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Self-consistent density functional calculations of the hydroxylated (0001) corundum surfaces are presented. It is demonstrated that the hydroxylated surfaces are the most stable under most, but not all, conditions. Hydroxylation significantly lowers the surface free energy of α-alumina. The stability of the hydrated surface resolves the discrepancies between the morphology of the α-alumina (0001) surface observed under ultra-high vacuum, and at ambient conditions. A method for the calculation of the equilibrium surface stoichiometry is proposed. The proposed approach provides a valuable connection between theoretical calculations and experiments with metal oxides. [ABSTRACT FROM AUTHOR]

We report density functional theory calculations of the interaction of Cu and Pd with the (0001) surface of α-Al[sub 2]O[sub 3]. The interaction of those metals with the oxide surface varies from covalent-like for the aluminum rich surface to ionic-like for the oxygen terminated surface. Stoichiometric hydroxylation of the surface does not increase the metal-oxide interaction significantly. We suggest that defects created upon hydroxylation are the main mechanism to explain the observed metal wetting of the oxide. © 2001 American Institute of Physics. [DOI: 10.1063/1.1421107]. [ABSTRACT FROM AUTHOR]

Effective equations of motion of a surface atom placed in an anharmonic surface potential are derived for three kinds of substrates (i) continuous dispersionless, (ii) discrete with nearest neighbours harmonic interactions, and (iii) a substrate showing a strong spatial dispersion. The resulting equations of motion are differential equations in the case (i) and Volterra Integro-Differential Equations (VIDE) in the cases (ii) and (iii). Examples of non-linear surface response to phonons coming from the bulk are given for the dispersionless substrate. [ABSTRACT FROM AUTHOR]

LiBH4 has been discussed as a promising hydrogen storage material and as a solid-state electrolyte in lithium-ion batteries. It contains 18.5 wt% hydrogen and undergoes a structural phase transition at 381 K which is associated with a large increase in rotational disorder of the [BH4]− anion and the increase of [Li]+ conductivity by three orders of magnitude. We investigated the [BH4]− anion dynamic in bulk LiBH4, in LiBH4-LiI solid solutions and in nano-confined LiBH4 by quasielastic neutron scattering, complemented by DFT calculations. In all cases the H-dynamics is dominated by thermally activated rotational jumps of the [BH4]− anion in the terahertz range. The addition of LiI as well as nano-confinement favours the disordered high temperature phase and lowers the phase transition below room temperatures. The results are discussed on the basis of first principles calculations and in relation to ionic conductivity of [Li]+.

Provision of atomic hydrogen by hydrogen dissociation catalysts only moderately accelerates the hydrogenation rate of magnesium. They shed light on this well-known but technically challenging fact through a combined approach using an unconventional surface science technique together with Density Functional Theory (DFT) calculations. The calculations demonstrate the drastic electronic structure changes during transformation of Mg to MgH 2 , which make fractional hydrogen coverage on the surface, as well as substoichiometric hydrogen content in the bulk energetically unfavorable. Reflecting Electron Energy Loss Spectroscopy (REELS) is used to measure the surface and bulk plasmon during hydrogen sorption in magnesium. The measurements show that the hydrogenation proceeds via the growth of magnesium hydride without the presence of chemisorbed hydrogen on the metallic magnesium surface exactly as indicated by the calculations. This is due to the low stability of sub-stoichiometric amounts of chemisorbed H correlating with the unfavorable charge state of Mg. They are merely bound to the unchanged adjacent Mg layers, thereby explaining the failure of classical hydrogenation catalysts, which effectively only hydrogenate Mg in their direct vicinity. The acceleration of hydrogen sorption kinetics in Mg must affect the polarization in the interface between Mg and MgH 2 during hydrogenation., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

