• Solid-State Chemistry, Especially of actinides
• Nuclear Waste Disposal
• Structure-Property Relationships in Novel Crystalline Solids
• Environmental Chemistry
• Crystal Structure Analysis

Our group is dedicated to understanding the fundamental and applied chemistry of the lanthanides and actinides.  In particular, we focus on the preparation of new crystalline solids with unusual structures and properties.

Actinide Phosphonates

Phosphonates have played an essential role in actinide partitioning in advanced nuclear fuel cycles.  Despite their critical nature and the necessity for understanding bonding in actinide phosphonates, little is known about the solid-state structures of these compounds with all actinides except U(VI).  Quite recently we have undertaken the task of elucidating structural tendencies in transuranium phosphonates with the goal of developing periodic trends and structure-property relationships.  To this end, an in situ redox method has been developed for preparing crystalline neptunium(IV) compounds.  These studies reveal that neptunium phosphonates have their own unique crystal chemistry that is not mimicked by early transition metals, lanthanides, or uranium.  An explicit aim of this work is to illuminate the differences in bonding between transuranium elements, especially neptunium and plutonium, and elements that have been used as their surrogates, e.g. Zr⁴⁺, Ce⁴⁺, and Th⁴⁺.

Lanthanide Chalcogenides

The role of reaction conditions (pressure, temperature, flux, etc.) in determining composition and structure in both binary and ternary interlanthanide sesquichalcogenides cannot be over emphasized.  We have focused on the use of the new flux, Sb₂S₃, which has been used to prepare g-LnLn'S₃ (Ln = La, Ce; Ln' = Er, Tm, Yb) in high yield.  In contrast, when KI is employed as a flux, b-LaYbS₃ can only be prepared in low yield, and this medium is more amenable for the syntheses of selenide compounds.  Some fluxes, such as CsCl, are in fact reactive, resulting in Cs⁺ incorporation into the lanthanide chalcogenide phases, and the formation of quaternary compounds that are illustrated by Cs_0.14-0.17Ln_0.26-0.33YbS₂ (Ln = La – Yb).  The use of fluxes provides access to mixed-lanthanide compounds with novel structure types, and is a means of understanding structure-property relationships.<

            Currently, the most promising application of the mixed-lanthanide chalcogenides is the ability to change, and perhaps systematically vary, the band gap of these semiconductors.  This can be accomplished by both changing the lanthanide, as we demonstrated in this report, and by changing the chalcogenide, whereby the band gap should decrease when heavier chalcogenides are substituted into the structure.  Doing this allows materials to be prepared that range from nearly colorless to black.  This property is not unique to the present system, but rather applies to many lanthanide compounds.