Spectroscopic studies of the vibrational and electronic properties of solid hydrogen to 285 GPa

Abstract

We report Raman scattering and visible to near-infrared absorption spectra of solid hydrogen under static pressure up to 285 GPa between 20 and 140 K. We obtain pressure dependences of vibron and phonon modes consistent with results previously determined to lower pressures. The results indicate the stability of the ordered molecular phase III to the highest pressure reached and provide constraints on the insulator-to-metal transition pressure. R ecent theoretical predictions for the transformation pressure of solid hydrogen to its metallic state are still uncertain and range between 260 and 410 GPa (1-3). Metallization by band overlap is predicted to occur before breakdown to a monoatomic solid. The ability of this theory to calculate the band gap at relevant densities is substantially impaired by the uncertainty of the structure of the high-pressure molecular modification of solid hydrogen-phase III (e.g., ref. 4). The nature of this phase and its related infrared activity have been the topic of numerous theoretical and experimental studies (e.g., ref. 5), but definitive knowledge of its crystal and electronic structure is not yet in hand.Diamond-anvil cells have been used successfully to reach static pressures on the order of 360 GPa for compressed metals (6-8). However, numerous attempts to compress solid hydrogen to transform it to conducting states (9-15) have not been successful in reaching the critical pressure range, while at the same time characterizing the sample and pressure definitively. The claim of compressing solid hydrogen to 342 GPa (15), for instance, showed no evidence for the presence of hydrogen in the sample chamber, indicating that instead the ''soft'' hydrogen was most likely lost by developing small leaks in diamonds and gasket or by reaction with the gasket material (see ref. 4). In addition, the large stress-induced increase in optical absorption and fluorescence in diamond anvils (6, 16, 17) poses a major obstacle in optical measurements of hydrogen samples and pressure calibration by ruby fluorescence. Control measurements for optical absorption experiments are essential. For example, comparison of the absorption of the transparent ruby grains adjacent to hydrogen can be used to ascertain that the pressure-induced changes in absorption occurs is in hydrogen but not in the diamond windows (9). At pressures beyond 180 GPa, the ruby fluorescence becomes extremely weak and can be overwhelmed by diamond fluorescence, thus presenting another serious problem for pressure calibration. As a result, the pressure for our previously reported onset of absorption in hydrogen could be determined as being above 200 GPa, but the upper limit could not be established. Indirect methods of pressure calibration, such as x-ray diffraction measurements on the gasket (15), do not indicate the pressure of the sample, and pressure calibration based on the pressure shift of diamond Raman band (18) may vary by as much as a factor of 3, depending on the local nonhydrostatic stress condition on the pressure-bearin...

Structure of phase III of solid hydrogen

“…The infrared activity of the strong infrared-active vibrons in C2/c increases with pressure, as is observed in phase III 17 . Figure 5 shows the variation with pressure of both the strong infrared peak and the Raman-active vibron frequency of C2/c to be in good agreement with experiment 18 . Figure 5 also shows a fairly strongly infraredactive phonon mode between 1,600 and 2,000 cm −1 , as seen in experiment 18 .…”

“…Figure 5 shows the variation with pressure of both the strong infrared peak and the Raman-active vibron frequency of C2/c to be in good agreement with experiment 18 . Figure 5 also shows a fairly strongly infraredactive phonon mode between 1,600 and 2,000 cm −1 , as seen in experiment 18 . The Supplementary Information shows that the calculated libron frequencies (those up to about 600 cm −1 ) appear in the correct frequency range and gently increase with pressure in a similar fashion to those observed in experiment 18 .…”

The properties of hydrogen and helium under extreme conditions

“…The planetary isentropes lie outside the realm of static diamond anvil cell (DAC) experiments, which while they reliably can extend to pressures of ∼320 GPa (Goncharov et al, 2001;Loubeyre et al, 2002), can only do so at relatively low temperatures, such as 1115 K at 73 GPa . The experimental range of static measurements is depicted in Fig.…”

Sound velocities in solid hydrogen under pressure

“…The frequency range of this Raman mode is extremely large, from 36 cm -1 at zero pressure [27][28][29][30][31][32][33][34][35][36] to 1100 cm -1 at 250 GPa. These measurements show that hcp-based structures are stable in this pressure range.…”