173 research outputs found

    B-12(SCN)(12)(-): An Ultrastable Weakly Coordinating Dianion

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    Stable dianions that are weakly coordinating with metal ions are not common. In this work, we show that the thiocyanate SCN- anion, known for its detoxification property of cyanide CN- and antidegradation property of perovskite solar cell materials, can also be used to produce a new set of weakly coordinating B-12(SCN)(12)(-) dianion complexes which are potential candidates for the anionic part inside the electrolytes of metal-ion, especially the magnesium-ion-based, batteries

    Rational design of super-alkalis and their role in CO2 activation

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    Super-alkalis are clusters of atoms. With ionization potentials smaller than those of the alkali atoms, they are playing an increasing role in chemistry as highlighted by recent applications in solar cells as well as in Li-ion batteries. For the past 40 years superalkalis were designed using inorganic elements with the sp orbital character. Here, we show that a large class of superalkalis composed of only simple metal atoms, transition metal complexes as well as organic molecules can be designed by making use of electron counting rules beyond the octet rule. Examples include Al-3(+), Mn(B3N3H6)(2)(+), B9C3H12+, and C5NH6+ which obey the jellium shell closure rule, the 18-electron rule, the Wade-Mingos rule, and Huckel\u27s aromatic rule, respectively. We further show that the ability of superalkalis to transfer an electron easily can be used to activate a CO2 molecule by transforming it from a linear to a bent structure. These results, based on density functional theory with generalized gradient approximation for exchange-correlation potential, open the door to a new class of catalysts for CO2 activation

    Li- and B-decorated cis-polyacetylene: A computational study

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    By using density functional theory and the generalized gradient approximation, we show that Li-decorated cis-polyacetylene meets some of the requirements of an ideal hydrogen storage material. Unlike Ti-doped cis-polyacetylene, Li resists clustering and can reversibly store up to 10.8 wt %hydrogen in molecular form. However, molecular dynamics simulations show that Li can retain hydrogen only at cryogenic temperatures. On the other hand, B-doped cis-polyacetylene can store up to 7.5 wt % hydrogen, but it binds to hydrogen too strongly to be suitable for room temperature applications. The results are compared to those in Ti-doped cis-polyacetylene

    Origin of the anatase to rutile conversion of metal-doped TiO2

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    Extensive calculations using density functional theory enable us to explain the origin of the surprising room-temperature conversion of anatase to rutile phase of TiO2 when doped with Co and Ni, but not with Cu. Contrary to earlier suggestion, neither high spin nor strain of the transition metals is found to be responsible for this phase conversion. The driving mechanism, instead, is attributed to the increased interaction between Co and Ni atoms forming a linear chain in the rutile phase. We predict that Cr and Mn which have even larger spins than Co and Ni cannot induce this phase conversion

    First-principles study of magnetism in (112̄0) Zn1−xMnxO thin film

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    First-principles calculations of total energies and magnetism of Zn1−xMnxOthin film are performed by simulating it with a slab consisting of seven layers along (112̄0). It is shown that a single Mn atom shows very little preference for the site it occupies. This is consistent with the experimental finding that Mn atoms are homogeneously distributed in ZnO films. As the concentration of Mn atoms increases, antiferromagnetic coupling between Mn atoms becomes more favorable, and there is a tendency for Mn atoms to form clusters around oxygen, in agreement with recent experiments

    Effect of Ti and metal vacancies on the electronic structure, stability, and dehydrogenation of Na3AlH6: Supercell band-structure formalism and gradient-corrected density-functional theory

