Native Defects and the Dehydrogenation of NaBH₄

Chemical reactions of hydrogen storage materials often involve mass transport through a bulk solid. Diffusion in crystalline solids proceeds by means of lattice defects. Using density functional theory (DFT) calculations, we identify the stability and the mobility of the most prominent lattice defects in the hydrogen storage material NaBH4. At experimental dehydrogenation conditions, the Schottky defects of missing Na+ and BH4– ions form the main vehicle for mass transport in NaBH4. Substituting a BH4– by a H– ion yields the most stable defect, locally converting NaBH4 into NaH. Such a substitution most likely occurs at the surface of NaBH4, releasing BH3. Adding Mg or MgH2 to NaBH4 promotes this scenario

Hydrogen Storage by Polylithiated Molecules and Nanostructures

We study polylithiated molecules as building blocks for hydrogen storage materials, using first-principles calculations. CLi4 and OLi2 bind 12 and 10 hydrogen molecules, respectively, with an average binding energy of 0.10 and 0.13 eV, leading to gravimetric densities of 37.8 and 40.3 wt % of H2. Bonding between Li and C or O is strongly polar and H2 molecules attach to the partially charged Li atoms without dissociating, which is favorable for (de)hydrogenation kinetics. CLin and OLim molecules can be chemically bonded to graphene sheets to hinder the aggregation of such molecules. In particular B- or Be-doped graphene strongly bind the molecules without seriously affecting the hydrogen binding energy. This still leads to a hydrogen storage capacity in the range of 5−8.5 wt % of H2

First-Principles Study of LiBH₄ Nanoclusters and Their Hydrogen Storage Properties

Recent experimental studies suggest faster desorption kinetics, improved reversibility, and more favorable thermodynamics for confined LiBH₄ nanoparticles as compared to bulk. We study the structures, total energies, and decomposition reactions of LiBH₄ nanoparticles using density functional theory calculations. We find that the reaction energies of nanoclusters with a diameter ≳2 nm are very close to that of bulk LiBH₄. Only very small clusters with a diameter <1 nm are significantly destabilized relative to the bulk. The thermodynamics of such small clusters is unfavorable, however, and leads to dehydrogenation temperatures that are higher than that of the bulk. Although small (LiBH₄)_<i>n</i> nanoclusters exhibit a number of different geometries, they show only little variation in the total energy per formula unit. Of all possible decomposition reactions of (LiBH₄)_<i>n</i>, the reaction where diborane is released, is unfavorable for most cluster sizes, whereas the hydrogen desorption reaction to Li₂H₁₂B₁₂ is most favorable. This suggests that the experimentally observed improvement of the (de)­hydrogenation properties of LiBH₄ can be attributed to an improvement of the kinetics of the latter reaction

Model for the Formation Energies of Alanates and Boranates

We develop a simple model for the formation energies (FEs) of alkali and alkaline earth alanates and boranates, based upon ionic bonding between metal cations and AlH4- or BH4- anions. The FEs agree well with values obtained from first principles calculations and with experimental FEs. The model shows that details of the crystal structure are relatively unimportant. The small size of the BH4- anion causes a strong bonding in the crystal, which makes boranates more stable than alanates. Smaller alkali or alkaline earth cations do not give an increased FE. They involve a larger ionization potential that compensates for the increased crystal bonding

A Density Functional Study of α-Mg(BH₄)₂

Boranates (tetrahydroborates) are studied intensively because of their potential use as hydrogen storage materials. In this Article, we present a first-principles study of α-Mg(BH4)2 at the level of density functional theory. We optimize the complex structure of α-Mg(BH4)2, starting from the experimental crystal structures with 30 formula units per unit cell. From total energy calculations, incorporating vibrational contributions at finite temperature, we show that the hydrogen desorption reaction α-Mg(BH4)2 → MgB2 + 4H2 has a reaction enthalpy of 38 kJ/mol H2 at room temperature. This makes Mg(BH4)2 an interesting candidate as a hydrogen storage material

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

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