Evidence for the Insulator-Metal Transition in Xenon from Optical, X-Ray, and Band-Structure Studies to 170 GPa

Abstract

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...

Refractive index of lithium fluoride ramp compressed to 800 GPa

“…[1][2][3][4] Lithium fluoride's (LiF's) large band-gap and ionic crystalline structure produces its uniquely high ultraviolet transmissivity. Two rare-gas solids, He and Ne, have anomalously high metallization pressures due to the predicted intershell band overlap, which is unique among monatomic materials.…”

The fusion curves of xenon, krypton, and argon

“…For such reasons, neon and helium will not be considered in the present work. The second exception is concerned with the fusion of the heavier rare gases, argon, krypton, and xenon, for which a number of new experimental results have been obtained more recently at much higher pressures than before [7][8][9][10], partly a result of the advancement in laboratory techniques, namely the diamond-anvil cell (DAC), partly from the efforts of geologists to investigate the so-called ''missing xenon problem" in the atmosphere of the earth [11,12], and also from the interest of many other scientists to attain and observe the metallic phase predicted by theory to exist for Xe above about (137 to 150) GPa [13,14].…”

Metallic high-pressure modifications of main group elements

“…14) is observed for iodine at pressures between ambient and 55 GPa [104][105][106]. Finally, in the noble gas xenon the transformation from hcp to fcc between 70 GPa and 90 GPa precedes the onset of an insulator-metal transition at 150 GPa as evidenced by optical absorption data [107].…”