Syntheses, Crystal Structures, and Physical Properties of the New Thorium Chalcogenides CuTh₂Te₆ and SrTh₂Se₅

Single crystals of CuTh₂Te₆ form by a stoichiometric reaction of the elements at 1000 °C. The compound crystallizes in the space group −P2₁/m with unit cell parameters a = 6.170(2) Å, b = 4.332(1) Å, c = 10.424(3) Å, β = 98.85(1)°, and Z = 1 at 113 K. The structure was solved from single-crystal X-ray data. It consists of layers of [Th₂Te₆^-] double chains joined by Cu⁺ cations. Each Th atom is coordinated to eight Te atoms in a bicapped trigonal prismatic arrangement. There are three crystallographically unique Te atoms. Each ThTe₈ unit is bridged through one distinct Te atom, such that the capping Te atom of one unit forms the vertex of its neighbor. The two nonbridging Te atoms form infinite Te−Te chains along the exterior of the [Th₂Te₆^-] layer. Copper atoms are coordinated to these Te atoms in a tetrahedral arrangement. Owing to the existence of Te−Te bonds of intermediate length, the assignment of formal oxidation states in this compound is not possible. Four-probe dc electrical conductivity measurements of a single crystal of CuTh₂Te₆ indicate the compound is a semiconductor along [010]. Magnetic susceptibility measurements in the range 2−300 K show CuTh₂Te₆ to be paramagnetic with μ_eff = 2.06 μ_B at 300 K. Single crystals of SrTh₂Se₅ form from the reaction SrSe + Th + 3Sn + 3Se at 1000 °C. EDAX experiments show no tin present in several crystals analyzed. Transparent red blocks of SrTh₂Se₅ crystallize in the space group −P2₁/c with unit cell parameters a = 8.704(2) Å, b = 7.861(2) Å, c = 12.458(4) Å, β = 90.00(2)°, and Z = 4 at 113 K. The structure, which is related to that of U₃S₅, is a three-dimensional framework with Sr cations located in one-dimensional channels. There are two distinct Th environments, bicapped trigonal prismatic and distorted monocapped octahedral. There are no Se−Se bonds and so formal oxidation states of Sr²⁺, Th⁴⁺, and Se^2- may be assigned.