Jorge Botana

Mercury under Pressure acts as a Transition Metal: Calculated from First Principles†

The inclusion of Hg among the transition metals is readily debated. Recently, molecular HgF4 was synthesized in a low-temperature noble gas but the potential of Hg to form compounds beyond a +2 oxidation state in a stable solid remains unresolved. We propose high-pressure techniques to prepare unusual oxidation states of Hg-based compounds. Using an advanced structure search algorithm and first-principles electronic structure calculations, we find that under high pressure Hg in Hg-F compounds transfers charge from the d orbitals to the F, thus behaving as a transition metal. Oxidizing Hg to +4 and +3 yielded the thermodynamically stable compounds HgF4 and HgF3. The former consists of HgF4 planar molecules, a typical geometry for d(8) metal centers. HgF3 is metallic and ferromagnetic owing to the d(9) configuration of Hg, with a large gap between its partially occupied and unoccupied bands under high pressure.

Pressure-stabilized lithium caesides with caesium anions beyond the −1 state

Main group elements usually assume a typical oxidation state while forming compounds with other species. Group I elements are usually in the +1 state in inorganic materials. Our recent work reveals that pressure may make the inner shell 5p electrons of Cs reactive, causing Cs to expand beyond the +1 oxidation state. Here we predict that pressure can cause large electron transfer from light alkali metals such as Li to Cs, causing Cs to become anionic with a formal charge much beyond -1. Although Li and Cs only form alloys at ambient conditions, we demonstrate that these metals form stable intermetallic LinCs (n=1-5) compounds under pressures higher than 100 GPa. Once formed, these compounds exhibit interesting structural features, including capped cuboids and dimerized icosahedra. Finally, we explore the possibility of superconductivity in metastable LiCs and discuss the effect of the unusual anionic state of Cs on the transition temperature.

Inner-shell Chemistry under High Pressure

Chemistry at ambient conditions has implicit boundaries rooted in the atomic shell structure: the inner-shell electrons and the unoccupied outer-shell orbitals do not contribute as the major component to chemical reactions and in chemical bonds. These general rules govern our understanding of chemical structures and reactions. We review the recent progresses in high-pressure chemistry demonstrating that the above rules can be violated under extreme conditions. Using a first principles computation method and crystal structure search algorithm, we demonstrate that stable compounds involving inner shell electrons such as CsF3, CsF5, HgF3, and HgF4 can form under high external pressure and may present exotic properties. We also discuss experimental studies that have sought to confirm these predictions. Employing our recently developed hard X-ray photochemistry methods in a diamond anvil cell, we show promising early results toward realizing inner shell chemistry experimentally.

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 (vf) 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.

Reactivity of He with ionic compounds under high pressure

Until very recently, helium had remained the last naturally occurring element that was known not to form stable solid compounds. Here we propose and demonstrate that there is a general driving force for helium to react with ionic compounds that contain an unequal number of cations and anions. The corresponding reaction products are stabilized not by local chemical bonds but by long-range Coulomb interactions that are significantly modified by the insertion of helium atoms, especially under high pressure. This mechanism also explains the recently discovered reactivity of He and Na under pressure. Our work reveals that helium has the propensity to react with a broad range of ionic compounds at pressures as low as 30 GPa. Since most of the Earth’s minerals contain unequal numbers of positively and negatively charged atoms, our work suggests that large quantities of He might be stored in the Earth’s lower mantle.Helium was long thought to be unable to form stable solid compounds, until a recent discovery that helium reacts with sodium at high pressure. Here, the authors demonstrate the driving force for helium reactivity, showing that it can form new compounds under pressure without forming any local chemical bonds.

Unexpected Xe anions in XeLin intermetallic compounds

The reactivity of Xe is important in both fundamental chemistry and geological science. The discovery of the reductive reactivity of Xe extended the doctrinal boundary of chemistry for which a completed shell is inert to reaction. The oxidation of Xe by various elements has been explored. On the other hand, the opposite chemical inclination, i.e. , gaining electrons and forming anions, has not been thoroughly studied for Xe or other noble-gas elements. In this work, we demonstrate, using first-principles calculations and an efficient structure prediction method, that Xe can form stable compounds under high pressure. These compounds are intermetallic and Xe are negatively charged. The stability of these compounds indicates that atoms or ions with completely filled shell may still gain electrons in chemical reactions.

Microporosity as a new property control factor in graphene-like 2D allotropes

We have performed a systematic study on the effect of microporosity on the energy stability and electronic properties of graphene-like materials, including 2D C, B and B–C allotropes. We used a modified crystal structure-search method to yield planar structures by systematically varying microporosity. Our results show that the energy stability of allotropes is strongly correlated with the microporosity and that the trends for C and B allotropes are qualitatively different. The formation energies of the most stable C allotropes with the same microporosity increase quickly with the microporosity, which shows a parabolic relationship. In contrast, B allotropes show a much weaker and linear dependence on microporosity. Our calculations also reveal that basic electronic properties such as metallicity and band gaps also depend strongly on the porosity. The allotropes with low microporosity and low formation energies tend to be metallic, and the semiconducting allotropes appear more often with high microporosity. In contrast to the general trend, our study identified a C allotrope with low microporosity that is quite stable and has a large band gap of 0.58 eV.

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

Novel phases of LinI (n = 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.