Iodine Anions beyond -1: Formation of LinI (n = 2-5) and Its Interaction with Quasiatoms

Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na2Cl3 and Na4Cl5 and Bromide Na4Br5

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures (tP10-Na₂Cl₃, hP18-Na₄Cl₅, and hP18-Na₄Br₅), a series of isostructural cubic cP8-AX₃ halides (NaCl₃, KCl₃, NaBr₃, and KBr₃), and a trigonal potassium chloride (hP24-KCl₃). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl₃^- anion in hP24-KCl₃ and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl]_∞^n- and [Br]_∞^n-, in the structures of cP8-AX₃ compounds and in hP18-Na₄Cl₅ and hP18-Na₄Br₅. In Na₄Cl₅ and Na₄Br₅, we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides.

Mechanochemistry and the Evolution of Ionic Bonds in Dense Silver Iodide

External mechanical stress alters the nature of chemical bonds and triggers novel reactions, providing interesting synthetic protocols to supplement traditional solvent- or thermo-based chemical approaches. The mechanisms of mechanochemistry have been well studied in organic materials made of a carbon-centered polymeric framework and covalence force field. They convert stress into anisotropic strain which will engineer the length and strength of targeted chemical bonds. Here, we show that by compressing silver iodide in a diamond anvil cell, the external mechanical stress weakens the Ag-I ionic bonds and activate the global diffusion of super-ions. In contrast to conventional mechanochemistry, mechanical stress imposes unbiased influence on the ionicity of chemical bonds in this archetypal inorganic salt. Our combined synchrotron X-ray diffraction experiment and first-principles calculation demonstrate that upon the critical point of ionicity, the strong ionic Ag-I bonds break down, leading to the recovery of elemental solids from a decomposition reaction. Instead of densification, our results reveal the mechanism of an unexpected decomposition reaction through hydrostatic compression and suggest the sophisticated chemistry of simple inorganic compounds under extreme conditions.

The Exotically Stoichiometric Compounds and Superconductivity of Lithium-Copper Systems under High Pressure

Pressure, as a useful tool, can push elements to new oxidation states by altering the stoichiometry of compounds, leading to materials with exotic physical and chemical properties. Herein, structure searches for Li-Cu systems were carried out under pressure. Three Li-rich Li-Cu compounds with exotic stoichiometries (i.e., Li₄Cu, Li₅Cu, and Li₆Cu) are predicted at high pressure. Remarkably, the Li₆Cu consists of a Cu-centered face-sharing icosahedron. Further simulations reveal that the captured electrons from Li atoms prompt Cu atoms to achieve high negative oxidation states beyond -1 and to act as a 4p group element. Moreover, our results unravel the superconductivity of the Li-rich Li-Cu system and the R3̅ phase of Li₆Cu with T_c of ∼15 K at 50 GPa. The present results can greatly improve the understanding of the exotic electronic behavior of Li-Cu systems under high pressure.

Superconducting Li10Se electride under pressure

Achieving a compound with interesting multiple coexisting states, such as electride, metallicity, and superconductivity, is of great interest in basic research and practical application. Pressure has become an effective way to realize high-temperature superconductivity in hydrides, whereas most electrides are semiconducting or insulating at high pressure. Here, we have applied swarm-intelligence structural search to identify a hitherto unknown C2/m Li₁₀Se electride that is superconducting at high pressure. More interestingly, Li₁₀Se is estimated to exhibit the highest T_c value of 16 K at 50 GPa, which is the lowest pressure among Li-based chalcogen electrides. This superconducting transition is dominated by Se-related low frequency vibration modes. The increasing electronic occupation of the Se 4d orbital and the decreasing amount of interstitial anion electrons with pressure heighten their coupling with low-frequency phonons, which is responsible for the enhancement of the T_c value. The finding of Li-based chalcogen superconducting electrides provides a reference for the realization of other superconducting electrides at lower pressures.

Predicted Stable Structures of the Li-Ag System at High Pressures

In this letter, we have performed extensive swarm-intelligence structure searching simulations on the Li-Ag system at high pressures. As a result, Li₄Ag is predicted to become stable at high pressures. Moreover, we have also identified several pressured-stabilized structures for the Li-Ag system. The further electronic density of states and electron localization function calculations reveal the metallic feature of predicted structures. Crystal orbital Hamilton population (COHP) calculations indicate the strong interaction between Li atoms, leading to the formation of Li-ring configurations in Li-rich Li-Ag structures. Our current results highlight the role of pressure in determining the stability for unexpected stoichiometry in the Li-Ag system.

Pressure-induced unexpected -2 oxidation states of bromine and superconductivity in magnesium bromide

Finding novel compounds with unusual crystal structures and physical properties is always an important goal for the materials and chemistry community. Pressure becomes attractive due to its unique ability to break down many fundamental rules by modifying the chemical properties of elements, overcoming reaction barriers and shortening interatomic distances, leading to the formation of some novel materials with unexpected properties. In this work, for the first time we have analyzed the high-pressure phase diagram, crystal structures and electron properties of the Mg-Br system up to 200 GPa using unbiased structure searching techniques. Besides the already known MgBr₂, here we report that three unusual stoichiometries of Mg-Br compounds can be stabilized at high pressures as MgBr₃, MgBr and Mg₄Br. Firstly, among the predicted stable compounds, we find that the Mg₄Br in the I4/mmm structure stabilized at 178 GPa behaves as a typical electride, indicating that the formation pressure of an electride for Mg can be significantly reduced by bonding with Br atoms. Secondly, it is surprising that the unexpected oxidation states of Br approaching -2 are observed in the predicted I4/mmm Mg₄Br and Pm3[combining macron]m MgBr compounds. Furthermore, P2₁/m MgBr₃ and I4[combining macron]2m MgBr₃ phases are predicted as superconductors with an estimated T_c of 23.2 and 0.49 K, respectively. Our work represents a significant step toward understanding the high pressure behaviors of alkaline earth halides and searching for novel high temperature superconductors.