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We report a study of high pressure x-ray absorption (XAS) performed in the partial fluorescence yield mode (PFY) at the U L3 edge (0–28.2 GPa) and single crystal x-ray diffraction (SXD) (0–20 GPa) on the UCd11 heavy fermion compound at room temperature. Under compression, the PFY-XAS results show that the white line is shifted by +4.1(3) eV at the highest applied pressure of 28.2 GPa indicating delocalization of the 5f electrons. The increase in full width at half maxima and decrease in relative amplitude of the white line with respect to the edge jump point towards 6d band broadening under high pressure. A bulk modulus of K0 = 62(1) GPa and its pressure derivative, K0 = 4.9(2) was determined from high pressure SXD results. Both the PFY-XAS and diffraction results do not show any sign of a structural phase transition in the applied pressure range.

Large-volume cubic presses are widely used in scientific research and industrial applications. However, their pressure capability is often limited to 6 GPa, which severely restricts their applications under extended pressures. In this work, we report a newly designed hybrid cell assembly for cubic presses by embedding six WC pressure-enhancing blocks into the pyrophyllite pressure-transmitting medium, leading to profoundly increased pressure efficiency (i.e. more than 40%), hence largely extended pressure conditions up to 8–9 GPa without sacrificing sample volume. Because of the optimized design, the expensive first-stage WC anvils are effectively protected, which makes the press routinely operated up to 8–9 GPa in avoiding damage of anvils. Through optimization of heating efficiency, temperature of the new assembly can reach above 2500°C. Using this high pressure cell, we have synthesized centimetre-sized polycrystalline cubic BN bulk sample by the direct transformation of hexagonal BN at 7.7 GPa and 2000°C. Successful implementation of large-volume cubic press up to 8–9 GPa and 2500°C would provide many opportunities for the synthesis of new materials on massive scale (e.g. novel superhard compounds) and for the study of materials at extended pressures. [ABSTRACT FROM AUTHOR]

We report experimental determination of compressibility and thermoelasticity for CrN based on in situ high pressure/high temperature synchrotron x-ray diffraction measurements. The pressure-induced cubic-to-orthorhombic phase transition is observed at ∼ 5 GPa in high-quality CrN samples synthesized using a high pressure reaction route. Equation of state, bulk modulus and axial compressibility of this material are obtained by analysis of high pressure x-ray diffraction data collected at room temperature. Both cubic and orthorhombic phases have a similar bulk modulus value of 257 (5) and 262 (6) GPa, respectively. For orthorhombic CrN, the b-axis has the most axial incompressibility, which should be associated with magnetic properties. In addition, the isothermal equation of states is also determined at different temperatures of 300, 400, 600, 800, 1000, and 1200 K, respectively. Some of the important thermoelastic properties of cubic CrN are thus derived, including temperature derivative of bulk modulus ∂ K T / ∂ T = − 2.95 (5) × 10 − 2 G P a ⋅ K − 1 and volumetric thermal expansivity a T ≈ a 0 + a 1 ⋅ T where a0 = 1.95 (16)×10−5 K−1 and a1 = 4.39 (3)×10−9 K−1. [ABSTRACT FROM AUTHOR]

Materials with an abrupt volume collapse of more than 20 % during a pressure-induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin-state of the transition metal. A combined in situ crystallography and electron spin-state study to probe the mechanism of the pressure-driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt-to-MnP phase transition under compression with ca. 22 % unit-cell volume changes, which was found to be coupled with the Mn2+(d5) spin-state transition from S=5/2 to S=1/2 and the formation of Mn−Mn intermetallic bonds as supported by the metallic transport behavior of their high-pressure phases. Our results reveal the mutual relationship between pressure-driven lattice collapse and the orbital/spin-state of Mn2+ in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities. [ABSTRACT FROM AUTHOR]

Structural behavior of bulk 2 under high pressure was investigated using synchrotron X-ray diffraction and diamond anvil cell up to 52 GPa along with high temperature X-ray diffraction and high pressure Raman spectroscopy analysis. The high pressure results obtained from X-ray diffraction and Raman analysis did not show any pressure induced structural phase transformations up to 52 GPa. The high temperature results show that the 2 crystal structure is stable upon heating up to 600°C. Furthermore, the powder X-ray diffraction obtained on shock subjected 2 to high pressures up to 10 GPa also did not reveal any structural changes. Our results suggest that even though 2 is less compressible than the isostructural 2, its crystal structure is stable under static and dynamic compressions up to the experimental limit. [ABSTRACT FROM AUTHOR]

We studied polycrystalline tetragonal LaFeAsO0.9F0.1 up to 9 GPa using time-of-flight neutron and synchrotron X-ray diffraction. Consistent with previous studies, the layer-structured LaFeAsO0.9F0.1 has a bulk modulus of 74–79 GPa, with the c−axis being twice as compressible as the a−axis. The refined structural parameters under pressure show non-monotonic variation with kink points at 4 GPa, well correlating with a maximum superconducting temperature (Tc) of 43 K previously reported for LaFeAsO0.9F0.1. At this pressure, however, both the As–Fe–As bond angle and anion height from the Fe layers deviate substantially from the respective ideal values of 109.47° and 1.38 Å for a regular FeAs4 tetrahedron. These findings indicate that the correlation between the maximum Tc and the geometry of conductive Fe–As layers is material dependent and may also be sensitive to atomic doping in the parent Fe-based superconductors. [ABSTRACT FROM PUBLISHER]

