Exploring the Limits of Transition-Metal Fluorination at High Pressures

Theoretical Study on the Structures and Electronic Properties of Tungsten Fluorides at High Pressures

Transition metal fluorides are a series of strong oxidizing agents. Tungsten (W) fluorides, particularly WF6, have shown broad applications such as luminescence and fluorinating agent. However, other stoichiometries of W fluorides have rarely been studied. It is well-known that pressure can induce structural phase transition, stabilize new compounds, and produce novel properties. In this work, the high-pressure phases of W-F were searched systematically at the pressure range of 0-200 GPa through first-principles swarm-intelligence structural search calculations. A new stoichiometry of WF4 has been predicted to be stable under high pressures. On the other hand, two new high-pressure phases of WF6 with the symmetries ofP 2 1 ${{P2}_{1}}$ /m and P ${P}$ -1 were found with decahedral structural units. The electronic properties of the W-F compounds were then investigated. The predicted stable WF6 high-pressure phases maintain semiconducting features, since the W atom provides all its valence electrons to fluorine. We evaluated the oxidizing ability of WF6 by calculating its electron affinity potential. The high pressureP 2 1 ${{P2}_{1}}$ /m WF6 molecular phase shows higher oxidation capacity than the ambient phase. The built pressure-composition phase diagram and the theoretical results of W-F system provide some useful information for experimental synthesis.

Potential rules for stable transition metal hexafluorides with high oxidation states under high pressures

High pressure is a powerful tool in material sciences which can lead to the discovery of novel inorganic species in high oxidation states. Based on the prediction of the stability of PdF₆ with a high Pd oxidation state of +6, we propose three potential guiding rules for finding stable transition metal (TM) fluorides with high +6 oxidation states: (1) the existence of a large (>7 eV) valence orbitals energy differences of atoms between the TM d orbital and the F 2p orbital; (2) an appropriate number of valence electrons within the range of 6-11; and (3) suitable electronegativity values less than 2.3 on the Pauli scale. More importantly, by synergistically invoking all of these rules, we predict, by combining a particle swarm optimization algorithm with first-principles calculation on the phase stabilities of the various TM-F compounds, a collection of new TMF₆ species with the space group Pnma that have a +6 oxidation state. Subsequently, we develop an understanding of the high +6 oxidation state for the TM elements. These findings are expected to play a crucial role in the predictive discoveries of new fluorides with high oxidation states of +6.

Pressure-stabilized hexafluorides of first-row transition metals

Fluorine chemistry was demonstrated to show the importance of stretching the limits of chemical synthesis, oxidation state, and chemical bonding at ambient conditions. Thus far, the highest fluorine stoichiometry of a neutral first-row transition-metal fluoride is five, in VF₅ and CrF₅. Pressure can stabilize new stoichiometric compounds that are inaccessible at ambient conditions. Here, we attempted to delineate the fluorination limits of first-row transition metals at a high pressure through first-principles swarm-intelligence structure searching simulations. Besides reproducing the known compounds, our extensive search has resulted in a plethora of unreported compounds: CrF₆, MnF₆, FeF₄, FeF₅, FeF₆, and CoF₄, indicating that the application of pressure achieves not only the fluorination limit (e.g., hexafluoride) but also the long-sought bulky tetrafluorides. Our current results provide a significant step forward towards a comprehensive understanding of the fluorination limit of first-row transition metals.

Au with sp3 Hybridization in Li5AuP2

The achievement of new bonding patterns of atoms in compounds is of great importance, which usually induces interesting physical and chemical properties. Rich oxidation states, diverse bonding types, and unique aurophilic attraction endow gold (Au) as a distinctive element. Here we report that a pressure-induced Li₅AuP₂, identified by a swarm intelligence-based structural prediction, becomes the first example of Au with sp³ hybridization. The most remarkable feature of Li₅AuP₂ is that it contains various frameworks made by AuP₄, AuLi₄, LiP₄, and blende-like Li-P units, exhibiting noncentrosymmetry. The charge transfer from Li to Au makes Au 6p orbitals activate and hybridize with the 6s one. On the other hand, Li donating electrons to P and polar Au-P covalence make the constituent atoms satisfy the octet rule, rendering Li₅AuP₂ with a semiconducting character and a large second-order nonlinear optical response in the near-infrared region. Our work represents a significant step toward extending the understanding of gold chemistry.

