Structure and Thermodynamics of the Mixed Alkali Alanates

The thermodynamics and structural properties of the hexahydride alanates (M2M'AlH6) with the elpasolite structure have been investigated. A series of mixed alkali alanates (Na2LiAlH6, K2LiAlH6 and K2NaAlH6) were synthesized and found to reversibly absorb and desorb hydrogen without the need for a catalyst. Pressure-composition isotherms were measured to investigate the thermodynamics of the absorption and desorption reactions with hydrogen. Isotherms for catalyzed (4 mol% TiCl3) and uncatalyzed Na2LiAlH6 exhibited an increase in kinetics, but no change in the bulk thermodynamics with the addition of a dopant. A structural analysis using synchrotron x-ray diffraction showed that these compounds favor the Fm-3m space group with the smaller ion (M') occupying an octahedral site. These results demonstrate that appropriate cation substitutions can be used to stabilize or destabilize the material and may provide an avenue to improving the unfavorable thermodynamics of a number of materials with promising gravimetric hydrogen densities.Comment: 6 pages, 7 figures,3 tables, submitted to PR

How inert, perturbing, or interacting are cryogenic matrices? A combined spectroscopic (infrared, electronic, and x-ray absorption) and DFT investigation of matrix-isolated Iron, Cobalt, Nickel, and Zinc Dibromides

The interactions of FeBr2, CoBr2, NiBr2 and ZnBr2 with Ne, Ar, Kr, Xe, CH4 and N2 matrices have been investigated using IR, electronic absorption and X-ray absorption spectroscopies, as well as DFT calculations. ZnBr2 is linear in all the matrices. NiBr2 is linear in all but N2 matrices where it is severely bent. For FeBr2 and CoBr2 there is a more gradual change, with evidence of non-linearity in Xe and CH4 matrices as well as N2. In the N2 matrices the presence of νNN modes blue shifted from the “free” N2 values indicates the presence of physisorbed species, and the magnitude of the blue-shift correlates with the shift in the ν3 mode of the metal dibromide. In the case of NiCl2 and NiBr2 chemisorbed species are formed after photolysis, but only if deposition takes place below 10 K. There was no evidence for chemisorbed species for NiF2 and FeBr2 and in the case of CoBr2 the evidence was not strong

Metal Hydrides and Related Materials - Energy Carriers for Novel Hydrogen and Electrochemical Storage

The seventh edition of the International Renewable and Sustainable Energy Conference (IRSEC) was held in Agadir (Sofitel Royal Bay, November 27–30, Morocco) under the Program Chair of Prof. Ahmed Ennaoui (IRESEN). IRSEC, as one of the biggest conferences in north Africa, aims at creating an international forum to facilitate discussions and exchanges in all aspects of renewable and sustainable energy. This Viewpoint will summarize the scientific presentations and stimulated discussions during the Special Session (November 28–29) on Metal Hydrides’ Energy covering topics of metal hydrides and energy related issues for innovative processes and technologies, with a focus on magnesium-based hydrides, intermetallic hydrides, complex and melt hydrides, porous materials, and thin films

Competing magnetostructural phases in a semiclassical system

The interplay between charge, structure, and magnetism gives rise to rich phase diagrams in complex materials with exotic properties emerging when phases compete. Molecule-based materials are particularly advantageous in this regard due to their low energy scales, flexible lattices, and chemical tunability. Here, we bring together high pressure Raman scattering, modeling, and first principles calculations to reveal the pressure-temperature-magnetic field phase diagram of Mn[N(CN)2]2. We uncover how hidden soft modes involving octahedral rotations drive two pressure-induced transitions triggering the low ??? high magnetic anisotropy crossover and a unique reorientation of exchange planes. These magnetostructural transitions and their mechanisms highlight the importance of spin-lattice interactions in establishing phases with novel magnetic properties in Mn(II)-containing systems

Stable Lithium Argon compounds under high pressure

High pressure can fundamentally alter the bonding patterns of chemical elements. Its effects include stimulating elements thought to be “inactive” to form unexpectedly stable compounds with unusual chemical and physical properties. Here, using an unbiased structure search method based on CALYPSO methodology and density functional total energy calculations, the phase stabilities and crystal structures of Li−Ar compounds are systematically investigated at high pressure up to 300 GPa. Two unexpected Li(m)Ar(n) compounds (LiAr and Li(3)Ar) are predicted to be stable above 112 GPa and 119 GPa, respectively. A detailed analysis of the electronic structure of LiAr and Li(3)Ar shows that Ar in these compounds attracts electrons and thus behaves as an oxidizing agent. This is markedly different from the hitherto established chemical reactivity of Ar. Moreover, we predict that the P4/mmm phase of Li(3)Ar has a superconducting transition temperature of 17.6 K at 120 GPa

Reactivity of He with ionic compounds under high pressure

Helium was long thought to be unable to form stable solid compounds, until a recent discovery that helium reacts with sodium at high pressure. Here, the authors demonstrate the driving force for helium reactivity, showing that it can form new compounds under pressure without forming any local chemical bonds