Diamond-anvil-cell and shock-wave technologies now permit the study of matter under multimegabar pressure (that is, of several hundred GPa). The properties of matter in this pressure regime differ drastically from those known at 1 atm (about 10(5) Pa). Just how different chemistry is at high pressure and what role chemical intuition for bonding and structure can have in understanding matter at high pressure will be explored in this account. We will discuss in detail an overlapping hierarchy of responses to increased density: a) squeezing out van der Waals space (for molecular crystals); b) increasing coordination; c) decreasing the length of covalent bonds and the size of anions; and d) in an extreme regime, moving electrons off atoms and generating new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.

化学講話 ロアルド・ホフマン博士講演録 狭い空間で働く化学概念--The Chemical Imagination at Work in Very Tight Places

https://iopscience.iop.org/article/10.1088/1674-1056/ab3f91/pdf

Pressure-induced new chemistry

Unexpected persistence of cis-bridged chains in compressed AuF3

Much effort has been devoted to the investigation of the physical and chemical properties of the Au-O system over a range of pressures, owing to the considerable importance of these materials in fundamental and practical applications. To date, however, only Au1+, Au2+, Au3+, and Au5+ oxidation states have been identified in the Au-O system, but tetravalent Au4+ has not been found. Here, we report the results of structure prediction for the Au-O system at high pressure via the effective structure prediction methodology within a first-principles electronic structure framework. We have uncovered an intriguing structure with AuO2 composition and tetravalent Au, stable at high pressures. This phase shows an electronic transition from a metal to a semiconducting phase as a function of pressure. The present results provide fundamental understanding of the structural and physicochemical properties of compressed Au-O compounds.

Computational prediction of a +4 oxidation state in Au via compressed AuO2 compound.

In Ag-Cu oxides possible to fabricate so far, superconductivity has not been detected, but high conductivity was. In the quest for superconductivity the demand is to create a high and peculiar copper-oxygen coordination. Such coordination makes it non-trivial to determine Cu oxidation states, which may be several and co-existing. Another reason for uncertainty is in oxygen deficiency typical for superconducting crystals. Finally, Cu oxidation is influenced by the other metals in the substance. For chemical fabrication the difficulty is to tune the relative abundances of elements in a fine way. Ag-Cu oxides have been also produced by reactive co-sputtering of Cu and Ag, but the composition with high Cu oxidation states necessary for high conductivity has not been realized. In the present work we have fabricated Ag-Cu-oxide nanoparticles containing Cu and Ag in high oxidation states actual for superconductivity. The fabrication includes reactive sputtering of Ag and Cu metals, their vapour oxidation and aggregation into nanoparticles. The ability to create different and high oxidation states, also co-existing, is demonstrated. The fabrication approach also allows overcoming the poor miscibility of Cu and Ag. The nanoparticle composition and the oxidation states could be determined due to an experimental arrangement in which photoelectron spectroscopy is applied to free nanoparticles in a beam in vacuum, what allows avoiding any contact of the particles to a substrate or atmosphere. The combination of the fabrication and characterization methods has proven to be a powerful approach when fine composition tuning and control are desirable.

Ag-Cu oxide nanoparticles with high oxidation states : towards new high T c materials

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔGH* value. Its active site density counts up to 1.46 × 1015 site/cm2, larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H2 spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction.

Mixed-valence compounds are of great interest due to their interesting properties and wide applications Recently, gold (Au) chemistry has experienced an unprecedented development However, Au with mixed-valence states in Au-O compounds has not been reported thus far Here, two hitherto unknown AuO2 and AuS compounds with mixed-valence character were identified with the aid of first-principles swarm structure searching calculations AuO2 consists of quasi-square AuO4 moiety and AuO6 octahedron in which Au shows the mixed-valence states of III and V, the first example in Au-O binary compounds AuS contains the linear AuS2 and quasi-square AuS4 units exhibiting AuI/III mixed-valence states The analysis of electronic property demonstrates that AuO2 and AuS are narrow band gap semiconductors with strong hybridization between Au 5d and O 2p or S 3p With the increase of pressure, Au-O and Au-S compounds show completely different thermodynamic stabilities, resulting from distinct shifting of pressure-induced atomic orbital energy levels of O and S atoms Our work provides an opportunity for understanding mixed-valence character in Au-O and Au-S compounds

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/cphc.201800984

Mixed-valence Compounds: AuO2 and AuS

K-ion batteries (KIBs) have been considered as appealing alternatives to Li ion batteries due to the high abundance of K, their high working voltages, and allowing the use of mature LIB technology. Thus far, anode materials that can meet the rigorous requirements of KIBs are still rather rare. Here, we have identified a desirable anode material, a metallic C3NS monolayer with high stability, a high storage capacity of 980 mAh/g, a low diffusion barrier of 0.24 eV, and a low open-circuit voltage of 0.36 V, through first-principles calculations. Metallic C3NSKn (n = 1-3) can ensure a high electron conductivity during the charge/discharge process. Valence electrons of the N atom in a triangular bipyramid configuration favor the formation of a planar edge-sharing hexagonal C4N2 unit and delocalized π bonding with C 2p electrons. The lone pair electrons of the S atom induce strong interactions with K atoms, facilitating storage capacity. These interesting properties make the C3NS monolayer a promising anode for KIBs.

Meng Tang

Papers

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction.

Mixed-valence Compounds: AuO2 and AuS

Conductive C3NS Monolayer with Superior Properties for K Ion Batteries.