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    Electronic and structural properties of sodium-aluminum hexahydride (Na3AlH6) formed during the decomposition reaction of sodium alanate (NaAlH4) and the effects of Ti catalyst are studied using supercell approach and density-functional theory. The preferred site of Ti has been determined by substituting it at both the Na and Al sites and comparing the respective formation energies. The least unfavorable site for Ti is found to be the Al site. To examine the role of Ti substitution on the desorption of hydrogen, the energy cost to remove a H atom from the vicinity of Ti was calculated and compared with that from the pure Na3AlH6 The improvement in dehydrogenation of Na3AlH6 was found to be due to the weakening the Al-H bond caused by Ti substitution. We also studied the role of metal vacancies on hydrogen desorption. Although this desorption was exothermic, the energies to create these vacancies are high

    Magnetic hollow cages with colossal moments

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    A comprehensive study of the interaction of transition metal clusters with B, C, N, O, and Si reveal novel structure and properties: Co12C6, Mn12C6, and Mn24C18 clusters form stable ferromagnetic hollow cages with total magnetic moments of 14 μB, 38 μB, and 70 μB, respectively. Replacement of C with B, N, O, or Si has significant impact on their structure and magnetic properties. For example, Mn20Si12 cluster forms a ferrimagnetic dodecahedral hollow cage with a total magnetic moment of 36 μB while Mn12N6, X12C6 (X = Ni, Cu, Pd, Pt), and Cu12O6 possess no magnetic moment, although they retain hollow cage structures. Mn12B6 and Mn24Si18, on the other hand, form compact ferrimagnetic structures. Synthesis of hollow cage clusters with unique magnetic properties may lead to important applications

    Molecular view of the interfacial adhesion in aluminum‐silicon carbide metal‐matrix composites

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    The binding energies, electron charge transfer,bond lengths, and core level shifts of Al‐Al, Al‐Si, Al‐C, and Si‐C dimers have been calculated self‐consistently using the linear combination of atomic orbitals‐molecular orbital theory. The exchange interactions are treated using the unrestricted Hartree–Fock theory and correlation corrections are included through the Möller–Plesset perturbation scheme up to fourth order. The results are used to understand the nature and strength of bonding at the interface of Al and SiC crystals. The strong bonding of Al‐C dimers compared to Al‐Al and Al‐Si is shown to be responsible for the aluminumcarbide formation at the interface. The charge transfer between the constituent atoms in the dimer and the accompanying core level shifts are also shown to be characteristic of what has been observed at the Al/SiC interface

    (110) Facet of MgTe Zinc Blende Semiconductor: A Holy Grail for Modern Spintronics

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    In this work, we propose mono/few layers of MgTe (110) facets of zinc-blende structure for a unique next-generation ferromagnet-free nonballistic spin-field effect transistor. The atomic arrangement in this facet exhibits a unique combination of three basic symmetries found in nature: rotation, reflection, and translation. The electronic properties based on the density functional theory (DFT) reveal momentum-independent unidirectional spin polarization known as persistent spin texture (PST) that leads to an infinite spin lifetime in this particular facet. PST in these 2D structures originating from crystal symmetry makes them an ideal alternative to quantum well structures. Although the proposed Datta-Das s-FET is applicable to nonballistic s-FET, there has been little technological progress in this rapidly growing field of research and only a trivial amount of 2D semiconductors have been identified that exhibit PST intrinsically. In this regard, this particular facet studied in this work will be a great asset. A detailed theoretical insight into the origin of PST and its application in a ferromagnet-free s-FET has been proposed combining both the spin-Hall effect and inverse spin-Hall effects, harmonizing spintronics with conventional electronics

    Dehydrogenation mechanism in catalyst-activated MgH2

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    A small amount of Nb2O5 catalyst is known to substantially improve the desorption thermodynamics and kinetics of MgH2. Using density functional theory in combination with ab initio molecular dynamics simulation, we provide theoretical understanding of the mechanism of dehydrogenation in Nb doped MgH2. We show that the substitution of Nb at the Mg site followed by the clustering of H around Nb is a likely pathway for hydrogen desorption. We also find that dehydrogenation from the vicinity of Mg vacancies is exothermic. However, the vacancies are not likely to play a significant role in hydrogen desorption due to their high formation energy (3.87eV)
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