High-Pressure Electrides: The Chemical Nature of Interstitial Quasiatoms

Magnetic order of Nd5Pb3 single crystals

We report millimeter-sized Nd₅Pb₃ single crystals grown out of a Nd-Co flux. We experimentally study the magnetic order of Nd₅Pb₃ single crystals by measuring the anisotropic magnetic properties, electrical resistivity under high pressure up to 8 GPa, specific heat, and neutron single crystal diffraction. Two successive magnetic orders are observed at T _N1  =  44 K and T _N2  =  8 K. The magnetic cells can be described with a propagation vector [Formula: see text]. Cooling below T _N1, Nd1 and Nd3 order forming ferromagnetic stripes along the b-axis, and the ferromagnetic stripes are coupled antiferromagnetically along the a-axis for the [Formula: see text] magnetic domain. Cooling below T _N2, Nd2 orders antiferromagnetically to nearby Nd3 ions. All ordered moments align along the crystallographic c-axis. The magnetic order at T _N1 is accompanied by a quick drop of electrical resistivity upon cooling and a lambda-type anomaly in the temperature dependence of specific heat. At T _N2, no anomaly was observed in electrical resistivity but there is a weak feature in specific heat. The resistivity measurements under hydrostatic pressures up to 8 GPa suggest a possible phase transition around 6 GPa. Our first-principles band structure calculations show that Nd₅Pb₃ has the same electronic structure as does Y₅Si₃ which has been reported to be a one-dimensional electride with anionic electrons that do not belong to any atom. Our study suggests that R ₅Pb₃ (R  =  rare earth) can be a materials playground for the study of magnetic electrides. This deserves further study after experimental confirmation of the presence of anionic electrons.

Tiered Electron Anions in Multiple Voids of LaScSi and Their Applications to Ammonia Synthesis

Electrides-compounds in which electrons localized in interstitial spaces periodically serve as anions-have attracted broad attention for their exotic properties, such as extraordinary electron-donating ability. In our efforts to expand this small family of phases, LaScSi emerges as a promising candidate. Its electron count is 2e^- f.u.^-1 in excess of that expected from the Zintl concept, while its structure offers interstitial spaces that can accommodate these extra electrons. Herein, this potential is explored through density functional theory (DFT) calculations and property measurements on LaScSi. DFT calculations (validated by heat capacity and electrical transport measurements) reveal electron density peaks at two symmetry-distinct interstitial sites. Importantly, this electride-like character is combined with chemical stability in air and water, an advantage for catalysis. Ru-loaded LaScSi shows outstanding catalytic activity for ammonia synthesis, with a turnover frequency (0.1 s^-1 at 0.1 MPa, 400 °C) an order of magnitude higher than those of oxide-based Ru catalysts, e.g., Ru/MgO. As with other electrides, LaScSi's ability to reversibly store hydrogen prevents the hydrogen poisoning of Ru surfaces. The better performance of LaScSi, however, hints at the importance of the high concentration (>1.6 × 10²² cm^-3 ) and tiered nature of its anionic electrons, which offers guidance toward new catalysts.

Cubic Fluorite-Type CaH2 with a Small Bandgap

A cubic variant of CaH₂ adopting a fluorite-type crystal structure was synthesized by cationic substitution with La or Y, yielding the first alkaline earth hydride-based with fluorite-type framework. The material has a bandgap of ∼2.5 eV (greenish yellow in color), which is much smaller than that of orthorhombic PbCl₂-type CaH₂ (4.4 eV) and is, in fact, the smallest among alkaline or alkaline earth metal hydrides reported to date. Analysis of the density functional theory band structure of cubic-CaH₂ indicates that its conduction band minimum is formed mainly by the interaction between the Ca 3d e_g orbitals around the crystallographic cavity defined by cubes of H^- ions. The use of such cavities in the creation of low-lying conduction band minima by semiconductors is extremely rare, and has similarities to inorganic electrides.

Cesium's Off-the-Map Valence Orbital

The Td -symmetric [CsO4 ]+ ion, featuring Cs in an oxidation state of 9, is computed to be a minimum. Cs uses outer core 5s and 5p orbitals to bind the oxygen atoms. The valence Cs 6s orbital lies too high to be involved in bonding, and contributes to Rydberg levels only. From a molecular orbital perspective, the bonding scheme is reminiscent of XeO4 : an octet of electrons to bind electronegative ligands, and no low-lying acceptor orbitals on the central atom. In this sense, Cs+ resembles hypervalent Xe.

The exemplary role of nanoconfinement in the proton transfer from acids to ammonia

Proton transfer processes from mineral acids to bases (HX, where X = F, Cl, Br and I to ammonia) are normally feasible in solution and they cannot spontaneously occur in the gas phase. We demonstrate that this process can be feasible under nanoconfinement without using any solvent molecules. More interestingly, in contrast to the general observation, halide ions except fluoride behave like protons under high confinement, leading to the formation of NH₃X instead of NH₄ ions. The triggering transformation of hydrogen bonded to the proton transferred complex under nanoconfinement is explained based on the thermodynamic quantity, static pressure.

Crystal Structures of CaB3N3 at High Pressures

Using global structure searches, we have explored the structural stability of CaB₃N₃, a compound analogous to CaC₆, under pressure. There are two high-pressure phases with space groups R3c and Amm2 that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC₆. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the R3c phase is a semiconductor with a band gap of 2.21 eV and that the Amm2 phase is a semimetal. These findings help advance our understanding of the Ca-B-N ternary system.

The future of topological analysis in experimental charge-density research

In a recent paper, Dittrich (2017) critically discussed the benefits of analysing experimental electron density within the framework of the quantum theory of atoms in molecules, often called simply the topological analysis of the charge density. The point he raised is important because it challenges the scientific production of a very active community. The question whether this kind of investigation is still sensible is intriguing and it fosters a multifaceted answer. Granted that none can predict the future of any field of science, but an alternative point of view emerges after answering three questions: Why should we investigate the electron charge (and spin) density? Is the interpretative scheme proposed by the quantum theory of atoms in molecules useful? Is an experimental charge density necessary?

Thermodynamic cycles of the alkali metal–ligand complexes central to electride formation

Alkali metal–ligand complexes are the building blocks of the exotic organic alkalide and electride materials.

The explicit examination of the magnetic states of electrides

Electrides are a unique class of ionic solids in which the anions are stoichiometrically replaced by electrons localised within the crystal voids. We present the first all electron magnetic state calculations for electrides and show the magnetic properties of these materials come from the localised electrons.

Theoretical study of electronic and mechanical properties of Fe2B

Fe₂B is ultra-incompressible under static pressure but rather flexible under uniaxial or shear strain conditions.