The crystal structures of three thermal polymorphs (I, II, and III) for each isomer of closo -dicarbadodecaboranes C 2 B 10 H 12 ( ortho , meta, and para ) have been determined by combining synchrotron radiation X-ray powder diffraction and density functional theory calculations. The structures are in agreement with previous calorimetric and spectroscopic studies. The difference between rotatory phases (plastic crystals) I and II lies in isotropic rotations in the former and anisotropic rotations of the icosahedral clusters in the latter. Phase I is the cubic close packing ( ccp ) of rotating closo -molecules C 2 B 10 H 12 in the space group Fm 3̅. Phase II is the ccp of rotating closo -molecules C 2 B 10 H 12 in the cubic space group Pa 3̅. The preferred rotational axis in II varies with the isomer. The ordered phases III are orthorhombic ( meta ) or monoclinic ( ortho and para ) deformations of the cubic unit cell of the disordered phases I and II. The ordering in the phase III of the ortho -isomer carrying the biggest electrical dipole moment creates a twofold superstructure w.r.t. the cubic unit cell. The thermal polymorphism for C 2 B 10 H 12 and related metal salts can be explained by division of the cohesive intercluster interactions into two categories (i) dispersive cohesive interaction with additional Coulombic components in the metal salts and (ii) anisotropic local interaction resulting from nonuniform charge distribution around icosahedral clusters. The local interactions are averaged out by thermally activated cluster dynamics (rotations and rotational jumps) which effectively increase the symmetry of the cluster. The C 2 B 10 H 12 molecules resist at least as well as the CB 11 H 12 - anion to the oxidation, and both clusters form easily a mixed compound. This allows designing solid electrolytes such as Na x (CB 11 H 12 ) x (C 2 B 10 H 12 ) 1 - x , where the cation content may be varied and the temperature of transition into the disordered conducting phase is decreased.

In this work, we report on the structural properties of alkali hydrido- closo -(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K 2 B 12 H 12 from Fm 3̅ to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li 2 B 10 H 10 was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group-subgroup relationship, with the notable exception of the room-temperature phase of Li 2 B 10 H 10 .

Titanium is an excellent getter material, catalyzes gas-solid reactions such as hydrogen absorption in lightweight metal hydrides and complex metal hydrides and has recently been shown as a potential ammonia synthesis catalyst. However, knowledge of the surface properties of this metal is limited when it absorbs large quantities of hydrogen at operation conditions. Both the conceptual description of such a surface as well as the experimental determination of surface hydrogen concentration on hydride-forming metals is challenging due to the dynamic bulk properties and the incompatibility of traditional surface science methods with the hydrogen pressure needed to form the metal hydride, respectively. In this paper, the surface pressure-composition isotherms of the titanium-hydrogen system are measured by operando reflecting electron energy loss spectroscopy (REELS). The titanium thin films were deposited on and hydrogenated through a palladium membrane, which provides an atomic hydrogen source under ultrahigh vacuum conditions. The measurements are supported by density functional theory calculations providing a complete picture of the hydrogen-deficient surface of TiH 2 being the basis of its high catalytic activity., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Li et al commented that our report claims that methods reported thus far cannot enable the production of high-purity corundum with surface areas greater than 100 m 2 g -1 , and that our obtained material could be porous aggregates rather than nanoparticles. We disagree with both of these suggestions., (Copyright © 2020, American Association for the Advancement of Science.)

Hydrogen-containing materials are of fundamental as well as technological interest. An outstanding question for both is the amount of hydrogen that can be incorporated in such materials, because that determines dramatically their physical properties such as electronic and crystalline structure. The number of hydrogen atoms in a metal is controlled by the interaction of hydrogens with the metal and by the hydrogen-hydrogen interactions. It is well established that the minimal possible hydrogen-hydrogen distances in conventional metal hydrides are around 2.1 Å under ambient conditions, although closer H-H distances are possible for materials under high pressure. We present inelastic neutron scattering measurements on hydrogen in [Formula: see text] showing nonexpected scattering at low-energy transfer. The analysis of the spectra reveals that these spectral features in part originate from hydrogen vibrations confined by neighboring hydrogen at distances as short as 1.6 Å. These distances are much smaller than those found in related hydrides, thereby violating the so-called Switendick criterion. The results have implications for the design and creation of hydrides with additional properties and applications., Competing Interests: The authors declare no competing interest.

