7 Search Results

LaRu₃⁢Si₂ is of current research interest as a kagome metal with a superconducting transition temperature, 𝑇_𝑐∼7 K, and higher-temperature charge density wave orders. Here we report on electrical transport and x-ray diffraction measurements on LaRu₃⁢Si₂ under pressure up to 65 GPa and 35 GPa, respectively. The superconducting transition temperature 𝑇_𝑐 first gets slightly enhanced and reaches a maximum ∼8.7 K at ∼8.5 GPa. With further applied pressure, 𝑇_𝑐 is initially gradually suppressed, then more rapidly suppressed, followed by gradual suppression, revealing a superconducting dome. Two possible pressure-induced structural phase transitions are also observed at room temperature, from the original hexagonalmore » phase to another hexagonal structure above ∼11.5 GPa, and further to a structure with lower symmetry above ∼23.5 GPa. These transition pressures roughly correlate with features found in our pressure-dependent transport data.« less

Brillouin scattering spectroscopy has been used to obtain an accurate (<1%) ρ-P equation of state (EOS) of 1:1 and 9:1 H₂-He molar mixtures from 0.5 to 5.4 GPa at 296 K. Our calculated equations of state indicate close agreement with the experimental data right to the freezing pressure of hydrogen at 5.4 GPa. The measured velocities agree on average, within 0.5%, of an ideal mixing model. The ρ-P EOSs presented have a standard deviation of under 0.3% from the measured densities and under 1% deviation from ideal mixing. Furthermore, a detailed discussion of the accuracy, precision, and sources of errormore » in the measurement and analyses of our equations of state is presented.« less

Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogen-dominated atmosphere and a high-temperature heavier-element-dominant core. From an assumption that there is no chemical reaction between hydrogen and silicates/metals at the atmosphere–interior boundary, the cores of sub-Neptunes have been modeled with molten silicates and metals (magma) in previous studies. In large sub-Neptunes, pressure at the atmosphere–magma boundary can reach tens of gigapascals where hydrogen is a dense liquid. A recent experiment showed that hydrogen can induce the reduction of Fe²⁺ in (Mg,Fe)O to Fe⁰ metal at the pressure–temperature conditions relevant to the atmosphere–interior boundary. However, it ismore » unclear whether Mg, one of the abundant heavy elements in the planetary interiors, remains oxidized or can be reduced by H. Our experiments in the laser-heated diamond-anvil cell found that heating of MgO + Fe to 3,500 to 4,900 K (close to or above their melting temperatures) in an H medium leads to the formation of Mg₂FeH₆ and H₂O at 8 to 13 GPa. At 26 to 29 GPa, the behavior of the system changes, and Mg–H in an H fluid and H₂O were detected with separate FeH_x. The observations indicate the dissociation of the Mg–O bond by H and subsequent production of hydride and water. Therefore, the atmosphere–magma interaction can lead to a fundamentally different mineralogy for sub-Neptune exoplanets compared with rocky planets. The change in the chemical reaction at the higher pressures can also affect the size demographics (i.e., “radius cliff”) and the atmosphere chemistry of sub-Neptune exoplanets.« less

A Paris-Edinburgh press combined with a multi-channel collimator assembly has been commissioned at the GeoSoilEnviro Center for Advanced Radiation Sources (GSECARS) beamline for monochromatic X-ray scattering, with an emphasis on studying low-Z liquids, especially silicate liquids at high pressure. The Paris-Edinburgh press is mounted on a general-purpose diffractometer, with a pixel array detector mounted on the detector arm. The incident monochromatic undulator beam with energies up to 60 keV is focused both horizontally and vertically to a beam size about 30 × 30 µm. With this setup, background scattering from the surrounding pressure media is completely removed at 2θ anglesmore » above 10° for samples larger than 1.05 mm in diameter. Thirty minutes is typically sufficient to collect robust X-ray scattering signals from a 1.6 mm diameter amorphous silicate sample. Cell assemblies for the standard Paris-Edinburgh anvils have been developed and pressures and temperatures up to 7 GPa and 2300 K, respectively, have been maintained steadily over hours. We have also developed a cupped-toroidal Drickamer anvil to further increase pressure and temperature capabilities. The cupped-toroidal Drickamer anvil combines features of a modified Drickamer anvil and the traditional Paris-Edinburgh anvil. Pressures up to 12 GPa have been generated at temperatures up to 2100 K.« less

Synthesis of novel extended forms of nitrogen and nitrogen-rich materials has been a topic of interest in development of high-energy-density materials. Here, we present the formation of high-density (3.983 g/cm³) copolymer CON₂, formed in crystalline form by laser heating of CO–N₂ mixtures above 1700 K and 45 GPa—a substantially lower pressure–temperature condition than those required for converting pure nitrogen (above 110 GPa and 2000 K). It can be made even at lower pressures ~20 GPa at ambient temperature for amorphous solid. According to the refined structure, the crystalline polymer is made of nitrogen-hybridized, eight-membered rings of singly bonded CON₂ inmore » a three-dimensional framework structure in the space group of P4₃, as one of the previously predicted structures. Furthermore, unlike the predicted structures, the present P4₃ solid converts back to ε-N₂-like and δ-N₂-like molecular phases as pressure unloads to 20 and 10 GPa, respectively.« less

Hydrazinium azide (HA) has been investigated at high pressures to 68 GPa using confocal micro-Raman spectroscopy and synchrotron powder x-ray diffraction. The results show that HA undergoes structural phase transitions from solid HA-I to HA-II at 13 GPa, associated with the strengthening of hydrogen bonding, and then to N₈ at 40 GPa. The transformation of HA to recently predicted N₈ (N≡N⁺—N–—N=N—–N—+N≡N) is evident by the emergence of new peaks at 2384 cm^–1, 1665 cm^–1, and 1165 cm^–1, arising from the terminal N≡N stretching, the central N=N stretching, and the N—N stretching, respectively. Furthermore, upon decompression, N₈ decomposes to ε-N₂ belowmore » 25 GPa, but the remnant can be seen as low as 3 GPa.« less