Pressure calibration of diamond anvil Raman gauge to 410 GPa

"Chemical precompression" through introducing impurity atoms into hydrogen has been proposed as a method to facilitate metallization of hydrogen under external pressure. Here we selected Ar(H 2 ) 2 , a hydrogen-rich compound with molecular hydrogen, to explore the effect of "doping" on the intermolecular interaction of H 2 molecules and metallization at ultrahigh pressure. Ar(H 2 ) 2 was studied experimentally by synchrotron X-ray diffraction to 265 GPa, by Raman and optical absorption spectroscopy to 358 GPa, and theoretically using the density-functional theory. Our measurements of the optical bandgap and the vibron frequency show that Ar(H 2 ) 2 retains 2-eV bandgap and H 2 molecular units up to 358 GPa. This is attributed to reduced intermolecular interactions between H 2 molecules in Ar(H 2 ) 2 compared with that in solid H 2 . A splitting of the molecular vibron mode above 216 GPa suggests an orientational ordering transition, which is not accompanied by a change in lattice symmetry. The experimental and theoretical equations of state of Ar(H 2 ) 2 provide direct insight into the structure and bonding of this hydrogen-rich system, suggesting a negative chemical pressure on H 2 molecules brought about by doping of Ar.ultrahigh pressure | hydrogen-rich compound | intermolecular interaction | metallization C hemistry under extreme conditions continues to inspire generations of researchers as it charts excitingly new realms of structures, properties, and phenomena in molecular systems (1). As an extreme case of tuning properties of simple molecules using pressure, metallic hydrogen has been keenly pursued for decades due to the predicted intriguing properties, i.e., room-temperature superconductivity (2) and superfluidity (3), and its fundamental scientific implications (4-6). However, for solid hydrogen, neither the predicted metallization in the atomic form through molecular dissociation (7) nor that in the molecular form through bandgap closure (8) has been achieved experimentally up to ∼400 GPa (9, 10). This is almost the pressure limit of studying solid hydrogen using the current static high-pressure techniques.It has been proposed theoretically that the attainment of metallic hydrogen can be facilitated through chemical precompression in the form of hydrogen-rich materials (11-13). As one of the hydrogenrich van der Waals compounds found under high pressure (14-19), Ar(H 2 ) 2 has been considered as a viable candidate. An experimental study revealed a phase transition of Ar(H 2 ) 2 associated with a molecular dissociation and possible Drude-type metallic behavior above 175 GPa at 100 K (20). A follow-up high-pressure IR absorption study up to 220 GPa at 100 K showed a persistence of molecular H 2 questioning the proposed metallization (21). Further theoretical studies of Ar(H 2 ) 2 also showed conflicting results. New phases were predicted at pressures ranging from 55 to 250 GPa (22-24). Based on different crystal structures, some calculation predicted metallization at lower pressure (22), whereas others...

Section: Methodsmentioning

confidence: 99%

Stability of Ar(H ₂ ) ₂ to 358 GPa

Goncharov

Shukla

et al. 2017

“…XAS spectra are collected every few seconds before, during, and after heating in four different runs in the pressure range 63-103 GPa and temperatures up to 3,530 K. For each heating cycle, the laser power is ramped up incrementally and kept constant for several seconds to record the XAS spectrum and light emission to measure the temperature. Pressures are determined before and after heating cycles using both ruby and the Raman spectra of the diamond anvil tips (24), with the accuracy within 2 GPa at the highest pressure achieved. After several heating cycles, these pressures are within 5 GPa.…”

Section: Methodsmentioning

confidence: 99%

Melting of iron determined by X-ray absorption spectroscopy to 100 GPa

Aquilanti

Trapananti

Karandikar

et al. 2015

Temperature, thermal history, and dynamics of Earth rely critically on the knowledge of the melting temperature of iron at the pressure conditions of the inner core boundary (ICB) where the geotherm crosses the melting curve. The literature on this subject is overwhelming, and no consensus has been reached, with a very large disagreement of the order of 2,000 K for the ICB temperature. Here we report new data on the melting temperature of iron in a laser-heated diamond anvil cell to 103 GPa obtained by X-ray absorption spectroscopy, a technique rarely used at such conditions. The modifications of the onset of the absorption spectra are used as a reliable melting criterion regardless of the solid phase from which the solid to liquid transition takes place. Our results show a melting temperature of iron in agreement with most previous studies up to 100 GPa, namely of 3,090 K at 103 GPa.iron melting curve | XAS | megabar range

“…A Raman system, mounted on the same bench, enables measurement of the highfrequency edge of the T 2g Raman band of the diamond at the anvil/sample interface and estimation from it of the sample pressure by using the calibration of ref. 24.…”

Section: Methodsmentioning

confidence: 99%

Synthesis of lithium polyhydrides above 130 GPa at 300 K

Pépin

Loubeyre

Occelli

et al. 2015

The prediction of novel lithium hydrides with nontraditional stoichiometries at high pressure has been seminal for highlighting a promising line of research on hydrogen-dense materials. Here, we report the evidences of the disproportionation of LiH above 130 GPa to form lithium hydrides containing H 2 units. Measurements have been performed using the nonperturbing technique of synchrotron infrared absorption. The observed vibron frequencies match the predictions for LiH 2 and LiH 6 . These polyhydrides remain insulating up to 215 GPa. A disproportionation mechanism based on the diffusion of lithium into the diamond anvil and a stratification of the sample into LiH 6 /LiH 2 /LiH layers is proposed. Polyhydrides containing an H 2 sublattice do exist and could be ubiquitously stable at high pressure.high pressure | hydride chemistry | hydrogen stoichiometry