Goncharov et al., Phys. Rev. Lett., in press

1. 100 mm Polymorphism and melting of nitrogen at high pressureRussell J. Hemley (Carnegie Institution of Washington) DMR 0508988 As an archetypal diatomic system, nitrogen is an excellent testbed for refining models of physical and chemical processes that occur under extreme pressures and temperatures. Raman measurements in the diamond anvil cell (DAC) reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied. These results provide an explanation for complex kinetic effects inthe experimentally observed phase changesand amorphization of nitrogen at low and moderate temperatures above 120 GPa. (a) (b) (c) (d) • Phase diagram of nitrogen. • Diamond anvil cell (DAC) • Sample cavity in a DAC at • 100 GPa. The rectangle with a • small hole is a coupler to absorb • the laser radiation. • (d) Hot sample under pressure Goncharov et al., Phys. Rev. Lett., in press

2. Polymorphism and melting of nitrogen at high pressureRussell J. Hemley (Carnegie Institution of Washington) DMR 0508988 Iridium temperature coupler inside DAC This work involved close collaboration between academia and national laboratories, in particular, Lawrence Livermore National Laboratory which greatly benefited from the results and technological advances. The experiments resulted in the development of new instrumentation, i.e., combined pulsed laser heating – Raman techniques that allow optical spectra to be obtained in materials for the first time at simultaneous high pressures (>100 GPa) and temperatures (to 2500 K). This opens new avenues for studies of molecular materials and alloys with implications for planetary interiors and for high energy-density materials sciences. As a result, our laboratory is now frequently used by visitors, including students and senior scientists; see: http://cdac.gl.ciw.edu/content/view/12/38/ (a) (b) • Temperature distribution in the • laser heated DAC sample cavity • (courtesy of W. M. Howard, LLNL) • (b) Spectroradiometry showing the very high temperatures achieved. • (c) Raman spectra of hot compressed nitrogen