Solid State Research

Differences between transition-metal oxide structures and chalcogenide structures are striking. These include polyhedra other than tetrahedra and octahedra as building blocks in most chalcogenide structures; the absence of closest packing in many chalcogenide structures; the common occurrence of layered structures for transition-metal chalcogenides; the presence of Q-Q bonds (Q = S, Se, Te), that is Q_n^2- species, n = 2, in many chalcogenides; and finally the wide range of possible Q-Q bond lengths, especially for the tellurides. Because of these differences, metal chalcogenides, in particular the tellurides, show a very rich structural chemistry that is fundamentally different from that of the oxides. Metal chalcogenides show a wide range of physical properties, including low-temperature superconductivity and phenomena, such as charge-density waves and highly anisotropic behavior, associated with their low-dimensionality. Such systems have received little attention compared with the oxides.

Our current investigations concentrate on the chalcogenides and oxychalcogenides of ternary and quaternary transition-metal thorium, uranium and neptunium compounds, as well as related rare-earth compounds. The synthesis of thorium- and uranium-containing compounds is done at Northwestern University whereas the synthesis of neptunium compounds is carried out by us at the Actinide Facility at Argonne National Laboratory. We determine the crystal structures of these materials, measure their transport and magnetic properties, and in selected cases carry out theoretical calculations to correlate structure, bonding, and physical properties. A few techniques employed include X-ray crystallography (single-crystal and powder), high-temperature solid-state reaction methods, EDX, magnetic measurements, optical measurements (diffuse reflectance and single-crystal absorption measurements), and transport measurements (thermopower and electrical resistivity).