Syntheses, crystal structures, and resistivities of the two new ternary uranium selenides, Er₃USe₈ and Yb₃USe₈

Two new ternary lanthanide (Ln) uranium selenides, Er₃USe₈ and Yb₃USe₈, were synthesized at 1198 K using NaI as a flux. Single-crystal X-ray studies show these two compounds to be isostructural and to crystallize in space group D2h11-Pbcm of the orthorhombic crystal system. The Ln and U atoms are disordered on the same crystallographic site in these crystal structures. Each Ln/U atom is coordinated to eight Se atoms in a bicapped trigonal prism, and sharing of these (Ln/U)Se₈ units creates a three-dimensional network. Se2 atoms are connected to each other to form infinite one-dimensional chains along the c axis. In these chains, the two Se atoms are separated by about 2.74 Å, a distance intermediate to those of a Se-Se single bond and a van der Waals interaction. Temperature-dependent resistivity measurements show that Er₃USe₈ and Yb₃USe₈ are semiconductors with activation energies of 0.08(1) and 0.17(1) eV, respectively.