46 Search Results

The high proclivity of x rays to destabilize and distort molecular structures has been previously utilized in the synthesis of novel compounds. Here, we show that x-ray induced decomposition of cadmium oxalate induces chemical and structural transformations only at 0.5 and 1 GPa. Using x-ray diffraction and Raman spectroscopy, the synthesized product is identified as cadmium carbonate with cadmium oxalate remnants, which is stable under ambient conditions. At ambient and >1 GPa pressures, only degradation of the electronic density distribution is observed. The transformation kinetics are examined in terms of Avrami’s model, which demonstrates that despite the necessity of highmore » pressure for efficient x-ray induced synthesis of cadmium carbonate, the rate and geometry of structural synthesis in the 0.5–1 GPa pressure range do not depend on the applied pressure. In addition, the possible role of intermolecular distance and molecular mobility in transformation yield is also discussed. Our experimental results indicate that x-ray induced photochemical synthetic pathways can be modulated and optimized by specific parameter selection such as high pressure.« less

Zirconium carbide (ZrC) is commonly used for energy sector research, as well as a surrogate for the proposed advanced nuclear fuel candidate uranium carbide. Here this study investigates structural modifications to nanocrystalline and microcrystalline ZrC resulting from dense electronic excitations induced by swift heavy ion exposure. Samples were irradiated with 946 MeV Au ions to various fluences up to 6 × 10¹³ ions cm^–2 and characterized using synchrotron-based x-ray diffraction. The evolution of the unit-cell parameter and heterogeneous microstrain were evaluated as a function of fluence and compared with those of nanocrystalline and microcrystalline CeO₂ (a surrogate for UO₂ fuel)more » irradiated under identical conditions. Distinct differences were observed in the radiation responses of the carbide and oxide across both grain sizes. Most notably, microcrystalline ZrC exhibits swelling characterized by two distinct regimes, which does not result in saturation at the ion fluences achieved. This contrasts with CeO₂, which exhibits the well-documented direct-impact defect accumulation mechanism, reaching a steady-state saturation of swelling at higher fluences. Nanocrystalline CeO₂ undergoes more pronounced swelling compared with microcrystalline CeO₂, in contrast to nanocrystalline ZrC, which exhibits only minimal unit-cell changes. These results demonstrate that swift heavy ion-induced structural changes can be quite different in carbides and oxides, which must be considered when extrapolating fission-fragment type damage in current fuels to advanced fuels.« less

We have analyzed SnO₂ with a combination of synchrotron X-ray diffraction and X-ray absorption spectroscopy across a pressure range of 0 → 82.9 GPa with thermal annealing by a CO₂ laser allowing access to all of the known high-density polymorphs of SnO₂, and here report their crystallographic information. The metastability of the post-rutile α -PbO₂ and PdF₂ structures in SnO₂ are investigated by experiment and PW-DFT simulations, revealing a complex energetic landscape and suggesting a significant dependence of the observed phases on the pressure–temperature pathway taken in experiment.

Pressure is a unique tuning parameter for probing the properties of materials, and it has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. In this work we report the effects of quasihydrostatic compression produced by a neon pressure medium on the structures of bismuth-based high-T_c cuprate superconductors with the nominal composition Bi₂Sr₂Ca_n-1Cu_nO_2n+4+δ (n=1, 2, 3) up to 155 GPa. The structures of all three compositions obtained by synchrotron x-ray diffraction can be described as pseudotetragonal over the entire pressure range studied. We show that previously reported pressure-induced distortions and structural changes arise from the largemore » strains that can be induced in these layered materials by nonhydrostatic stresses. The pressure-volume equations of state (EOS) measured under these quasihydrostatic conditions cannot be fit to single phenomenological formulation over the pressure ranges studied, starting below 20 GPa. This intrinsic anomalous compression as well as the sensitivity of Bi₂Sr₂Ca_n-1Cu_nO_2n+4+δ to deviatoric stresses provide explanations for the numerous inconsistencies in reported EOS parameters for these materials. We conclude that the anomalous compressional behavior of all three compositions is a manifestation of the changes in electronic properties that are also responsible for the remarkable nonmonotonic dependence of T_c with pressure, including the increase in T_c at the highest pressures studied so far for each. Transport and spectroscopic measurements up to megabar pressures are needed to fully characterize these cuprates and explore higher possible critical temperatures in these materials.« less

The critical roles of high pressure and the presence of water molecules in an initial solid-state compound (Cs ₂ C ₂ H ₂ O ₅ ) in the monochromatic X-ray induced synthesis of cesium superoxide (CsO ₂ ) are experimentally demonstrated.

