Four New Actinide Chalcogenides Ba₂Cu₄USe₆, Ba₂Cu₂ThSe₅, Ba₂Cu₂USe₅, and Sr₂Cu₂US₅: Crystal Structures and Physical Properties

Four new actinide chalcogenidesnamely, Ba₂Cu₄USe₆, Ba₂Cu₂ThSe₅, Ba₂Cu₂USe₅, and Sr₂Cu₂US₅were synthesized via solid-state methods at 1173 K. Single-crystal X-ray diffraction studies show that Ba₂Cu₄USe₆ crystallizes in a new structure type in space group <i>C</i>_2<i>h</i>⁵–<i>P</i>2₁/<i>c</i> of the monoclinic system, whereas the three other compounds are isostructural and adopt the Ba₂Cu₂US₅ structure type in space group <i>C</i>_2<i>h</i>³–<i>C</i>2/<i>m</i>, also of the monoclinic system. These Ak/Cu/An/Q structures (Ak = alkaline-earth metal; An = actinide; Q = chalcogen) have no short Q–Q interactions and, hence, are charge-balanced with Ak²⁺, Cu¹⁺, An⁴⁺, and Q^2–. Crystal structures of all these compounds are two-dimensional and feature layers that are separated by Ba²⁺ cations. The compositions of these layers differ. In the structure of Ba₂Cu₄USe₆, the _∞²[Cu₄USe₆^4–] layers comprising USe₆ octahedra and CuSe₄ tetrahedra stack perpendicular to the <i>a</i>-axis. These _∞²[Cu₄USe₆^4–] layers show short Cu–Cu interactions. In the three isostructural Ak₂Cu₂AnQ₅ compounds, AnQ₆ octahedra and CuQ₄ tetrahedra are connected along the <i>c</i>-axis in the sequence “...<i>oct tet tet oct tet tet</i>...” to form the _∞²[Cu₂AnQ₅^4–] layers. Resistivity, optical, and DFT calculations show semiconducting behavior for these compounds