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Pressure-Induced Amorphization and Crystallization of Heterophase Pd Nanostructures
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High-pressure phase transformation of carbonate malachite Cu2(CO3)(OH)2 driven by [CuO6] regularization and [CO3] rotation
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Probing matter at Gbar pressures at the NIF
Here, we describe a platform to measure the material properties, specifically the equation of state and electron temperature, at pressures of 100 Mbar to a Gbar at the National Ignition Facility (NIF). In these experi- ments we launch spherically convergent shock waves into solid CH, CD, or diamond samples using a hohlraum radiation drive, in an indirect drive laser geometry. X-ray radiography is applied to measure the shock speed and infer the mass density profile, enabling determination of the material pressure and Hugoniot equation of state. X-ray scattering is applied to measure the electron temperature through probing of the electron velocity distribution via Doppler broadening.
High-pressure high-temperature stability of hcp-Ir Os1− (x = 0.50 and 0.55) alloys
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Shear induced weakening of the hydrogen bonding lattice of the energetic material 5,5'-Hydrazinebistetrazole at high-pressure
5,5'-Hydrazinebistetrazole (HBTA) has been studied by in-situ x-ray diffraction and vibrational spectroscopy to pressures near 25 GPa at room temperature. Analysis of the x-ray diffraction pattern of HBTA collected at ambient pressure and temperature revealed a monoclinic structure consistent with that previously reported. Under compression, the x-ray diffraction reveals little evidence of a phase transition over the pressure range studied. Slight anisotropy in response to compression was noted and the β angle decreased moderately, suggesting geometry modifications occur in the hydrogen bonding lattice and between neighboring HBTA molecules as a result of compression along the c axis. Blue shifts in the Infrared active N-H stretching modes were observed, implying a weakening of the hydrogen bond with compression. The weakening of the hydrogen bonding lattice with pressure may lead to an increase in the bending angle of the C-N=N-C bridge between the tetrazole rings and an increased overlap between the π-bonding orbitals. The Raman spectra showed a number of modes associated with H-N=N-H motions of the bridge become more prominent in the spectra under compression. Additionally, the possibility that the increased bend in the angle of the C-N=N-C bridge results from a shearing deformation is discussed.
Weak interactions between water and clathrate-forming gases at low pressures
Using scanning probe microscopy and temperature programed desorption we examined the interaction between water and two common clathrate-forming gases, methane and isobutane, at low temperature and low pressure. Water co-deposited with up to 10-1 mbar methane or 10-5 mbar isobutane at 140 K onto a Pt(111) substrate yielded pure crystalline ice, i.e., the exposure to up to ~107 gas molecules for each deposited water molecule did not have any detectable effect on the growing films. Exposing metastable, less than 2 molecular layers thick, water films to 10-5 mbar methane does not alter their morphology, suggesting that the presence of the Pt(111) surface is not a strong driver for hydrate formation. This weak water-gas interaction at low pressures is supported by our thermal desorption measurements from amorphous solid water and crystalline ice where 1 ML of methane desorbs near ~43 K and isobutane desorbs near ~100 K. Similar desorption temperatures were observed for desorption from amorphous solid water.
