Nitrogen is observed to remain a molecular solid up to 130 GPa, contrary to recent theoretical predictions of metallization below 100 GPa. Raman scattering reveals three new phases at 20, 66, and 100 GPa, which are distinguished by branching of existing vibronic modes. One mode increases in frequency to a broad maximum at 66 GPa and then decreases, similar to the case of H 2 . In H2, such behavior was attributed to a weakening of the H-H bond. In N2, three other vibronic frequencies continue to increase, showing the situation to be more complicated than just a weakening of the N-N bond.PACS 78.30.Gt There has been a great deal of recent interest in insulator-metal transitions, particularly in diatomic molecules such as N 2 , H 2 , and I2. 1 Recent experimental and theoretical results for nitrogen indicate that N 2 might transform to a monatomic metallic state at pressures below 100 GPa (1 Mbar). The results of experimental studies by Nellis and co-workers 2 ' 3 of singleand double-shocked nitrogen appear to show a continuous dissociation of molecular nitrogen to a monatomic state starting at 30 GPa and 6000 K. Theoretical calculations by McMahan and LeSar 4 indicate that crystal structures composed of N 2 molecules may be less stable than a monatomic simple cubic structure at 0 K and 77 to 94 GPa. They suggested that structures observed in other group-V elements might have even lower cohesive energies than the simple cubic structure at high pressures. Martin 5 has subsequently made calculations which show that nitrogen would have a lower cohesive energy in the arsenic (A 7) rhombohedral structure at pressures around 100 GPa.Our work was intended to investigate the prediction that molecular nitrogen would transform to a monatomic structure, possibly metallic, at pressure accessible to the diamond-anvil cell. Preliminary experiments which we performed at pressure above 110 GPa showed a color change in nitrogen, but no dramatic transition to a metallic phase. Concurrently, Hemley, Mao, and Bell 6 made preliminary Raman measurements on nitrogen up to pressures of 160 GPa. They showed that the frequency of the v 2 vibron increases with pressure to a maximum at about 80 GPa and then drops, in a manner similar to the behavior of the vibrational mode of molecular hydrogen at 50 GPa. 7,8 These experimental developments prompted us to study the optical absorption spectrum of nitrogen and to investigate the N 2 vibron region carefully with high-resolution Raman spectroscopy in order to detect any subtle phase changes in molecular nitrogen at pressure.Our high-pressure experiments were made with "megabar" diamond-anvil cells. 9 We employed the techniques described by Goettel, Mao, and Bell, 10 and used beveled-diamond anvils for generating pressures above 100 GPa. The anvils used were standard brilliant-cut diamonds with sixteen facets, a central flat of 50 jxm, a 5° bevel, and a 300-/xm total culet diameter.The sample chamber was formed by preindenting a 250-/xm-thick 7-301 stainless-steel gasket to a thickness of 15 to...

X ray diffraction and infrared spectroscopic measurements to pressures of 45–50 GPa, electrical resistivity and optical absorption to 70–80 GPa, and reflectance measurements to 225 GPa are presented for Fe2SiO4 fayalite at 300 K. Diffraction results document that Fe2SiO4 fayalite becomes amorphous on static loading to pressures in excess of 39 (±3) GPa, a pressure identical to that at which the “mixed phase” regime of fayalite commences under shock compression. As with more polymerized silicates, the high‐pressure amorphization of metastable fayalite is associated with the SiO4 tetrahedron becoming unstable relative to higher coordinations of silicon. Infrared absorption spectroscopy reveals the pressure‐induced change in coordination through a decline in intensity of the tetrahedral asymmetric stretching vibration, accompanied by an increase in amplitude at frequencies characteristic of SiO6 vibrations. The coordination change is not quenchable to zero pressure, and infrared spectra of amorphous fayalite quenched from pressure document that the local structure of the sample is similar to that of crystalline Fe2SiO4. This represents the first example of the static synthesis of a glass without fusion in a silicate containing isolated tetrahedra. The electrical resistivity of initially crystalline Fe2SiO4 drops from approximately 2×105 ohm m at zero pressure to 10−3 ohm m at 79 GPa, with the pressure dependence of the resistivity decreasing at approximately the pressure of the coordination change (and amorphization). The electrical properties are quantitatively consistent with previous measurements, both under static and under shock wave loading. Optical transmission experiments demonstrate that in accord with the rapidly changing electron‐transport properties of Fe2SiO4, a strong absorption edge decreases in energy from the ultraviolet to infrared energies of about 3500 cm−1 (0.4 eV) on compression to 70 GPa. However, the reflectance of Fe2SiO4 is less than 3.1% between 780 and 2100 nm at all pressures to 225 GPa, indicating that amorphous fayalite does not metallize to at least this pressure. We interpret our resistivity and optical results in terms of increased interactions between iron ions with increasing pressure in Fe2SiO4 and an approach toward a Fe3++Fe1+ metallic configuration at high pressures. When combined with previous observations of amorphization at low temperatures and high pressures, our results suggest that transition to an amorphous phase is likely to be a general phenomenon in metastable silicates when the distortion of SiO4 tetrahedra becomes a major mechanism of compression.

