'... a metal conducts and a non-metal doesn't'

Challenges for fluoride superionic conductors: fundamentals, design, and applications

Electronics, which harnesses the properties of electrons, has made remarkable progress since its inception and is a cornerstone of modern society. Ionics, which exploits the properties of ions, has also had a profound impact, as demonstrated by the award of the Nobel Prize in Chemistry in 2019 for achievements related to lithium-ion batteries (LIBs). Ionic conduction in solids is the flow of carrier ions through a solid owing to an electrical or chemical bias. Some ionic materials have been studied intensively because their ionic conductivities are higher than those of liquids, even though they are solids. Among various conductive species, fluoride ions are the most promising charge carriers for fluoride-ion batteries (FIBs) as post LIBs. Increasing fluoride-ion conductivity toward the superionic conductive region at room temperature would be a breakthrough for the room-temperature operation of all-solid-state FIBs. This review focuses on fluoride-ion conductors, from the general concept of ions to the characteristics of fluoride ions. Fluoride-ion conductors are classified according to material type and form, and our current understanding, identification of problems, and future directions are discussed from experimental and theoretical physics perspectives.

Ultrafast multi-cycle terahertz measurements of the electrical conductivity in strongly excited solids

Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity.

The electrical conductivity is critical to understand warm dense matter, but the accurate measurement is extremely challenging. Here the authors use multi-cycle THz pulses to measure the conductivity of gold foils strongly heated by free-electron laser, determining the individual contributions of electron-electron and electron-ion scattering.

Metals and non-metals in the periodic table

The demarcation of the chemical elements into metals and non-metals dates back to the dawn of Dmitri Mendeleev's construction of the periodic table; it still represents the cornerstone of our view of modern chemistry. In this contribution, a particular emphasis will be attached to the question 'Why do the chemical elements of the periodic table exist either as metals or non-metals under ambient conditions?' This is perhaps most apparent in the p-block of the periodic table where one sees an almost-diagonal line separating metals and non-metals. The first searching, quantum-mechanical considerations of this question were put forward by Hund in 1934. Interestingly, the very first discussion of the problem-in fact, a pre-quantum-mechanical approach-was made earlier, by Goldhammer in 1913 and Herzfeld in 1927. Their simple rationalization, in terms of atomic properties which confer metallic or non-metallic status to elements across the periodic table, leads to what is commonly called the Goldhammer-Herzfeld criterion for metallization. For a variety of undoubtedly complex reasons, the Goldhammer-Herzfeld theory lay dormant for close to half a century. However, since that time the criterion has been repeatedly applied, with great success, to many systems and materials exhibiting non-metal to metal transitions in order to predict, and understand, the precise conditions for metallization. Here, we review the application of Goldhammer-Herzfeld theory to the question of the metallic versus non-metallic status of chemical elements within the periodic system. A link between that theory and the work of Sir Nevill Mott on the metal-non-metal transition is also highlighted. The application of the 'simple', but highly effective Goldhammer-Herzfeld and Mott criteria, reveal when a chemical element of the periodic table will behave as a metal, and when it will behave as a non-metal. The success of these different, but converging approaches, lends weight to the idea of a simple, universal criterion for rationalizing the instantly-recognizable structure of the periodic table where …the metals are here, the non-metals are there … The challenge of the metallic and non-metallic states of oxides is also briefly introduced. This article is part of the theme issue 'Mendeleev and the periodic table'.

Global aromaticity at the nanoscale

Aromaticity can be defined by the ability of a molecule to sustain a ring current when placed in a magnetic field. Hückel’s rule states that molecular rings with [4n+2] π-electrons are aromatic, with an induced magnetisation that opposes the external field inside the ring, whereas those with 4n π-electrons are antiaromatic, with the opposite magnetisation. This rule reliably predicts the behaviour of small molecules, typically with fewer than 22 π-electrons (n = 5). It is not clear whether aromaticity has a size limit, or whether Hückel’s rule extends to much larger macrocycles. Here, we present evidence for global aromaticity in porphyrin nanorings with circuits of up to 162 π-electrons (n = 40); aromaticity is controlled by changing the constitution, oxidation state and conformation. Whenever a ring current is observed, its direction is correctly predicted by Hückel’s rule. The largest ring currents occur when the porphyrins units have fractional oxidation states.

