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- Lazor, Peter, et al.
(författare)
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Stability of iron oxides in Earth's mantle
- 2004
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Ingår i: Proceedings of the 32nd International Geological Congress: Abstracts.
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Konferensbidrag (refereegranskat)abstract
- Synchrotron x-ray diffraction, Raman spectroscopic experiments, and thermodynamic analysis aiming at the investigation of stability of iron oxides at pressures and temperatures reaching the deep lower mantle conditions were carried out using diamond anvil cell technique combined with laser and resistive heating. In the x-ray experiments, the thermodynamically predicted breakdown of magnetite to hematite and wüstite was not observed, but the trace amounts of hematite detected in the sample assemblage may signify the presence of nucleation centers of breakdown products, the growth of which is kinetically hindered due to the energetic requirements for the reaction. This is corroborated by the occurrence of bands of the locally ordered units of hematite in the Raman spectra. Raman spectra of wüstite compressed to 80 GPa and subjected to an intense laser heating show several new strong bands in the range 200-350 , originating presumably from the transformation to a polymorph with the NiAs structure. The transition is reversible as evidenced by the recovery of wüstite upon decompression of sample to a zero pressure. The 298 K isotherm of magnetite derived from the compression experiments is in a very good agreement with the results of earlier studies. The 1-confidence ellipsoid shows large negative correlations for the fit parameters K0, K0', and V0. The transformation of magnetite to a dense polymorph was observed already at 19 GPa. The Rietveld refinement of the diffraction pattern of is consistent with the -type structure. Thermodynamic assessment shows that on the pressure - temperature phase diagram the stability field for the mixture of oxides + FeO has maximum temperature of only 850 K at 14.6 GPa. At 298 K, the equilibrium pressure for the breakdown of magnetite is 13.3 GPa, while the pressure for the synthesis of FeO + to is located between 35 and 47 GPa, depending on the choice of the equation of state. The calculations also predict that the becomes unstable with respect to h- + FeO at pressures higher than 50 GPa. The values of Gibbs formation energies at standard conditions for the high-pressure polymorphs and h- were estimated to be -962 kJ/mol and -610 kJ/mol, respectively. The standard-state entropy of is 172.4 J/K/mol.
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| 2. |
- Lundin, Filippa, 1992, et al.
(författare)
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Pressure and Temperature Dependence of Local Structure and Dynamics in an Ionic Liquid
- 2021
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 125:10, s. 2719-2728
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Tidskriftsartikel (refereegranskat)abstract
- A detailed understanding of the local dynamics in ionic liquids remains an important aspect in the design of new ionic liquids as advanced functional fluids. Here, we use small-angle X-ray scattering and quasi-elastic neutron spectroscopy to investigate the local structure and dynamics in a model ionic liquid as a function of temperature and pressure, with a particular focus on state points (P,T) where the macroscopic dynamics, i.e., conductivity, is the same. Our results suggest that the initial step of ion transport is a confined diffusion process, on the nanosecond timescale, where the motion is restricted by a cage of nearest neighbors. This process is invariant considering timescale, geometry, and the participation ratio, at state points of constant conductivity, i.e., state points of isoconductivity. The connection to the nearest-neighbor structure is underlined by the invariance of the peak in the structure factor corresponding to nearest-neighbor correlations. At shorter timescales, picoseconds, two localized relaxation processes of the cation can be observed, which are not directly linked to ion transport. However, these processes also show invariance at isoconductivity. This points to that the overall energy landscape in ionic liquids responds in the same way to density changes and is mainly governed by the nearest-neighbor interactions.
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