ABSTRACT
A polymer electrolyte fuel cell has been designed to allowoperandox-ray absorption spectroscopy (XAS) measurements of catalysts. The cell has been developed to operate under standard fuel cell conditions, with elevated temperatures and humidification of the gas-phase reactants, both of which greatly impact the catalyst utilisation. X-ray windows in the endplates of the cell facilitate collection of XAS spectra during fuel cell operation while maintaining good compression in the area of measurement. Results of polarisation curves and cyclic voltammograms showed that theoperandocell performs well as a fuel cell, while also providing XAS data of suitable quality for robust XANES analysis. The cell has produced comparable XAS results when performing a cyclic voltammogram to an establishedin situcell when measuring the Pt LIII edge. Similar trends of Pt oxidation, and reduction of the formed Pt oxide, have been presented with a time resolution of 5 s for each spectrum, paving the way for time-resolved spectral measurements of fuel cell catalysts in a fully-operating fuel cell.
ABSTRACT
Two structural transitions in covalent aluminum hydride AlH3 were characterized at high pressure. A metallic phase stable above 100 GPa is found to have a remarkably simple cubic structure with shortest first-neighbor H-H distances ever measured except in H2 molecule. Although the high-pressure phase is predicted to be superconductive, this was not observed experimentally down to 4 K over the pressure range 120-164 GPa. The results indicate that the superconducting behavior may be more complex than anticipated.
ABSTRACT
At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.
Subject(s)
Sodium/chemistry , X-Ray Diffraction/methods , Models, MolecularABSTRACT
The compressibilities of the nitridosilicate SrYb[Si(4)N(7)] and the oxonitridoaluminosilicates MYb[Si(4-x)Al(x)O(x)N(7-x)] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42 GPa were generated using the diamond-anvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order Birch-Murnaghan equation-of-state to the p-V data results in V(0) = 302.91 (6) A(3), B(0) = 176 (2) GPa and B' = 4.4 (2) for SrYb[Si(4)N(7)]; V(0) = 310.4 (1) A(3), B(0) = 161 (2) GPa and B' = 4.6 (2) for SrYb[Si(4-x)Al(x)O(x)N(7-x)]; and V(0) = 317.3 (5) A(3), B(0) = 168 (2) GPa and B' = 4.7 (2) for BaYb[Si(4-x)Al(x)O(x)N(7-x)]. While the linear compressibilities of the a and c axes of BaYb[Si(4-x)Al(x)O(x)N(7-x)] are very similar up to 30 GPa, distinct differences were observed for SrYb[Si(4)N(7)] and SrYb[Si(4-x)Al(x)O(x)N(7-x)], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role.
ABSTRACT
Comprehensive x-ray powder diffraction studies were carried out in magnetite in the 80-150 K and 0-12 GPa ranges with a membrane-driven diamond anvil cell and helium as a pressure medium. Careful data analyses have shown that a reversible, cubic to a distorted-cubic, structural transition takes place with increasing pressure, within the (P,T) regime below the Verwey temperature TV(P). The experimental documentation that TV(P)=Tdist(P) implies that the pressure-temperature-driven metal-insulator Verwey transition is caused by a gap opening in the electronic band structure due to the crystal-structural transformation to a lower-symmetry phase. The distorted-cubic insulating phase comprises a relatively small pressure-temperature range of the stability field of the cubic metallic phase that extends to 25 GPa.