In its nanoparticulate form, corundum (α-Al 2 O 3 ) could lead to several applications. However, its production into nanoparticles (NPs) is greatly hampered by the high activation energy barrier for its formation from cubic close-packed oxides and the sporadic nature of its nucleation. We report a simple synthesis of nanometer-sized α-Al 2 O 3 (particle diameter ~13 nm, surface areas ~140 m 2 g -1 ) by the mechanochemical dehydration of boehmite (γ-AlOOH) at room temperature. This transformation is accompanied by severe microstructural rearrangements and might involve the formation of rare mineral phases, diaspore and tohdite, as intermediates. Thermodynamic calculations indicate that this transformation is driven by the shift in stability from boehmite to α-Al 2 O 3 caused by milling impacts on the surface energy. Structural water in boehmite plays a crucial role in generating and stabilizing α-Al 2 O 3 NPs., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

closo-Borates, such as Na 2 B 12 H 12 , are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na 2 B 12 H 12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.

Solid state closo-borane salts of alkali metals have very high ionic conductivity. This makes them interesting for practical applications as solid state electrolytes, and has triggered extensive research efforts. Improvement and understanding of their properties require accurate theoretical description of their static and dynamical properties. In this work, we report accuracy assessment of density functional theory in the description of solids with B12H122- anions. We show that these aromatic anions interact via weak dispersive forces. For that reason, non-local exchange-correlation functionals give better description of structural properties and phonons in Li2B12H12 and Na2B12H12. Numerically efficient semi-local methods provide satisfactory results when applied in structure volumes obtained in a non-local method. An extensive structural search for stable crystalline phases of MgB12H12 predicts a new denser lattice with C2/c symmetry that is stabilized by van der Waals interactions. These structures might be discovered as anhydrous MgB12H12 in high pressure experiments, avoiding the amorphous state at ambient pressures.

Coordination complexes of magnesium borohydride show promising properties as solid electrolytes for magnesium ion batteries and warrant a thorough microscopic description of factors governing their mobility properties. Here, the dynamics of Mg(BH 4 ) 2 -diglyme 0.5 on the atomic level are investigated by means of quasielastic neutron scattering supported by density functional theory calculations and IR and NMR spectroscopy. Employing deuterium labeling, we can unambiguously separate all the hydrogen-containing electrolyte components, which facilitate Mg 2+ transport, and provide a detailed analytical description of their motions on the picosecond time scale. The planar diglyme chain coordinating the central Mg atom appears to be flexible, while two dynamically different groups of [BH 4 ] - anions undergo reorientations. The latter has important implications for the thermal stability and conductivity of Mg(BH 4 ) 2 -diglyme 0.5 and demonstrates that the presence of excess Mg(BH 4 ) 2 units in partially chelated Mg complexes may improve the overall performance of related solid-state electrolytes.

Nickel closo-dodecaborate NiB12H12 was prepared by mechanosynthesis (ball milling) of mixtures of Na2B12H12 + NiCl2 followed by hydration and drying under dynamic vacuum. The crystal structures of hydrated and anhydrous closo-dodecaborates were characterized by temperature dependent synchrotron radiation X-ray powder diffraction, ab initio calculations, thermal analysis and infrared spectroscopy. Three different water containing complexes were found: a homoleptic octahedral complex in Ni(H2O)6B12H12 crystallizing in two different deformation variants of a complex centred closo-dodecaborate cube, and a heteroleptic octahedral complex in Ni(H2O)4B12H12 containing four water molecules and two hydrogens and centring also a deformed closo-dodecaborate cube. Anhydrous nickel closo-dodecaborate was obtained by drying the hydrated sample under dynamic vacuum. It crystallizes with bcc packing of B12H122- anions and Ni2+ is disordered close to the triangular face of the tetrahedral interstice coordinated by a H5 square pyramid.