Nadine Rademacher

The Local Atomic Structures of Liquid CO at 3.6 GPa and Polymerized CO at 0 to 30 GPa from High‐Pressure Pair Distribution Function Analysis

The local atomic structures of liquid and polymerized CO and its decomposition products were analyzed at pressures up to 30 GPa in diamond anvil cells by X-ray diffraction, pair distribution function (PDF) analysis, single-crystal diffraction, and Raman spectroscopy. The structural models were obtained by density functional calculations. Analysis of the PDF of a liquid CO-rich phase revealed that the local structure has a pronounced short-range order. The PDFs of polymerized amorphous CO at several pressures revealed the compression of the molecular structure; covalent bond lengths did not change significantly with pressure. Experimental PDFs could be reproduced with simulations from DFT-optimized structural models. Likely structural features of polymerized CO are thus 4- to 6-membered rings (lactones, cyclic ethers, and rings decorated with carbonyl groups) and long bent chains with carbonyl groups and bridging atoms. Laser heating polymerized CO at pressures of 7 to 9 GPa and 20 GPa resulted in the formation of CO(2).

Experimental and predicted crystal structures of Pigment Red 168 and other dihalogenated anthanthrones

The crystal structures of 4,10-dibromo-anthanthrone (Pigment Red 168; 4,10-dibromo-dibenzo[def,mno]chrysene-6,12-dione), 4,10-dichloro- and 4,10-diiodo-anthanthrone have been determined by single-crystal X-ray analyses. The dibromo and diiodo derivatives crystallize in P2(1)/c, Z = 2, the dichloro derivative in P1, Z = 1. The molecular structures are almost identical and the unit-cell parameters show some similarities for all three compounds, but the crystal structures are neither isotypic to another nor to the unsubstituted anthanthrone, which crystallizes in P2(1)/c, Z = 8. In order to explain why the four anthanthrone derivatives have four different crystal structures, lattice-energy minimizations were performed using anisotropic atom-atom model potentials as well as using the semi-classical density sums (SCDS-Pixel) approach. The calculations showed the crystal structures of the dichloro and the diiodo derivatives to be the most stable ones for the corresponding compound; whereas for dibromo-anthanthrone the calculations suggest that the dichloro and diiodo structure types should be more stable than the experimentally observed structure. An experimental search for new polymorphs of dibromo-anthanthrone was carried out, but the experiments were hampered by the remarkable insolubility of the compound. A metastable nanocrystalline second polymorph of the dibromo derivative does exist, but it is not isostructural to the dichloro or diiodo compound. In order to determine the crystal structure of this phase, crystal structure predictions were performed in various space groups, using anisotropic atom-atom potentials. For all low-energy structures, X-ray powder patterns were calculated and compared with the experimental diagram, which consisted of a few broad lines only. It turned out that the crystallinity of this phase was not sufficient to determine which of the calculated structures corresponds to the actual structure of this nanocrystalline polymorph.

Viscosity and phase separations of binary CO–He and CO–Ar mixtures

Binary mixtures of 10 and 25 vol% CO in He and 10 vol% CO in Ar have been studied at high pressures and ambient temperature in diamond anvil cells. Phase separations were observed at 5.7(3) GPa, 3.6(2) GPa and 1.6(1) GPa. Earlier studies of –He mixtures of comparable concentrations revealed phase separations at significantly larger pressures, while –Ar mixtures separate at pressures comparable to those observed in the CO–Ar system here. The viscosity of a CO-rich fluid phase was determined by measuring the velocities of rising He bubbles. After corrections for the influence of the finite container size and of remaining helium in CO, the viscosity of the CO-rich fluid at 3.8(1) GPa was ≈3(1) mPa s, similar to what would be expected for isoelectronic liquid under the same conditions.

Decomposition of W(CO)[subscript 6] at high pressures and temperatures

The decomposition of hexacarbonyltungsten, W(CO){sub 6}, has been studied. The decomposition was induced by heating W(CO){sub 6} in an autoclave at 523 K and pressures up to 1.8 MPa, and by laser heating in a diamond anvil cell at pressures between 5 and 18 GPa. The products have been characterized using synchrotron X-ray diffraction, pair distribution function analysis, Raman spectroscopy and scanning electron microscopy. Decomposition in the autoclave at the lower pressures resulted in the formation of a metastable tungsten carbide, W{sub 2}C, with an average particle size of 1-2 nm, and an unidentified nanocrystalline tungsten oxide and nanocrystalline graphite with average particle sizes of 1-2 and 11 nm, respectively. The existence of nanocrystalline graphite was deduced from micro-Raman spectra and the graphite particle size was extracted from the intensities of the Raman modes. The high-pressure decomposition products obtained in the diamond anvil cell are the monoclinic tungsten oxide phase WO{sub 2} and the high-pressure phase W{sub 3}O{sub 8}(I). The approximate average size of the graphite particles formed here was 6-8 nm. The bulk modulus of W(CO){sub 6} is B{sub 0} {approx_equal} 13 GPa.

High-pressure X-ray single-crystal and powder diffraction of SF6up to 14 GPa

It is well known that at low temperatures the molecular crystal SF6 exists in an orientationally disordered body-centered cubic phase (from 90 – 230 K) and an ordered monoclinic phase (below 90 K). The high-pressure behaviour has so far been only investigated by Raman spectroscopy with pressures up to 10 GPa. Sasaki et al. propose a phase transtition at 0.25 GPa from liquid SF6 to the so-called solid I phase and a second phase transition at 1.8 GPa to the solid II phase. Moreover the assumption is made that the solid I phase crystallizes in the bcc and the solid II phase in the monoclinic structure. This study presents an in-situ crystal-structure determination of SF6 in diamond anvil cells (DAC) up to 14 GPa. In order to ensure quasi-hydrostatic conditions, a mixture of 20 vol% SF6 in helium was loaded into the DAC. During compression of the gas mixture, SF6 separated from the He at around 0.5 GPa. Crystal growth was observed at 2 GPa and after a pressure increase to 4 GPa SF6 single crystals were grown using an external heating set-up. In-situ Raman measurements show the typical SF6 modes and indicate a phase transition between 1.6 and 2.2 GPa consistent with earlier results. X-ray diffraction experiments have been performed at the Extreme Conditions Beamline P02.2 at PETRA III. Single-crystal and powder data were measured using 43 keV radiation at four different pressure points: 1.5 GPa, 1.9 GPa, 4 GPa and 14 GPa. At 1.5 and 1.9 GPa the structure was determined to be the proposed orientationally disordered body-centered cubic and at 4 GPa SF6 crystallizes in the monoclinic structure. After further compression to 14 GPa only powder rings were observed and the monoclinic phase is still stable.