Composition, response to pressure, and negative thermal expansion in M II B IV F 6 (M = Ca, Mg; B = Zr, Nb) [Composition, response to pressure, and negative thermal expansion in A II B IV F 6 ; A - Ca, Mg, B - Zr, Nb]
CaZrF 6 has recently been shown to combine strong negative thermal expansion (NTE) over a very wide temperature range (at least 10–1000 K) with optical transparency from mid-IR into the UV range. Variable-temperature and high-pressure diffraction has been used to determine how the replacement of calcium by magnesium and zirconium by niobium(IV) modifies the phase behavior and physical properties of the compound. Similar to CaZrF 6 , CaNbF 6 retains a cubic ReO 3 -type structure down to 10 K and displays NTE up until at least 900 K. It undergoes a reconstructive phase transition upon compression to ~400 MPa at room temperature and pressure-induced amorphization above ~4 GPa. Prior to the first transition, it displays very strong pressure-induced softening. MgZrF 6 adopts a cubic ( Fm$\bar{3}$m ) structure at 300 K and undergoes a symmetry-lowering phase transition involving octahedral tilts at ~100 K. Immediately above this transition, it shows modest NTE. Its’ thermal expansion increases upon heating, crossing through zero at ~500 K. Unlike CaZrF 6 and CaNbF 6 , it undergoes an octahedral tilting transition upon compression (~370 MPa) prior to a reconstructive transition at ~1 GPa. Cubic MgZrF 6 displays both pressure-induced softening and stiffening upon heating. MgNbF 6 is cubic ( Fm$\bar{3}$m ) at room temperature, but it undergoes a symmetry-lowering octahedral tilting transition at ~280 K. It does not display NTE within the investigated temperature range (100–950 K). Furthermore the replacement of Zr(IV) by Nb(IV) leads to minor changes in phase behavior and properties, the replacement of the calcium by the smaller and more polarizing magnesium leads to large changes in both phase behavior and thermal expansion.
Pressure-Induced Spectroscopic Changes in a Californium 1D Material Are Twice as Large as Found in the Holmium Analog
Here, in this study, the synthesis, characterization, and pressure response of a 1D californium mellitate (mellitate = 1,2,3,4,5,6-benzenehexacarboxylate) coordination polymer, Cf 2 (mell)(H 2 O) 10 ·4H 2 O (Cf-1), are reported. The Cf–O lengths within the crystal structure are compared to its gadolinium (Gd-1) and holmium (Ho-1) analogs as well. These data show that the average Cf–O bond distance is slightly longer than the average Gd–O bond, consistent with trends in effective ionic radii. UV–vis-NIR absorption spectra as a function of pressure were collected using diamond-anvil techniques for both Cf-1 and Ho-1. These experiments show that the Cf(III) f → f transitions have a stronger dependence on pressure than that of the holmium analog. In the former case, the shift is nearly linear with applied pressure and averages 6.6 cm –1 /GPa, whereas in the latter, it is <3 cm –1 /GPa.
Pronounced Pressure Dependence of Electronic Transitions for Americium Compared to Isomorphous Neodymium and Samarium Mellitates
The mellitate ion is relevant in spent nuclear fuel processing and is utilized as a surrogate for studying the interactions of f elements with humic acids. A wealth of different coordination modes gives the potential for diverse structural chemistry across the actinide series. In this study, an americium mellitate, 243 Am 2 [(C 6 (COO – ) 6 ](H 2 O) 8 ·2H 2 O (1-Am), has been synthesized and characterized using structural analysis and spectroscopy at ambient and elevated pressures. 1-Am was then compared to isomorphous neodymium (1-Nd) and samarium (1-Sm) mellitates via bond-length analysis and pressure dependence of their Laporte-forbidden f → f transitions. Results show that the pressure dependence of the f → f transitions of 1-Am is significantly greater than that observed in 1-Nd and 1-Sm, with average shifts of 21.4, 4.7, and 3.6 cm –1 /GPa, respectively. Finally, this greater shift found in 1-Am shows further evidence that the 5f orbitals are more affected than the 4f orbitals when pressure is applied to isostructural compounds.
High-Pressure Studies of Cesium Uranyl Chloride
Here, the pressure behavior of crystalline Cs 2 UO 2 Cl 4 has been explored using a diamond anvil cell. The uranyl fluorescence intensity decreases dramatically with increasing pressure. Using the O–U–O symmetric stretching frequency, an apparent linear decrease in bond length with increasing pressure was observed. A linear decrease in fluorescence intensity with increasing pressure was attributed to a large growth in the nonradiative relaxation, likely attributed to increased relaxation through phonon modes. Quantum theory of atoms in molecules calculations and ab initio wave function methods (CASSCF) support the U≡O bond in UO 2 2+ being highly sensitive to the bond distance, but negligibly affected by the U–Cl bond length.