The generalized maximum hardness principle revisited and applied to solids (Part 2)

Building on Part 1 devoted to atoms and molecules (PCCP, in press 2017), we now focus on the crystal structure and electronic properties of solids as viewed from the Maximum Hardness Principle (MHP), first formulated by Pearson in 1987. The focus is on cases where nuclear potential acting on electrons does not remain constant and where substantial modifications of the nuclear geometry take place (Generalized MHP, GMHP). We present an overview of important manifestations of the (G)MHP for solids such as (i) a tendency of metals and doped-semiconductors to undergo superconducting transition at low temperatures, (ii) propensity of many types of alloys to develop a band gap or a pseudo-gap, (iii) preference for preserving the noble gas (octet, doublet) configuration of main block element ions in the solid state, (iv) preference of Jahn-Teller systems for band-gap-opening vibronic-coupling-related lattice distortions, (v) pressure phenomena leading to localization of the electronic density, (vi) tendency to annihilate the null band gap via phase separation (while preserving the nominal chemical composition), (vii) absence of a large number of families of high-T_C superconductors, (viii) resistance of most stable systems to chemical doping, etc. GMHP turns out to be an important qualitative guide in studies of solid state polymorphism and electronic phenomena. Exceptions from (G)MHP are discussed, and a more restrictive formulation of the principle is proposed.

On mobility of electrons in a shallow Fermi sea over a rough seafloor

Several doped semiconductors, in contrast to heavily-doped silicon and germanium, host extremely mobile carriers, which give rise to quantum oscillations detectable in relatively low magnetic fields. The small Fermi energy in these dilute metals quantifies the depth of the Fermi sea. When the carrier density exceeds a threshold, accessible thanks to the long Bohr radius of the parent insulator, the local seafloor is carved by distant dopants. In such conditions, with a random distribution of dopants, the probability of finding an island or a trench depends on on the effective Bohr radius, [Formula: see text] and the carrier density, n. This picture yields an expression for electron mobility with a random distribution of dopants: [Formula: see text] [Formula: see text], in reasonable agreement with the magnitude and concentration dependence of the low-temperature mobility in three dilute metals whose insulating parents are a wide-gap (SrTiO3), a narrow-gap (PbTe) and a 'zero'-gap (TlBiSSe) semiconductor.

On the occurrence of metallic character in the periodic table of the chemical elements

The classification of a chemical element as either 'metal' or 'non-metal' continues to form the basis of an instantly recognizable, universal representation of the periodic table (Mendeleeff D. 1905 The principles of chemistry, vol. II, p. 23; Poliakoff M. & Tang S. 2015 Phil. Trans. R. Soc. A 373: , 20140211). Here, we review major, pre-quantum-mechanical innovations (Goldhammer DA. 1913 Dispersion und Absorption des Lichtes; Herzfeld KF. 1927 Phys. Rev. 29: , 701-705) that allow an understanding of the metallic or non-metallic status of the chemical elements under both ambient and extreme conditions. A special emphasis will be placed on recent experimental advances that investigate how the electronic properties of chemical elements vary with temperature and density, and how this invariably relates to a changing status of the chemical elements. Thus, the prototypical non-metals, hydrogen and helium, becomes metallic at high densities; and the acknowledged metals, mercury, rubidium and caesium, transform into their non-metallic forms at low elemental densities. This reflects the fundamental fact that, at temperatures above the absolute zero of temperature, there is therefore no clear dividing line between metals and non-metals. Our conventional demarcation of chemical elements as metals or non-metals within the periodic table is of course governed by our experience of the nature of the elements under ambient conditions. Examination of these other situations helps us to examine the exact divisions of the chemical elements into metals and non-metals (Mendeleeff D. 1905 The principles of chemistry, vol. II, p. 23).

Quantum molecular dynamics study of expanded beryllium: evolution from warm dense matter to atomic fluid

By performing quantum molecular dynamics (QMD) simulations, we investigate the equation of states, electrical and optical properties of the expanded beryllium at densities two to one-hundred lower than the normal solid density, and temperatures ranging from 5000 to 30000 K. With decreasing the density of Be, the optical response evolves from the one characteristic of a simple metal to the one of an atomic fluid. By fitting the optical conductivity spectra with the Drude-Smith model, it is found that the conducting electrons become localized at lower densities. In addition, the negative derivative of the electrical resistivity on temperature at density about eight lower than the normal solid density demonstrates that the metal to nonmetal transition takes place in the expanded Be. To interpret this transition, the electronic density of states is analyzed systematically. Furthermore, a direct comparison of the Rosseland opacity obtained by using QMD and the standard opacity code demonstrates that QMD provides a powerful tool to validate plasma models used in atomic physics approaches in the warm dense matter regime.