1. |
- Hansen, Henriette Wase, 1988, et al.
(author)
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Density scaling of structure and dynamics of an ionic liquid
- 2020
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 22:25, s. 14169-14176
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Journal article (peer-reviewed)abstract
- Room temperature ionic liquids are salts with low melting points achieved by employing bulky and asymmetrical ions. The molecular design leads to apolar and polar parts as well as the presence of competing Coulomb and van der Waals interactions giving rise to nano-scale structure, e.g. charge ordering. In this paper we address the question of how these nano-scale structures influence transport properties and dynamics on different timescales. We apply pressure and temperature as control parameters and investigate the structure factor, charge transport, microscopic alpha relaxation and phonon dynamics in the phase diagram of an ionic liquid. Including viscosity and self diffusion data from literature we find that all the dynamic and transport variables studied follow the same density scaling, i.e. they all depend on the scaling variable Γ = ργ/T, with γ = 2.8. The molecular nearest neighbor structure is found to follow a density scaling identical to that of the dynamics, while this is not the case for the charge ordering, indicating that the charge ordering has little influence on the investigated dynamics.
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2. |
- Lundin, Filippa, 1992, et al.
(author)
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Pressure and Temperature Dependence of Local Structure and Dynamics in an Ionic Liquid
- 2021
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 125:10, s. 2719-2728
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Journal article (peer-reviewed)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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3. |
- Lundin, Filippa, 1992, et al.
(author)
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Structure and dynamics of highly concentrated LiTFSI/acetonitrile electrolytes
- 2021
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 23:25, s. 13819-13826
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Journal article (peer-reviewed)abstract
- High salt concentration has been shown to induce increased electrochemical stability in organic solvent-based electrolytes. Accompanying the change in bulk properties is a structural ordering on mesoscopic length scales and changes in the ion transport mechanism have also been suggested. Here we investigate the local structure and dynamics in highly concentrated acetonitrile electrolytes as a function of salt concentration. Already at low concentrations ordering on microscopic length scales in the electrolytes is revealed by small angle X-ray scattering, as a result of correlations of Li+ coordinating clusters. For higher salt concentrations a charge alternation-like ordering is found as anions start to take part in the solvation. Results from quasi-elastic neutron spectroscopy reveal a jump diffusion dynamical process with jump lengths virtually independent of both temperature and Li-salt concentration. The jump can be envisaged as dissociation of a solvent molecule or anion from a particular Li+ solvation structure. The residence time, 50-800 ps, between the jumps is found to be highly temperature and Li-salt concentration dependent, with shorter residence times for higher temperature and lower concentrations. The increased residence time at high Li-salt concentration can be attributed to changes in the interaction of the solvation shell as a larger fraction of TFSI anions take part in the solvation, forming more stable solvation shells.
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4. |
- Ţuţueanu, A. E., et al.
(author)
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Gradual emergence of superconductivity in underdoped La2-x Srx CuO4
- 2023
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 107:17
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Journal article (peer-reviewed)abstract
- We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La2-xSrxCuO4 with x=0.05, 0.06 and 0.07, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings x=0.06 and x=0.07, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO2 planes is detectable for x=0.05. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.
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