Fe0.36(4)Pd 0.64(4)Se 2 : Magnetic Spin-Glass Polymorph of FeSe2 and PdSe2 Stable at Ambient Pressure
We report the synthesis and characterization of Fe 0.36(4) Pd 0.64(4) Se 2 with a pyrite-type structure. Fe 0.36(4) Pd 0.64(4) Se 2 was synthesized using ambient pressure flux crystal growth methods even though the space group Pa3 is high-pressure polymorph for both FeSe 2 and PdSe 2 . Combined experimental and theoretical analysis reveal magnetic spin glass state below 23 K in 1000 Oe that stems from random Fe/Pd occupancies on the same atomic site. The frozen-in magnetic randomness contributes significantly to electronic transport. Electronic structure calculations confirm dominant d-electron character of hybridized bands and large density of states near the Fermi level. Flux-grown single crystal alloys in Pd-Fe-Se atomic system therefore open new pathway for exploring different polymorphs in crystal structures and their novel properties. © 2019 American Chemical Society.
Operando MAS NMR Reaction Studies at High Temperatures and Pressures
Operando MAS-NMR studies provide unique insights into the details of chemical reactions; comprehensive information about temperature- and time-dependent changes in chemical species is accompanied by similarly rich information about changes in phase and chemical environment. Here we describe a new MAS-NMR rotor (the WHiMS rotor) capable of achieving internal pressures up to 400 bar at 20 °C or 225 bar at 250 °C, a range which includes many reactions of interest. The MAS-NMR spectroscopy enabled by these rotors is ideal for studying the behavior of mixed-phase systems, such as reactions involving solid catalysts and volatile liquids, with the potential to add gases at high pressure. The versatile operation of the new rotors is demonstrated by collecting operando 1H and 13C spectra during the hydrogenolysis of benzyl phenyl ether, catalyzed by Ni/-Al2O3 at ca. 250 ºC, both with and without H2 (g) supplied to the rotor. The 2-propanol solvent, which exists in the supercritical phase under these reaction conditions, serves as an internal source of H2. The NMR spectra provide detailed kinetic profiles for the formation of the primary products toluene and phenol, as well as secondary hydrogenation and etherification products.
Negative Pressures and Spallation in Water Drops Subjected to Nanosecond Shock Waves
Abstract Not Provided
A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo2As2 (A = Eu and Ca)
We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo(2)As(2) (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 eleetron) change in the population of the 3d orbitals. The mixed valence state of En observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.
Pressure-induced metallization of the halide perovskite (CH 3 NH 3 )PbI 3
We report the metallization of the hybrid perovskite semiconductor (MA)PbI 3 (MA = CH 3 NH 3 + ) with no apparent structural transition. We tracked its bandgap evolution during compression in diamond-anvil cells using absorption spectroscopy and observed strong absorption over both visible and IR wavelengths at pressures above ca. 56 GPa, suggesting the imminent closure of its optical bandgap. The metallic character of (MA)PbI 3 above 60 GPa was confirmed using both IR reflectivity and variable-temperature dc conductivity measurements. The impressive semiconductor properties of halide perovskites have recently been exploited in a multitude of optoelectronic applications. Meanwhile, the study of metallic properties in oxide perovskites has revealed diverse electronic phenomena. Importantly, the mild synthetic routes to halide perovskites and the templating effects of the organic cations allow for fine structural control of the inorganic lattice. Lastly, pressure-induced closure of the 1.6 eV bandgap in (MA)PbI3 demonstrates the promise of the continued study of halide perovskites under a range of thermodynamic conditions, toward realizing wholly new electronic properties.
Irreversible xenon insertion into a small-pore zeolite at moderate pressures and temperatures
Abstract not provided