Engineering topics

Cheetham, Anthony K

Publications and source records attributed to Cheetham, Anthony K.

4 records

DOE OSTI2012

Preparation, magnetism and electronic structures of cadmium technetates

Due to the scarcity of the artificial transition metal technetium, studies on the solid-state properties of its oxides have been rarely undertaken. We have prepared a new technetium metal oxide system that include the 4d metal in two separate oxidation states and have characterized the new phases' crystal structures and magnetic properties. One phase Cd{sub 2}Tc{sub 2}O{sub 7} was prepared through the vapor-phase reaction of the heptaoxide Tc{sub 2}O{sub 7} with Cd metal; the other phase, CdTcO{sub 3}, was prepared through the solid state reaction of TcO{sub 2} with CdO. High-resolution synchrotron X-ray diffraction was used to characterize the crystal structures and stoichiometries of the two new technetates. At room temperature, Cd{sub 2}Tc{sub 2}O{sub 7} takes on the pyrochlore structure with a = 10.18118(1) {angstrom}, space group, Fd{bar 3}m and Z = 8. CdTcO{sub 3} has the GdFeO{sub 3}-type structure with space group Pbnm and a = 5.38881(1) {angstrom}, b = 5.46504(1) {angstrom}, and c = 7.71272(1){angstrom}. The magnetic susceptibility behavior of Cd{sub 2}Tc{sub 2}O{sub 7} is strikingly similar to that observed in Cd{sub 2}Re{sub 2}O{sub 7}, with a broad transition close to 200 K. The magnetic behavior of Cd{sub 2}Tc{sub 2}O{sub 7} is also compared with that of isomorphous Pb{sub 2}Tc{sub 2}O{sub 6} and Bi{sub 2}Tc{sub 2}O{sub 7}, also presented in this study. The magnetic susceptibility of the distorted perovskite phase CdTcO{sub 3} is weakly temperature dependent, with no obvious signs of an ordering transition below 300 K. Electronic band structure calculations performed to simulate electronic densities of states indicate that the Fermi level is located in a 't{sub 2g}' band of the octahedrally coordinated Tc cations and therefore metallic conductivity in both CdTcO{sub 3} and Cd{sub 2}Tc{sub 2}O{sub 7}.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Towards enhanced ligand-centred photoluminescence in inorganic-organic frameworks for solid state lighting

Three novel inorganic-organic framework compounds containing the organic chromophore ligand 9-fluorenone-2,7-dicarboxylic acid (abbreviated H2FDC) and barium (BaFDC), cadmium (CdFDC) and manganese (MnFDC), respectively, have been synthesized and evaluated for their use as phosphor materials for solid state lighting and other applications. The results are compared with two earlier reported structures containing the same ligand with calcium (CaFDC) and strontium (SrFDC). The barium- and cadmium-containing compounds both show blue excited, yellow photoluminescence, while the manganese structure does not. The trends in luminescent efficiency for the Ba, Cd, Ca, and Sr derivatives are discussed in relation to crystallographic, optical, and low-temperature specific heat considerations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2011

Understanding ligand-centred photoluminescence through flexibility and bonding of anthraquinone inorganic-organic frameworks

Five novel inorganic-organic framework compounds containing the organic chromophore ligand anthraquinone-2,3-dicarboxylic acid (abbreviated H 2 AQDC) and calcium (CaAQDC), zinc (ZnAQDC), cadmium (CdAQDC), manganese (MnAQDC), and nickel (NiAQDC), respectively, have been synthesized. The photoluminescence of these materials is only visible at low temperatures and this behaviour has been evaluated in terms of ligand rigidity. It is proposed that the 2,3 position bonding sites result in luminescence-quenching ligand motion, as supported by X-ray diffraction and temperature-dependent luminescence studies.

36 MATERIALS SCIENCE↗

DOE OSTI2011

FINAL REPORT

Hydrogen storage systems based on the readily reversible adsorption of H{sub 2} in porous materials have a number of very attractive properties with the potential to provide superior performance among candidate materials currently being investigated were it not for the fact that the interaction of H{sub 2} with the host material is too weak to permit viable operation at room temperature. Our study has delineated in quantitative detail the structural elements which we believe to be the essential ingredients for the future synthesis of porous materials, where guest-host interactions are intermediate between those found in the carbons and the metal hydrides, i.e. between physisorption and chemisorption, which will result in H{sub 2} binding energies required for room temperature operation. The ability to produce porous materials with much improved hydrogen binding energies depends critically on detailed molecular level analysis of hydrogen binding in such materials. However, characterization of H{sub 2} sorption is almost exclusively carried by thermodynamic measurements, which give average properties for all the sites occupied by H{sub 2} molecules at a particular loading. We have therefore extensively utilized the most powerful of the few molecular level experimental probes available to probe the interactions of hydrogen with porous materials, namely inelastic neutron scattering (INS) spectroscopy of the hindered rotations of the hydrogen molecules adsorbed at various sites, which in turn can be interpreted in a very direct way in by computational studies. This technique can relate spectral signatures of various H{sub 2} molecules adsorbed at binding sites with different degrees of interaction. In the course of this project we have synthesized a rather large number of entirely new hybrid materials, which include structural modifications for improved interactions with adsorbed hydrogen. The results of our systematic studies on many porous materials provide detailed information on the effects on hydrogen binding from framework modifications, including charged frameworks and extraframework cations, from reduction in pore sizes, functionalization of the organic linking group, and most importantly, that of the various types of metal sites. We provided a clear demonstration that metal sites are most effective if the metal is highly undercoordinated, open and completely accessible to the H{sub 2} molecule, a condition which is not currently met in MOFs with intra-framework metals. The results obtained from this project therefore will give detailed direction to efforts in the synthesis of new materials that can reach the goal of a practical sorption based hydrogen storage material.

08 HYDROGEN↗