"Unconventional covalent" KAgF3 is metallic above 50 K

Using microwave cavity perturbation and magnetic susceptibility measurements we demonstrate for the first time that for KAgF3 the antiferromagnet-paramagnet transition leads to metallic state above 50 K. © 2003 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

Chemical tuning of the thermal decomposition temperature of inorganic hydrides: Computational aspects

We show that chosen computationally-derived molecular features along several atomic descriptors of the metallic center, as well as the value of the standard redox potential of the metal center, E0, allow for the semiquantitative estimate of the ease of metal-hydrogen bond rupture for binary and multinary hydrides. Utility of E0 is illustrated for Group 2 hydrides, and further extended to complex systems such as amido- (NH2-), imido- (NH2-) and methyl anion (CH3-) model complexes of metal cations bound to tetrahydridoborate anion (BH4-). Such considerations may be utilized for the tuning of the thermal decomposition temperature, Tdec, of the chemical hydrogen store, and for the design of the l ow-temperature hydrogen fuel source, via deliberate choice of chemical elements constituting the hydrogen storage material. © 2005 Elsevier B.V. All rights reserved

Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen

This review focuses on key aspects of the thermal decomposition of multinary or mixed hydride materials, with a particular emphasis on the rational control and chemical tuning of the strategically important thermal decomposition temperature of such hydrides, Tdec. An attempt is also made to predict the thermal stability of as-yet unknown, elusive or even unknown hydrides. The future of a particularly promising class of materials for hydrogen storage, namely the catalytically enhanced complex metal hydrides, is discussed. The predictions are supported by thermodynamics considerations, calculations derived from molecular orbital (MO) theory and backed up by simple chemical insights and intuition

Vibronic coupling in molecules and in solids.

We utilize the experience gained in our previous studies on the "chemistry of vibronic coupling" in simple homonuclear and heteronuclear molecules to begin assembling theoretical guidelines for the construction of potentially superconducting solids exhibiting large electron-phonon coupling. For this purpose we analyze similarities between vibronic coupling in isolated molecules and in extended solids. In particular, we study vibronic coupling along the antisymmetric stretch coordinate (Q(as)) in linear symmetric AAA molecules, and along the optical phonon "pairing" mode coordinate (Q(opt)) in corresponding one-dimensional [A]( infinity ) chains built of equidistant A atoms. This is done for a broad range of chemical elements (A). The following similarities between vibronic coupling in molecules and phonon coupling in solids emerge from our calculations: 1) The HOMO/LUMO electronic energy gap in an AAA molecule increases along Q(as), and the highest occupied crystal orbital/lowest unoccupied crystal orbital gap in [A]( infinity ) chain increases along Q(opt). 2) The maximum vibronic instability is invariably obtained for a half-filled, singly occupied molecular orbital in AAA molecules, and for a corresponding half-filled band in [A]( infinity ) chains. 3) The vibronic stability of an AAA molecule increases with a decrease of the AA bond length, as does the vibronic stability of [A]( infinity ) chains (external pressure may lead to a reversal of a Peierls distortion). 4) The high degree of s-p mixing and ionic/covalent forbidden curve crossing dramatically enhance the vibronic instability of both AAA molecules and [A]( infinity ) chains. We also introduce one quantitative relationship: The parameter log(R) (where R is molar refractivity, a parameter used by Herzfeld to prescribe the conditions for the metallization of the elements) correlates with a parameter f(AA) (defined as twice the electronegativity of A, divided by the equilibrium AA bond length), used by two of us previously to describe vibronic coupling in AAA molecules for a broad range of elements (A=halogen, H, or an alkali metal). We hope to illustrate that key chemical aspects of vibronic coupling in simple molecules may thus be profitably transferred to corresponding materials in the solid state

Chemical control of the thermal decomposition temperature of multinary hydrides and complex hydrides

Hydrogen storage is regarded as the scientific and technological barrier inhibiting a transition to a hydrogen energy economy - certainly for the large scale utilization of PEM fuel cells in cars. The maximum hydrogen capacity of conventional (heavy) metal hydrides currently remains at around 2 wt %, which is inadequate for onboard storage of hydrogen which requires a target gravimetric storage density of ≤ 6.5 wt %. Simple atomic, mass-based calculations reveal that the main backbone of any efficient hydrogen storage material must only be built from targeted chemical elements from a short list, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, and P. Due to the toxicities and/or unfavorable chemical properties of hydrogen's compounds with Be, F, Si, and P, the effective list of chemical cogwheels constituting a Hydrogen Storage Material might only consist of only eight elemental apostles. A developing program centered upon an understanding of the thermal decomposition/sorption processes of multinary and complex hydride materials, with a particular emphasis on the rational control and chemical tuning of the strategically important thermal decomposition temperature of compounds and materials from the Light Periodic Table is presented. This is an abstract of a paper presented at the ACS Fuel Chemistry Meeting (Washington, DC Fall 2005)