Iodine Anions beyond −1: Formation of Li_<i>n</i>I (<i>n</i> = 2–5) and Its Interaction with Quasiatoms

Novel phases of Li_<i>n</i>I (<i>n</i> = 2, 3, 4, 5) compounds are predicted to form under high pressure using first-principles density functional theory and an unbiased crystal structure search algorithm. All of the phases identified are thermodynamically stable with respect to decomposition into elemental Li and the binary LiI at a relatively low pressure (≈20 GPa). Increasing the pressure to 100 GPa yields the formation of a high pressure electride where electrons occupy interstitial quasiatom (ISQ) orbitals. Under these extreme pressures, the calculated charge on iodine suggests the oxidation state goes beyond the conventional and expected −1 charge for the halogens. This strange oxidative behavior stems from an electron transfer going from the ISQ to I^– and Li⁺ ions as high pressure collapses the void space. The resulting interplay between chemical bonding and the quantum chemical nature of enclosed interstitial space allows this first report of a halogen anion beyond a −1 oxidation state

Nitrophosphorene: A 2D Semiconductor with Both Large Direct Gap and Superior Mobility

A new two-dimensional phosphorus nitride monolayer (<i>P</i>2₁/<i>c</i>-PN) with distinct structural and electronic properties is predicted based on first-principle calculations. Unlike pristine single-atom group V monolayers such as nitrogene, phosphorene, arsenene, and antimonene, <i>P</i>2₁/<i>c</i>-PN has an intrinsic direct band gap of 2.77 eV that is very robust against the strains. Strikingly, <i>P</i>2₁/<i>c</i>-PN shows excellent anisotropic carrier mobility up to 290 829.81 cm² V^–1 s^–1 along the <i>a</i> direction, which is about 18 times that in monolayer black phosphorus. This put <i>P</i>2₁/<i>c</i>-PN way above the general relation that carrier mobility is inversely proportional to bandgap, making it a very unique two-dimensional material for nanoelectronics devices

Unexpected Trend in Stability of Xe–F Compounds under Pressure Driven by Xe–Xe Covalent Bonds

Xenon difluoride is the first and the most stable of hundreds of noble-gas (Ng) compounds. These compounds reveal the rich chemistry of Ng’s. No stable compound that contains a Ng–Ng bond has been reported previously. Recent experiments have shown intriguing behaviors of this exemplar compound under high pressure, including increased coordination numbers and an insulator-to-metal transition. None of the behaviors can be explained by electronic-structure calculations with fixed stoichiometry. We therefore conducted a structure search of xenon–fluorine compounds with various stoichiometries and studied their stabilities under pressure using first-principles calculations. Our results revealed, unexpectedly, that pressure stabilizes xenon–fluorine compounds selectively, including xenon tetrafluoride, xenon hexafluoride, and the xenon-rich compound Xe₂F. Xenon difluoride becomes unstable above 81 GPa and yields metallic products. These compounds contain xenon–xenon covalent bonds and may form intercalated graphitic xenon lattices, which stabilize xenon-rich compounds and promote the decomposition of xenon difluoride

Electron Counting and a Large Family of Two-Dimensional Semiconductors

In comparison with conventional semiconductors, most two-dimensional semiconductor (2DSC) materials are dissimilar in structure and composition. Herein, we use electron-counting rules to propose a large family of 2DSCs, which all adopt the same structure and are composed of solely main group elements. Advanced density functional theory calculations are used to predict a number of novel 2DSCs, and we show that they span a large range of lattice constants, band gaps, and band edge states. As a result, they are good candidate materials for heterojunctions. This family of two-dimensional materials may be instrumental in the fabrication of 2DSC devices that may rival the currently employed 3D semiconductors

Honeycomb Boron Allotropes with Dirac Cones: A True Analogue to Graphene

We propose a series of planar boron allotropes with honeycomb topology and demonstrate that their band structures exhibit Dirac cones at the K point, the same as graphene. In particular, the Dirac point of one honeycomb boron sheet locates precisely on the Fermi level, rendering it as a topologically equivalent material to graphene. Its Fermi velocity (<i>v</i>_f) is 6.05 × 10⁵ m/s, close to that of graphene. Although the freestanding honeycomb B allotropes are higher in energy than α-sheet, our calculations show that a metal substrate can greatly stabilize these new allotropes. They are actually more stable than α-sheet sheet on the Ag(111) surface. Furthermore, we find that the honeycomb borons form low-energy nanoribbons that may open gaps or exhibit strong ferromagnetism at the two edges in contrast to the antiferromagnetic coupling of the graphene nanoribbon edges