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 C_2h⁵-P2₁/c of the monoclinic system, whereas the three other compounds are isostructural and adopt the Ba₂Cu₂US₅ structure type in space group C_2h³-C2/m, 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 a-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 c-axis in the sequence "...oct tet tet oct tet tet..." to form the _∞²[Cu₂AnQ₅^4-] layers. Resistivity, optical, and DFT calculations show semiconducting behavior for these compounds.