Formation of Energetic and Structural Extended Solids with Quench-recovery to Ambient Conditions

Carbon clathrate materials, comprised of sp3 carbon structures with light elements, as well as polymeric carbon monoxide, represent two classes of high-pressure materials with promising potential structural and energetic applications. A systematic exploration of PTx space was performed in carbon alkali and alkaline earth metals, under high-pressuretemperature conditions, as well as silicon-based systems, to establish the propensity to for sp3-based carbonsilicon networks with superlative properties. For the case of silicon, it was determined that silicon clathrates are thermodynamically stable at high-pressure conditions. This suggests that other high-pressure phases may be synthesized outside their thermodynamic stability regimes. We examined the sodium removal from a newly discovered NaSi6 Na4Si24 compound. Upon complete sodium removal, a new sp3 silicon allotrope was produced that possesses a quasidirect band gap with the ideal value for photovoltaic applications. For the case of carbon, mixtures with Li, Mg, Na, and Ca were investigated. All systems showed known carbide formation under certain conditions, but several new and recoverable phases were identified including Mg2C, Mg2C3, Ca2C, Ca2C3 and Ca3C2. Some of these, such as Mg2C, contain unusual anions like C4- that are recoverable to ambient conditions and display sp3 carbon hybridization. These may serve as useful precursors for carbon clathrate materials. The polymerization behavior of CO poly-CO, p-CO was examined over a range of conditions with different dopants. The presence of molecular nitrogen and acetylene did not significantly affect the polymerization pressure of CO or the molecular structure of the resulting polymer. The presence of amorphous carbon and SiO2 appear to reduce the polymerization pressure by a fraction of one GPa. The presence of metallic lithium and sodium appear to reduce the polymerization pressure substantially.