11 Search Results

The effects of pressure on the structure and electrical transport properties, including electrical resistance and critical magnetic field values for superconductivity in pristine and hydrogenated TiZr alloys, were investigated. As pressure increased from ambient to 55 GPa, the structure of the pristine TiZr alloys transformed from hcp to bcc via an intermediate ω phase. The superconducting transition temperature (T_c) significantly increased from 1.76 K at ambient pressure to 15.5 K at 52.2 GPa, accompanied by an extrapolated upper critical field of 24.5 T at T = 0 K. In contrast, the hydrogenated samples exhibited substantially lower T_c values compared tomore » the pristine ones, with a T_c of 4.6 K at 50.5 GPa and an extrapolated upper critical field of 1.8 T at T = 0 K. Upon decompression, the superconductivity in the hydrogenated sample persisted down to 25 GPa, disappearing when the pressure was reduced to 19 GPa. Synchrotron X-ray diffractions (XRD) analysis revealed the formation of three hydrides: I4/mmm-(Ti, Zr)H₂, P6₃/mmc-(Ti, Zr)H₃, and I4̅3d-(Ti,Zr)₄H₁₅. Finally, by combining XRD and electrical resistance data, we tentatively propose that the P6₃/mmc-(Ti, Zr)H₃ predominantly induces superconductivity. In contrast to superhydrides, where electron–phonon interactions increase the superconducting temperature at high pressure, hydrogen atoms in hcp-(Ti, Zr)H₃ reduce T_c.« less

Metal halide octahedra form the fundamental functional building blocks of metal halide perovskites, dictating their structures, optical properties, electronic structures, and dynamics. Here, in this study, we show that the connectivity of bismuth halide octahedra in Cs₃Bi₂Br₉ and Cs₃Bi₂I₉ quantum dots (QDs) changes with different halide elements. We use first-principles calculations to reveal the key role of the connectivity of bismuth halide octahedra on the wave function symmetry, Huang-Rhys factor, and exciton-phonon interaction strength. Following QD synthesis via a ligand-mediated transport method, the effect of connectivity is verified with transient absorption spectroscopy, where we contrast Cs₃Bi₂Br₉ and Cs₃Bi₂I₉ QD excitonmore » dynamics. In photoexcited Cs₃Bi₂I₉ QDs, phonons related to the vibrational motions of face-sharing [BiI₆]^3- bioctahedra couple strongly to the electronic state and drive rapid carrier relaxation. Equivalent signals are not observed for photoexcited Cs₃Bi₂Br₉ QDs, implying a lack of phonon involvement in band-edge absorption and subsequent exciton relaxation. Our findings suggest that structural engineering can effectively tune the exciton-phonon coupling and therefore influence exciton relaxation and recombination in perovskite nanomaterials.« 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

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

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

The ability to control materials stability, bonding, and transformation by thermo-mechanical and chemical means is significant for development of high-energy-density extended solids. We report that doping hydrogen (~10%) in carbon monoxide (CO) can greatly lower the polymerization pressure of CO and enhance the stability of recovered polymeric CO products at ambient conditions. Hydrogen-doped CO crystallizes into well-grown dendrites of β-CO-like phase at 3.2 GPa, which polymerizes to highly unsaturated black polymer (phase I) at ~4.7 (5.8) GPa. Upon further compression, this highly colored polymer transforms into a translucent 3D network structure (phase II) at 6–7 (10–17) GPa and then amore » transparent 2D layer structure (phase III) at 20–30 (30–60) GPa. A similar series of transformations are also found in pure CO but at considerably higher transition pressures, as noted in parentheses. All polymeric phases are recoverable at ambient conditions, exhibiting an array of phase stability and novel properties such as chemically unstable phase I, highly luminescent phase II, and highly transparent layered phase III. The density of recovered products ranges from ~2.3 g/cm³ to 3.6 g/cm³, depending on the pressure recovered. The recovered products are highly disordered but slowly decompose to crystalline solids of anhydrous polymeric oxalic acid while exhibiting interesting crystal morphologies such as nm-cobs, nm-lamellar layers, and μm-bales. Here, the present first-principles MD simulations suggest that the polymerization occurs at 6 (or 10) GPa in H₂-doped (or pure) CO. While not directly participating in the reaction, the role of H₂ molecules is to enhance the mobility of CO molecules leading to the polymerization.« less

Carbon monoxide (CO) is the first molecular system found to transform into a nonmolecular “polymeric” solid above 5.5 GPa, yet been studied beyond 10 GPa. Here, we show a series of pressure-induced phase transformations in CO to 160 GPa: from a molecular solid to a highly colored, low-density polymeric phase I to translucent, high-density phase II to transparent, layered phase III. The properties of these phases are consistent with those expected from recently predicted 1D P2₁/m, 3D I2₁2₁2₁, and 2D Cmcm structures, respectively. Furthermore, the present results advocate a stepwise polymerization of CO triple bonds to ultimately a 2D singlymore » bonded layer structure with an enhanced ionic character.« less