Konstantin Lokshin

Fast synthesis method and phase diagram of hydrogen clathrate hydrate

We report a very fast method for hydrogen clathrate production. The formation of hydrogen clathrate from H2 gas and ice-Ih at 77–273K occurs at least 100 times faster compared to the reaction with water. This effect is probably the result of a partial penetration of hydrogen into the ice-Ih body, which favors clathrate formation. With the ice method, the ice-Ih∕48H2*136H2O system can quickly trap and release up to 3.77 mass % of hydrogen, which gives it an advantage over other chemical-bonding hydrogen storage technologies. New data on the hydrogen-water phase diagram are presented and discussed.

Variable pressure-temperature neutron diffraction of wüstite (Fe1−xO): Absence of long-range magnetic order to 20GPa

Neutron diffraction measurements have been performed on polycrystalline Fe1−xO (wustite) up to 20.3GPa using a large-volume moissanite anvil cell at room temperature to examine the existence of long-range magnetic ordering in the high-pressure rhombohedral phase of the material. This investigation is crucial for understanding the nature of high-pressure phase transitions in Fe1−xO. Low temperature ambient pressure neutron diffraction measurements on the same sample were also conducted at 190, 180, and 85K to compare with the high-pressure results. Magnetic peaks are expected to be observed under pressure similar to those at 180K at ambient pressure. However, no magnetic peaks were evident in the high-pressure diffraction patterns (to 20.3GPa at 300K). The absence of magnetic peaks indicates the absence of long-range magnetic order in Fe1−xO under these conditions. This result indicates the need to reconsider the interpretation of high-pressure Mossbauer studies and the mechanism of the high-pressure phase...

Thermoelastic and texture behavior of aluminum at high pressure and high temperature investigated by in situ neutron diffraction

The behavior of aluminum under high pressures and temperatures was investigated by in situ time-of-flight neutron diffraction with a developed Toroidal Anvil Press (TAP-98). The effect of the displacement of the center of diffraction, which is caused by sample movement during compression, is corrected by an additional calibration. Unit-cell dimensions, measured up to P=5.7 GPa and T=900 K, were derived from the refinement results and fitted to the high-temperature Birch–Murnaghan equation of state. With (∂KT/∂P)T fixed at 4, we obtained K0=72.8(±2.4) GPa, (∂KT/∂T)P=−0.04(±0.01) GPa K−1, and αT (K−1)=3.7(±1.6)×10−5+9.7(±3.5)×10−8T. Our data are compared with previous experimental data involving shock wave, static compression, ultrasonic, and thermal-expansion measurements and with theoretical predictions. The results demonstrate that the newly developed high-pressure high-temperature system for in situ neutron diffraction is reliable. It was also found that the crystalline orientation of Al grains became h...

High-pressure/low-temperature neutron scattering of gas inclusion compounds: Progress and prospects

Alternative energy resources such as hydrogen and methane gases are becoming increasingly important for the future economy. A major challenge for using hydrogen is to develop suitable materials to store it under a variety of conditions, which requires systematic studies of the structures, stability, and kinetics of various hydrogen-storing compounds. Neutron scattering is particularly useful for these studies. We have developed high-pressure/low-temperature gas/fluid cells in conjunction with neutron diffraction and inelastic neutron scattering instruments allowing in situ and real-time examination of gas uptake/release processes. We studied the formation of methane and hydrogen clathrates, a group of inclusion compounds consisting of frameworks of hydrogen-bonded H2O molecules with gas molecules trapped inside the cages. Our results reveal that clathrate can store up to four hydrogen molecules in each of its large cages with an intermolecular H2–H2 distance of only 2.93 Å. This distance is much shorter than that in the solid/metallic hydrogen (3.78 Å), suggesting a strong densification effect of the clathrate framework on the enclosed hydrogen molecules. The framework-pressurizing effect is striking and may exist in other inclusion compounds such as metal-organic frameworks (MOFs). Owing to the enormous variety and flexibility of their frameworks, inclusion compounds may offer superior properties for storage of hydrogen and/or hydrogen-rich molecules, relative to other types of compounds. We have investigated the hydrogen storage properties of two MOFs, Cu3[Co(CN)6]2 and Cu3(BTC)2 (BTC = benzenetricarboxylate), and our preliminary results demonstrate that the developed neutron-scattering techniques are equally well suited for studying MOFs and other inclusion compounds.

