At ambient conditions, the light alkali metals are free-electron-like crystals with a highly symmetric structure. However, they were found recently to exhibit unexpected complexity under pressure. It ...was predicted from theory-and later confirmed by experiment-that lithium and sodium undergo a sequence of symmetry-breaking transitions, driven by a Peierls mechanism, at high pressures. Measurements of the sodium melting curve have subsequently revealed an unprecedented (and still unexplained) pressure-induced drop in melting temperature from 1,000 K at 30 GPa down to room temperature at 120 GPa. Here we report results from ab initio calculations that explain the unusual melting behaviour in dense sodium. We show that molten sodium undergoes a series of pressure-induced structural and electronic transitions, analogous to those observed in solid sodium but commencing at much lower pressure in the presence of liquid disorder. As pressure is increased, liquid sodium initially evolves by assuming a more compact local structure. However, a transition to a lower-coordinated liquid takes place at a pressure of around 65 GPa, accompanied by a threefold drop in electrical conductivity. This transition is driven by the opening of a pseudogap, at the Fermi level, in the electronic density of states-an effect that has not hitherto been observed in a liquid metal. The lower-coordinated liquid emerges at high temperatures and above the stability region of a close-packed free-electron-like metal. We predict that similar exotic behaviour is possible in other materials as well.
A multiscale coarse-graining model for ionic liquids has been extended to investigate the unique aggregation of cations in ionic liquids through computer simulation. It has been found that, with ...sufficiently long side chains, the tail groups of cations aggregate to form spatially heterogeneous domains, while headgroups of the cations and the anions distribute as uniformly as possible. This is understood as the result of competition between the charged electrostatic interactions between headgroups and anions and the collective short-range interactions between the neutral tail groups. This aggregation can help to explain a number of experimentally observed physical phenomena in ionic liquids.
X-ray scattering experiments and molecular dynamics simulations have been performed to investigate the structure of four room temperature ionic liquids (ILs) comprising the ...bis(trifluoromethylsulfonyl)amide (NTf2 –) anion paired with the triethyloctylammonium (N2228 +) and triethyloctylphosphonium (P2228 +) cations and their isoelectronic diether analogs, the (2-ethoxyethoxy)ethyltriethylammonium (N222(2O2O2) +) and (2-ethoxyethoxy)ethyltriethylphosphonium (P222(2O2O2) +) cations. Agreement between simulations and experiments is good and permits a clear interpretation of the important topological differences between these systems. The first sharp diffraction peak (or prepeak) in the structure function S(q) that is present in the case of the liquids containing the alkyl-substituted cations is absent in the case of the diether substituted analogs. Using different theoretical partitioning schemes for the X-ray structure function, we show that the prepeak present in the alkyl-substituted ILs arises from polarity alternations between charged groups and nonpolar alkyl tails. In the case of the diether substituted ILs, we find considerable curling of tails. Anions can be found with high probability in two different environments: close to the cationic nitrogen (phosphorus) and also close to the two ether groups. For the two diether systems, anions are found in locations from which they are excluded in the alkyl-substituted systems. This removes the longer range (polar/nonpolar) pattern of alternation that gives rise to the prepeak in alkyl-substituted systems.
Ionic liquids with intermediate nonpolar cationic side-chain lengths are known to have nanoscale spatial heterogeneities with nonpolar tail domains separated by a continuous polar network. In this ...work, we use coarse-grained molecular dynamics simulations to show that, when the nonpolar cationic side chain is sufficiently long, due to the stronger van der Waals interactions between the side chains, the structure of ionic liquids goes through a transition from spatially heterogeneous to liquid crystalline-like. For XMIm+/NO3 – ionic liquids, change occurs when the number of carbon groups on the cationic side chain varies from 14 to 16. In the liquid crystal-like phase, the cationic side chains tend to be parallel to each other, while the cationic head groups and anions, although being mostly layered perpendicularly to the direction along the side chains, still form a continuous polar network.
From the extension of the concepts used in our three previous works (B. Grosdidier 2018, 2019, 2021), we investigate the structure and calculate the order parameters in the quaternary alloy. We show ...that the order may be described by two conceptualizations as for the ternary alloy. All structural quantities and the relationships between them are calculated. In the frame of our approach, we propose a new and more concise formula of the order parameters. We check the consistency of our approach on several “toys models” having specific behaviour of their atomic species. All order parameters are calculated in both conceptualizations. Some of them are plotted on tetrahedral quaternary phase diagrams. We retrieve our results by studying the atom exchanges in several kinds of iso-stoichiometric quaternary alloys. We describe several properties specific to the quaternary alloy that do not exist in binary or in ternary alloys.
•Existence of two conceptualizations of the order in the quaternary alloy.•Calculation of the structure factors in both conceptualizations.•Elaboration of a new and more concise formula of the order parameters.•Calculation of the order in six “toy models” and in four iso-stoichiometric quaternary alloys.•Highlighting of order properties specific to the quaternary system.
X-ray absorption spectroscopy and x-ray Raman scattering were used to probe the molecular arrangement in the first coordination shell of liquid water. The local structure is characterized by ...comparison with bulk and surface of ordinary hexagonal ice Ih and with calculated spectra. Most molecules in liquid water are in two hydrogen-bonded configurations with one strong donor and one strong acceptor hydrogen bond in contrast to the four hydrogen-bonded tetrahedral structure in ice. Upon heating from 25°C to 90°C, 5 to 10% of the molecules change from tetrahedral environments to two hydrogen-bonded configurations. Our findings are consistent with neutron and x-ray diffraction data, and combining the results sets a strong limit for possible local structure distributions in liquid water. Serious discrepancies with structures based on current molecular dynamics simulations are observed.
When examining the formation energetics of a hydrogen-bonded complex R−X−H···Y−R‘, focus has been almost always on the atoms directly involved, namely the atoms X, Y, and H. Little attention has been ...paid to the effects of the secondary alkyl groups R and R'. Taking dimethyl sulfoxide (DMSO)−methanol binary system as an example, we have studied the roles of the alkyl groups in stabilizing the hydrogen bonds by employing FTIR and NMR techniques and quantum chemical calculations. We found that methyl groups play different roles in response to the hydrogen-bonding interactions. The methyl groups of DMSO are electron-donating, whereas that of methanol is electron-withdrawing, both making positive contributions. The findings reveal non-negligible effects of secondary alkyl groups in hydrogen bonding interaction and may shed light on the understanding of other more complicated hydrogen-bonded systems in chemical and biological systems.
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