Compressional and shear wave velocities of orthorhombic BaCe0.85 Y0.15 O2.925 perovskite were measured at high pressures up to 8.1 GPa by ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction. At room temperature, the adiabatic bulk modulus increases with increasing pressure, as expected for most polycrystalline materials. The shear modulus, however, show a negative dependence on pressure over a pressure range of ∼2-8.1 GPa. This anomalous elastic softening indicates destabilization of the orthorhombic structure in BaCe0.85Y0.15O2.925 perovskite over the experimental pressure range and is presumably the precursor to the first-order phase transition previously observed at 22 GPa using Raman spectroscopic study. [ABSTRACT FROM AUTHOR]

Abstract: We present a comparative study of the mechanical properties of body-centered cubic nanocrystalline iron (nano-Fe) and microcrystalline iron (micro-Fe) by in situ high-pressure synchrotron X-ray diffraction under triaxial compression. For nano-Fe with a starting high dislocation density of 1016 m−2, the peak broadening is almost reversible upon unloading from 8.6GPa to atmospheric pressure, indicating that no additional dislocations are built up during compressive deformation inside grains, at grain boundaries or twin boundaries. Furthermore, an orientation-dependent surface strain is found to be stored in the surface layer of the bcc nano-Fe, which is in agreement with the core–shell model of the nanocrystals. For micro-Fe, a significant and continuous peak sharpening and the associated work softening were observed after the sample is yielded at pressures above 2.0GPa, which can be presumably attributed to a pressure-induced dislocation annihilation. This finding/interpretation supports the hypothesis that the annihilation of dislocations is one of the dominant mechanisms underlying the plastic energy dissipation. The determined yield strength of 2.0GPa for nano-Fe is more than 15 times higher than that for micro-Fe (0.13GPa), indicating that the nanoscale grain-size reduction is a substantially more effective strengthening mechanism than conventional carbon infusion in iron. [Copyright &y& Elsevier]

Compressibility of periodically twinned silicon carbide nanowires is studied using in situ high pressure X-ray diffraction. Twinned SiC nanowires displayed a bulk modulus of 316GPa, ∼20–40% higher than previously reported values for SiC of other morphologies. This finding provides direct evidence of a significant effect of twinned structures on the elastic properties of SiC on the nano scale and supports previous molecular dynamics simulations of twin boundary/stacking fault-induced strengthening. Both experiments and simulations indicate that nanoscale twinning is an effective pathway by which to tailor the mechanical properties of nanostructures. [Copyright &y& Elsevier]

Abstract: In situ synchrotron X-ray diffraction experiments of ammonia borane synthesized using sonication assisted direct reaction (sodium borohydride and ammonium formate) were performed under high pressures up to 23 GPa at ambient temperature and up to 4 GPa at elevated temperatures (300 K–505 K). At ambient temperature, one first order phase transition (I4mm to Cmc21) was observed at 1.3 GPa and one second order phase transition was observed at about 5 GPa. Fitting the measured volumetric compression data to the third order Birch–Murnaghan equation of state reveals a bulk modulus of K = 9.3 ± 0.4 GPa (K′ = 4.8) for the I4mm phase, K = 11.9 ± 0.5 GPa (K′ = 4.6) for the Cmc21 phase below 5 GPa, and K = 37 ± 4 GPa (K′ = 4.6) for pressure above 5 GPa. There is a 6% volume drop at the first order phase transition. Transition pressures from I4mm to Cmc21 phases were determined at elevated temperatures. The phase boundary has a negative Clapeyron slope of −1.67 MPa/K. The experiments also reveal a new structural transition when temperature increases to 450 K at 4 GPa. [Copyright &y& Elsevier]

Abstract: Time-resolved in situ X-ray diffraction at simultaneous high pressures (P) and high temperatures (T) was used to monitor kinetics of the reaction between diamond and silicon. Analysis of the data indicated that the reaction was diffusion controlled, and the diffusion was taking place through grain boundaries. For the nm size diamond the activation energy (170 kJ/mol) was smaller than that for μm size diamond (260 kJ/mol), and the reaction started at a temperature below the melting point of silicon. These effects are attributed to nanocrystalline structure and strained bonds within grain boundaries. [Copyright &y& Elsevier]

The crystal structure of A-15 superconductor Nb 3 Ga with a critical temperature T c =16.5 K has been investigated by high pressure x-ray diffraction (HPXRD) using synchrotron x-rays and a diamond anvil cell under Ne pressure medium. The high pressure structural results indicate that Nb 3 Ga is stable up to 41 GPa. The P – V plot shows an anomaly around 15 GPa even though there are no pressure induced structural transitions are observed. High pressure resistance measurements were performed up to 0.5 GPa to understand the variation of T c under pressure. Our results show a positive pressure effect on T c . [ABSTRACT FROM AUTHOR]