The equation of state and optical absorption of condensed Xe have been measured in a diamond-anvil cell up to 172 GPa. Pressures were determined by use of the ruby-fluorescence technique or from the equation of state of the Re gasket used to confine the sample. Xe transformed to an hep structure between 70 and 90 GPa that remained stable to 172 GPa. At 150 GPa we observed sudden changes in absorption and reflection spectra of Xe that we attribute, on the basis of electron band calculations, to the onset of metallization. PACS numbers: 71.30,+h, 62.50.+p, 78.30.Hv In this paper we present evidence for the appearance of metallic Xe at 150 GPa. The pressure-induced transformation of condensed Xe from insulator to metal has long been the subject of intense interest. 1 " 4 Herzfeld 1 first predicted such a transition at a volume of 10.2 cmVmol, where the molar volume of Xe would become equal to its gas-phase molar refractivity. Electron band-structure calculations 3 have shown that transition of Xe to the metallic state at high pressure is due to crossing of the empty 5d conduction and full 5p valence bands, and that the insulating gap for volumes less than 30 cm 3 /mol is indirect. Subsequent calculations 4 produced an equation of state (EOS) and predicted the insulator-metal transition to occur, via band overlap, in the range from 11 to 9 cm 3 /mol, corresponding to pressures from 130 to more than 200 GPa. Recently, Goettel et al. 5 have measured an ir reflection spectrum of Xe at an estimated pressure of 230 GPa and used it to estimate metallization of Xe at 190 GPa. 5 Structural studies of Xe using x-ray techniques have shown that Xe freezes in an fee structure that has been reported to remain stable at room temperature up to a maximum pressure of between 23 and 55 GPa. 6 " 8 Recently, Jephcoat et al. 9 reported phase transitions in Xe at 14 GPa from fee to an intermediate, close-packed phase, and at 75 GPa, to an hep structure that remained stable to 137 GPa. Their measured EOS was found to be in close agreement with theory. 4 Transition of the heavy rare-gas solids to the hep phase prior to metallization was also predicted theoretically. 9 Optical absorption studies of Xe have been reported by Syassen 10 and Asaumi, Mori, and Kondo 11 at pressures up to 44 and 55 GPa, respectively. Both attributed weak uv absorption below the diamond absorption edge to interband transitions across the indirect insulating gap. Their findings, which show a decrease in the band gap with increasing pressure, are consistent with theoretical predictions, 4 and extrapolate to gap closure in the vicinity of 10 cm 3 /mol.In this Letter we describe the results of high-pressure optical absorption, ir reflection, and x-ray diffraction experiments on solid Xe using the diamond-anvil cell (DAC). Pressures were generated in Xe using "megabar" DAC's with either single-or double-beveled diamond anvils. 12 Both type-I and type-IIa diamonds were used in a series of experiments employing various anvil designs. Bevel angles as well as the flat an...

We have performed a comprehensive finite element analysis of the diamond anvil cell. Our analysis shows how beveled diamonds and material properties of the gasket affect diamond anvil cell performance. Using the results of the analysis, we have achieved 4.6 Mbar experimentally, which is the highest static pressure reported to date. Possible methods to increase the pressure further are discussed.

A four-probe technique is described for measuring the electrical resistance of metals in a diamond-anvil cell at pressures up to 40 GPa. The pressure range for electrical resistance measurements was extended by developing insulating gaskets that provide the necessary support for the diamonds and the electrical leads at the diamond edges. The various gasket materials and construction methods that were tested fall into two categories: (1) gaskets made entirely of insulating materials, and (2) gaskets made of metal coated with insulating materials. Gaskets developed in each category were used successfully in making resistance measurements up to 40 GPa. The most reliable gaskets were composites of sheet mica and MgO powder. This report describes the testing and development of the gaskets and presents electrical resistance data obtained for iron and beryllium to 40 GPa.