Experimental equations of state for cesium and lithium metals to 20 kbar and the high-pressure behavior of the alkali metals

Massive and massless charge carriers in an epitaxially strained alkali metal quantum well on graphene

We show that Cs intercalated bilayer graphene acts as a substrate for the growth of a strained Cs film hosting quantum well states with high electronic quality. The Cs film grows in an fcc phase with a substantially reduced lattice constant of 4.9 Å corresponding to a compressive strain of 11% compared to bulk Cs. We investigate its electronic structure using angle-resolved photoemission spectroscopy and show the coexistence of massless Dirac and massive Schrödinger charge carriers in two dimensions. Analysis of the electronic self-energy of the massive charge carriers reveals the crystallographic direction in which a two-dimensional Fermi gas is realized. Our work introduces the growth of strained metal quantum wells on intercalated Dirac matter.

Cesium atoms that are grown on intercalated bilayer graphene can create an ordered epitaxial film. Here, the authors report that such a strained film can host quantum well states with high electronic quality as characterized through angle-resolved photoemission spectroscopy.

The interaction of ethylammonium tetrafluoroborate [EtNH3+][BF4−] ionic liquid on the Li(001) surface: towards understanding early SEI formation on Li metal

Dissociation of an ionic liquid is not necessarily a requirement for the formation of an SEI layer.

First-Principles Investigations of the Working Mechanism of 2D h-BN as an Interfacial Layer for the Anode of Lithium Metal Batteries

An issue with the use of metallic lithium as an anode material for lithium-based batteries is dendrite growth, causing a periodic breaking and repair of the solid electrolyte interphase (SEI) layer. Adding 2D atomic crystals, such as h-BN, as an interfacial layer between the lithium metal anode and liquid electrolyte has been demonstrated to be effective to mitigate dendrite growth, thereby enhancing the Columbic efficiency of lithium metal batteries. But the underlying mechanism leading to the reduced dendrite growth remains unknown. In this work, with the aid of first-principle calculations, we find that the interaction between the h-BN and lithium metal layers is a weak van der Waals force, and two atomic layers of h-BN are thick enough to block the electron tunneling from lithium metal to electrolyte, thus prohibiting the decomposition of electrolyte. The interlayer spacing between the h-BN and lithium metal layers can provide larger adsorption energies toward lithium atoms than that provided by bare lithium or h-BN, making lithium atoms prefer to intercalate under the cover of h-BN during the plating process. The combined high stiffness of h-BN and the low diffusion energy barriers of lithium at the Li/h-BN interfaces induce a uniform distribution of lithium under h-BN, therefore effectively suppressing dendrite growth.

Extreme compressibility in LnFe(CN)6 coordination framework materials via molecular gears and torsion springs

The mechanical flexibility of coordination frameworks can lead to a range of highly anomalous structural behaviours. Here, we demonstrate the extreme compressibility of the LnFe(CN)6 frameworks (Ln = Ho, Lu or Y), which reversibly compress by 20% in volume under the relatively low pressure of 1 GPa, one of the largest known pressure responses for any crystalline material. We delineate in detail the mechanism for this high compressibility, where the LnN6 units act like torsion springs synchronized by rigid Fe(CN)6 units performing the role of gears. The materials also show significant negative linear compressibility via a cam-like effect. The torsional mechanism is fundamentally distinct from the deformation mechanisms prevalent in other flexible solids and relies on competition between locally unstable metal coordination geometries and the constraints of the framework connectivity, a discovery that has implications for the strategic design of new materials with exceptional mechanical properties.

Thermodynamic calculations of oxygen self-diffusion in mixed-oxide nuclear fuels

Molecular dynamics calculations are used to provide a self-consistent prediction of the elastic, thermal expansion and oxygen self-diffusion properties of mixed oxide nuclear fuels at arbitrary compositions.

Mixing unmixables: Unexpected formation of Li-Cs alloys at low pressure

Contrary to the empirical Miedema and Hume-Rothery rules and a recent theoretical prediction, we report experimental evidence on the formation of Li-Cs alloys at very low pressure (>0.1 GPa). We also succeeded in synthesizing a pure nonstoichiometric and ordered crystalline phase from an approximately equimolar mixture and resolved its structure using the maximum entropy method. The new alloy has a primitive cubic cell with the Li atom situated in the center and the Cs at the corners. This structure is stable to at least 10 GPa and has an anomalously high coefficient of thermal expansion at low pressure. Analysis of the valence charge density shows that electrons are donated from Cs to the Li "p"-orbitals, resulting in a rare formal oxidation state of -1 for Li. The observation indicates the diversity in the bonding of the seeming simple group I Li element.

An embedded-atom-method model for alkali-metal vibrations

We present an embedded-atom-method (EAM) model that accurately describes the vibrational dynamics in the alkali metals Li, Na, K, Rb and Cs. The bulk dispersion curves, frequency-moment Debye temperatures and temperature-dependent entropy Debye temperatures are all in excellent agreement with experimental results. The model is also well suited for studying surface vibrational dynamics in these materials, as illustrated by calculations for the Na(110) surface.

Compression dependence of the melting of elements

In normal melting there is no significant change in the electronic structure, while in anomalous melting the crystal and liquid have different electronic structures. For those elements that melt normally at zero pressure, the pressure derivative of the melting temperature is shown to follow the normal melting rule. For Ar, Na, K and Hg, the normal melting properties continue to hold to high compression, and in Hg it appears that the strong higher-order correlations in the liquid are gradually weakened by compression. The normal melting process is described by two sets of nearly independent parameters: universal statistical factors, and factors depending on the interatomic forces. Anomalous melting is related to a compression induced solid-solid-liquid triple point, and Cs is observed to change from normal to anomalous as its first triple point is approached.

Melting of elements

The aim is to identify and evaluate the most important factors that determine the melting temperature T m of the elements. Experimental data are analysed in terms of statistical mechanical theories of the energy and entropy of crystal and liquid phases. The elements are divided into two groups, according to their value of the entropy of fusion at constant density, ∆ S : the normal elements have ∆ S close to 0.79 k per atom,and the anomalous elements have ∆ S much larger. For the normal elements, a melting rule is constructed in term s of two factors, the lattice dynamics characteristic temperature for entropy, and the liquid correlation entropy. For the anomalous elements, the large ∆ S is attributed to a change in electronic groundstate upon melting, and the melting rule depends also on the corresponding electronic energy change. The melting rules give T m to an accuracy of around 20%, and at this level, anharmonicity and crystal symmetry effects are negligible. Two conclusions regarding the relative motion of ions in crystal and liquid phases are: ( a ) liquid higher order correlations lower T m because they lead to a more ordered liquid, and ( b ) the loss of long-range order in melting corresponds to ∆ S ≈ 0.79 k per atom for normal and anomalous elements alike.

Condensed-Matter Energetics from Diatomic Molecular Spectra

Analyses of molecular spectra and compression data from crystals show that a single function successfully describes the dependence on interatomic separation of both the potential energy of diatomic molecules and the cohesive binding energy of condensed matter. The empirical finding that one function describes interatomic energies for such diverse forms of matter and over a wide range of conditions can be used to extend condensed-matter equations of state but warrants further theoretical study.