Crystal structures and superconductivity of lithium and fluorine implanted gold hydrides under high pressures

The investigations on gold science have been capturing research interest due to its diverse physical and chemical properties. Gold hydrides in the solid state, as a member of the Au compound family, are rare since the reaction of Au with H is hindered in terms of their similar electronegativity. It is expected that Li and F can provide electrons and holes, respectively, to help stabilize gold hydrides under high pressure. Herein, by means of a crystal structural search based on particle swarm optimization methodology accompanied by first-principles calculations, four hitherto unknown Li-Au-H compounds (i.e., LiAuH, LiAu₂H, Li₂Au₂H, and Li₆AuH) are predicted to be stable under compression. Intriguingly, Au-H bonding is found in LiAuH, LiAu₂H, and Li₂Au₂H. As the gold content increases, Au atom arrangements exhibit diverse forms, from the chain in Li₆AuH, the square layer in LiAuH, the network in Li₂Au₂H, and eventually to the coexistence of square and pyramid layers in LiAu₂H. Additionally, Li₆AuH has a unique cage-type lithium structure. Furthermore, electron-phonon coupling calculations show that these Li-Au-H phases are phonon-modulated superconductors with a superconducting critical temperature of 1.3, 0.06, and 0.02 K at 25 GPa and 2.79 K at 100 GPa. In contrast, we also identified two solid F₄AuH and F₆AuH phases with unexpected semiconductivity. They have structural configurations of H-bridged AuF₄ quasi-square components and distorted AuF₆ octahedrons, respectively, and have no gold-to-hydrogen bonds. Our current results indicate that electron doping at suitable concentrations under pressure can stabilize unique gold hydrides, and provide deep insights into the structures, electron properties, bonding behavior, and stability mechanism of ternary Li-Au-H and F-Au-H compounds.

Electronic Structure and Spectroscopic Properties of Group-7 Tri-Oxo-Halides MO3X (M = Mn-Bh, X = F-Ts)

The 24 trioxide halide molecules MO₃X of the manganese group (M = Mn-Bh; X = F-Ts), which are iso-valence-electronic with the famous MnO_4^- ion, have been quantum-chemically investigated by quasi-relativistic density-functional and ab initio correlated approaches. Geometric and electronic structures, valence and oxidation numbers, vibrational and electronic spectral properties, energetic stabilities of the monomers in the gas phase, and the decay mode of MnO₃F have been investigated. The light Mn-3d species are most strongly electron-correlated, indicating that the concept of a closed-shell Lewis-type single-configurational structure [Mn⁺⁷(d⁰) O^-2(p⁶)₃ F^-(p⁶)] reaches its limits. The concept of real-valued spin orbitals φ(r)·α and φ(r)·β breaks down for the heavy Bh-6d, At-6p and Ts-7p elements because of the dominating spin-orbit coupling. The vigorous decomposition of MnO₃F at ambient conditions starts by the autocatalyzed release of n O₂ and the formation of Mn_mO_3m-2nF_m clusters, triggered by the electron-depleted "oxylic" character of the oxide ligands in MnO₃X.

IrN4 and IrN7 as potential high-energy-density materials

Transition metal nitrides have attracted great interest due to their unique crystal structures and applications. Here, we predict two N-rich iridium nitrides (IrN₄ and IrN₇) under moderate pressure through first-principles swarm-intelligence structural searches. The two new compounds are composed of stable IrN₆ octahedrons and interlinked with high energy polynitrogens (planar N₄ or cyclo-N₅). Balanced structural robustness and energy content result in IrN₄ and IrN₇ being dynamically stable under ambient conditions and potentially as high energy density materials. The calculated energy densities for IrN₄ and IrN₇ are 1.3 kJ/g and 1.4 kJ/g, respectively, comparable to other transition metal nitrides. In addition, IrN₄ is predicted to have good tensile (40.2 GPa) and shear strengths (33.2 GPa), as well as adequate hardness (20 GPa). Moderate pressure for synthesis and ambient pressure recoverability encourage experimental realization of these two compounds in near future.

Relating atomic energy, radius and electronegativity through compression

Trends in atomic properties are well-established tools for guiding the analysis and discovery of materials. Here, we show how compression can reveal a long sought-after connection between two central chemical concepts - van-der-Waals (vdW) radii and electronegativity - and how these relate to the driving forces behind chemical and physical transformations.

Understanding Periodic and Non-periodic Chemistry in Periodic Tables

The chemical elements are the "conserved principles" or "kernels" of chemistry that are retained when substances are altered. Comprehensive overviews of the chemistry of the elements and their compounds are needed in chemical science. To this end, a graphical display of the chemical properties of the elements, in the form of a Periodic Table, is the helpful tool. Such tables have been designed with the aim of either classifying real chemical substances or emphasizing formal and aesthetic concepts. Simplified, artistic, or economic tables are relevant to educational and cultural fields, while practicing chemists profit more from "chemical tables of chemical elements." Such tables should incorporate four aspects: (i) typical valence electron configurations of bonded atoms in chemical compounds (instead of the common but chemically atypical ground states of free atoms in physical vacuum); (ii) at least three basic chemical properties (valence number, size, and energy of the valence shells), their joint variation across the elements showing principal and secondary periodicity; (iii) elements in which the (sp)8, (d)10, and (f)14 valence shells become closed and inert under ambient chemical conditions, thereby determining the "fix-points" of chemical periodicity; (iv) peculiar elements at the top and at the bottom of the Periodic Table. While it is essential that Periodic Tables display important trends in element chemistry we need to keep our eyes open for unexpected chemical behavior in ambient, near ambient, or unusual conditions. The combination of experimental data and theoretical insight supports a more nuanced understanding of complex periodic trends and non-periodic phenomena.