Advanced setup for high-pressure and low-temperature neutron diffraction at hydrostatic conditions

We describe a design of the experimental setup for neutron diffraction studies at low temperatures and hydrostatic pressure. The significant benefit of the setup, compared to the previous methods, is that it makes possible the simultaneous collection of neutrons diffracted at the 30°–150° range with no contamination by the primary scattering from the sample surroundings and without cutting out the incident and diffracted beams. The suggested design is most useful for third-generation time-of-flight diffractometers and constant wavelength instruments. Application of the setup expands the capabilities of high-pressure neutron diffraction, allowing time-resolved kinetics and structural studies, multihistogram Rietveld, and pair distribution function and texture analyses. The high efficiency of the setup was proven for the HIPPO diffractometer at Los Alamos Neutron Science Center under pressures up to 10kbar and temperatures from 4to300K.

Chapter 23 – Development of high P–T neutron diffraction at LANSCE – toroidal anvil press, TAP-98, in the HiPPO diffractometer

The development of neutron diffraction under extreme pressure (P) and temperature (T) conditions is highly valuable to condensed matter physics, crystal chemistry, materials sciences, as well as the Earth and planetary sciences. We have incorporated a 500-ton press, TAP-98 (Toroidal Anvil Press, designed in 1998) into the HiPPO diffractometer at LANSCE to conduct in situ high P–T neutron diffraction experiments. The technical issues related to press design, P–T control, translation stage, diffraction optics, anvil package, and cell assembly are discussed in this chapter. Pressure and temperature calibrations and preliminary experimental data are also presented in this chapter.

Bulk metallic glass gasket for high pressure, in situ x-ray diffraction

Amorphous metallic alloys lack long-range atomic order and consequently exhibit excellent homogeneity, no microstructure discontinuities, and no sharp x-ray diffraction peaks. Moreover they have higher tensile fracture strength and hardness than those of traditional crystalline metals. These excellent physical properties make bulk metallic glasses good candidates for high pressure gaskets for in situ x-ray/neutron diffraction experiments. We tested the Pd40Ni40P20 amorphous alloy as a gasket material in three experiments. The behavior of the Pd40Ni40P20 amorphous alloy under a hydrostatic/nonhydrostatic pressure of over 100 GPa was investigated by in situ synchrotron x-ray diffraction. The pressure gradient of the amorphous metallic gasket in the diamond anvil cell was measured on the ruby fluorescence scale during compression and decompression. The results show that bulk metallic glasses are good high-pressure gasket materials for in situ x-ray diffraction experiments.

Synthesis, cation ordering, and magnetic properties of the (Sb_{1-x}Pb_{x})_{2}(Mn_{1-y}Sb_{y})O_{4} solid solutions with the Sb_{2}MnO_{4} -type structure

Single phase (Sb1-xPbx)2(Mn1-ySby)O4 (0.0 ≤ x ≤ 0.608, 0.0 ≤ y ≤ 0.372) samples with the Sb2MnO4−type structure were prepared at 650 °C by solid-state reaction in evacuated sealed silica tubes. A replacement of Sb by Pb results in the oxidation of Sb3+ to Sb5+, which in turn replaces Mn2+ cations in octahedrally coordinated positions within the infinite rutile-type chains. The crystal structures of Pb0.44Sb1.64Mn0.92O4, Pb0.75Sb1.48Mn0.77O4, Pb1.07Sb1.26Mn0.67O4, and Pb1.186Sb1.175Mn0.639O4 were refined from X-ray powder diffraction data. Increasing the Pb content leads to a decrease of the a parameter from a = 8.719(2) A to a = 8.6131(8) A and to an increase of the c parameter from c = 5.999(2) A to c = 6.2485(7) A (for Sb2MnO4 and Pb1.216Sb1.155Mn0.628O4, respectively). This occurs due to increasing average cation size at the Pb/Sb position and decreasing cation size at the Mn/Sb position that leads to strong deformation of the (Mn/Sb)O6 octahedra. Starting from the Pb0.75Sb1.48Mn0.77O4 composition a mo...