Thermodynamic Stability of Boron: The Role of Defects and Zero Point Motion

Its low weight, high melting point, and large degree of hardness make elemental boron a technologically interesting material. The large number of allotropes, mostly containing over a hundred atoms in the unit cell, and their difficult characterization challenge both experimentalists and theoreticians. Even the ground state of this element is still under discussion. For over 30 years, scientists have attempted to determine the relative stability of α- and β-rhombohedral boron. We use density functional calculations in the generalized gradient approximation to study a broad range of possible β-rhombohedral structures containing interstitial atoms and partially occupied sites within a 105 atoms framework. The two most stable structures are practically degenerate in energy and semiconducting. One contains the experimental 320 atoms in the hexagonal unit cell, and the other contains 106 atoms in the triclinic unit cell. When populated with the experimental 320 electrons, the 106 atom structure exhibits a band gap of 1.4 eV and an in-gap hole trap at 0.35 eV above the valence band, consistent with known experiments. The total energy of these two structures is 23 meV/B lower than the original 105 atom framework, but it is still 1 meV/B above the α phase. Adding zero point energies finally makes the β phase the ground state of elemental boron by 3 meV/B. At finite temperatures, the difference becomes even larger

Theoretical Study of the Stable Radicals Galvinoxyl, Azagalvinoxyl and Wurster’s Blue Perchlorate in the Solid State

Calculations on crystalline organic radicals were performed to establish the ground states of these materials. These calculations show that the radicals may interact, depending on their orientation in the crystal structure. For galvinxoyl, a second structure is proposed which is similar to that of azagalvinoxyl, in which the radicals form pairs. This structure accounts for the anomalous magnetic properties of galvinoxyl at low temperatures

Ferromagnetic Order from p-Electrons in Rubidium Oxide

Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO2). The anion vacancies in the resulting phase with composition RbO1.72 provide greater structural flexibility compared to RbO2 and facilitate a Jahn−Teller-driven order−disorder transition involving the anion orientations at ∼230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below ∼50 K. A ferromagnetic cluster-glass state then forms below ∼20 K, embedded in an antiferromagnetic matrix that orders at ∼5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides

Ferromagnetic Order from p-Electrons in Rubidium Oxide

Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO2). The anion vacancies in the resulting phase with composition RbO1.72 provide greater structural flexibility compared to RbO2 and facilitate a Jahn−Teller-driven order−disorder transition involving the anion orientations at ∼230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below ∼50 K. A ferromagnetic cluster-glass state then forms below ∼20 K, embedded in an antiferromagnetic matrix that orders at ∼5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides

Effect of Vacancies on Magnetism, Electrical Transport, and Thermoelectric Performance of Marcasite FeSe_2−δ (δ = 0.05)

The marcasite structure FeSe_2−δ was synthesized using a simple solvothermal method. Systematic study of the electrical transport properties shows that the transport is dominated by variable-range hopping (VRH), with a changeover from Mott VRH at higher temperature to Efros-Shklovskii VRH for temperatures lower than the width of the Coulomb gap. This also confirms the presence of a Coulomb gap in the density of states at the Fermi energy. We observe that Yttrium doping increases the electrical conductivity dramatically without significantly reducing the Seebeck coefficient. This results in remarkably high power factors for thermoelectric performance in the regime where the mean hopping energy shifts from defect dominated to Coulomb repulsion dominated. High resolution transmission electron microscopy, in combination with theoretical calculations, proves the narrowing of the band gap by introducing Se vacancies. This leads to a good conductivity and is responsible for the excellent thermoelectric performance. The formation of nanoclusters, resulting from Se vacancies, is responsible for a dense system of stacking faults and the generally reported weak ferrimagnetism. This also determines the transition between the different electrical transport mechanisms and contributes to the improved thermoelectric performance

Anionogenic Mixed Valency in K_<i>x</i>Ba_1–<i>x</i>O_2−δ

We have synthesized members of an isostructural solid solution series K_<i>x</i>Ba_1–<i>x</i>O_2−δ (<i>x</i> < 0.41, δ < 0.11) containing mixed-valent dioxygen anions. Synthesis in liquid ammonia solution allows a continuous range of compounds to be prepared. X-ray and neutron diffraction show that K_<i>x</i>Ba_1–<i>x</i>O_2−δ adopts the tetragonal rocksalt-derived structure of the end members KO₂ and BaO₂, without any structural phase transition down to 5 K, the lowest temperature studied here. We identify four oxygen–oxygen stretching modes above 750 cm^–1 in the measured Raman spectra, unlike the spectra of KO₂ and BaO₂ which both contain just a single mode. We use density functional theory calculations to show that the stretching modes in K_<i>x</i>Ba_1–<i>x</i>O_2−δ arise from in-phase and anti-phase coupling of the stretching of nearest-neighbor oxygen dimers when the valence state of the dimers lies between −1 and −2 because of mixed cation coordination. This coupling is a direct signature of a novel type of anionogenic mixed valency

Metal–Insulator Transition Induced by Spin Reorientation in Fe₇Se₈ Grain Boundaries

Fe₇Se₈ exists as a hexagonal NiAs-like crystal structure with a large number of ordered intrinsic vacancies. It is an ideal candidate for studying the effect of defects on properties such as magnetism and electrical transport. In this work, highly crystalline Fe₇Se₈ with the 3c crystal structure was synthesized by a solid-state reaction. Sharp changes in the magnetization at 100 K confirm a rotation of the spins from the <i>ab</i> plane to the <i>c</i> axis with decreasing temperature. We observe an interesting metal–insulator transition at the same temperature as the spin-direction changes. We propose that locked spins in the grain boundaries induce electron localization and result in the metal–insulator transition. Electron localization is confirmed by X-ray photoelectron spectroscopy of the Fe 2p peaks, which exhibit two characteristic satellite peaks. This mechanism is also verified by comparing it with the properties of the 4c-Fe₇Se₈ crystal structure

High-Purity Fe₃S₄ Greigite Microcrystals for Magnetic and Electrochemical Performance

High-purity Fe₃S₄ (greigite) microcrystals with octahedral shape were synthesized via a simple hydrothermal method using a surfactant. The as-prepared samples have the inverse spinel structure with high crystallinity. The saturation magnetization (<i>M</i>_s) reaches 3.74 μ_B at 5 K and 3.51 μ_B at room temperature, which is larger than all reported values thus far. Electrical transport measurements show metallic behavior with a resistivity 40 times lower than in any previous report. The potential use of greigite as an anode in lithium-ion batteries was investigated by cyclic voltammery and galvanostatic discharge–charge cycling on as-prepared samples. The discharge capacity was 1161 mAh/g in the first cycle and 563 mAh/g in the 100th cycle. This excellent electrochemical performance can be attributed to the high purity, crystallinity, and favorable morphology of the products