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB₂ during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, T_c of 17 K at 91 GPa. Upon further compression up to 187 GPa, the T_c gradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB₂ (hP3, space group 191, prototype AlB₂). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB₂) continues to persist to this pressure,more » consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.« less

High-Pressure Collaborative Access Team (HPCAT) is a synchrotron-based facility located at the Advanced Photon Source (APS). With four online experimental stations and various offline capabilities, HPCAT is focused on providing synchrotron x-ray capabilities for high pressure and temperature research and supporting a broad user community. Overall, the array of online/offline capabilities is described, including some of the recent developments for remote user support and the concomitant impact of the current pandemic. General overview of work done at HPCAT and with a focus on some of the minerals relevant work and supporting capabilities is also discussed. With the impending APS-Upgrade (APS-U),more » there is a considerable effort within HPCAT to improve and add capabilities. These are summarized briefly for each of the end-stations.« less

High-pressure single-crystal x-ray diffraction is used to experimentally map the electron-density distribution changes in (Fe,Mg)O as ferrous iron undergoes a pressure-induced transition from high- to low-spin states. As the bulk density and elasticity of magnesiowustite—one of the dominant mineral phases of Earth’s mantle—are affected by this electronic transition, our results have applications to geophysics as well as to validating first-principles calculations. Furthermore, the observed changes in diffraction intensities indicate a spin-transition-induced change in orbital occupancies of the Fe ion in general accord with crystal-field theory, illustrating the use of electron density measurements for characterizing high-pressure d-block chemistry and motivating furthermore » studies characterizing chemical bonding under pressure.« less

Uranium ditelluride (UTe₂) has attracted recent interest due to its unique superconducting properties, which include the potential for a topological odd-parity superconducting state. Recently, ac-calorimetry measurements under pressure indicate a change in the ground state of UTe₂ from superconducting to antiferromagnetic at 1.4 GPa. In this work, we investigate the effect of pressure on the crystal structure of UTe₂ up to 25 GPa at room temperature using x-ray diffraction. We find that UTe₂, which at ambient conditions has an orthorhombic (Immm) structure, transforms to a body-centered tetragonal (I4/mmm) structure at 5 GPa in a quasihydrostatic neon (Ne) pressure-transmitting medium. Inmore » the absence of a pressure-transmitting medium, this transformation occurs between 5 and 8 GPa. The data were fit with a third-order Birch-Murnaghan equation of state resulting in values of B₀ = 46.0 ± 0.6 GPa , B' = 9.3 ± 0.5 (no pressure medium), and B₀ = 42.5 ± 2.0 GPa , B' = 9.3 (fixed) (neon pressure medium) for the Immm phase. For the I4/mmm phase, B₀ = 78.9 ± 0.5 GPa GPa and B' = 4.2 ± 0.1 (no pressure-transmitting medium), and B₀ = 70.0 ± 1.1 GPa and B' = 4.1 ± 0.2 (neon pressure medium). The high-pressure tetragonal phase is retained after decompression to ambient pressure, with approximately 30% remaining after 2 days. We argue that the observed phase transition into a higher-symmetry structure at P ~ 5 GPa (orthorhombic to tetragonal) is accompanied by an increase in the shortest distance between uranium atoms from 3.6 Å (orthorhombic) to 3.9 Å (tetragonal), which suggests localization of the 5ƒ electrons, albeit with a 10.7% decrease in volume.« less

Here we present an experimental and theoretical study of dysprosium metal compressed in the soft pressure transmitting medium Ne up to 182 and 300 GPa, respectively. Angle-dispersive x-ray powder diffraction data from each of the high-pressure polymorphs shows anisotropic compression behavior indicating changes to the electron density distribution throughout its polymorphic landscape. We compare the monoclinic (mC4) and orthorhombic (oF16) structures for the collapsed structure for Dy above 82 GPa and verify that the oF16 structure offers a better fit to our data than the previously reported mC4 structure. Further, we have found that the oF16 structure undergoes similar anisotropicmore » compression of its lattice parameters, with a turning point above 160 GPa; suggesting a potential phase transition at pressures much higher than achieved in this study. Density functional theory calculations show the likely candidate for this new high-pressure phase is the isosymmetric oF8 structure, which is predicted to be lower in energy than the oF16 structure above 275 GPa.« less