Syntheses, structures, physical properties, and theoretical study of five rare-earth copper tellurides

Five rare-earth copper tellurides have been synthesized by the reactions of the elements at 1073 K. The isostructural compounds LaCu_0.40Te₂ (a = 7.7063(13) Å, b = 8.5882(14) Å, c = 6.3115(10) Å, T = 153 K), NdCu_0.37Te₂ (a = 7.6349(7) Å, b = 8.3980(8) Å, c = 6.18388(6) Å, T = 153 K), SmCu_0.34Te₂ (a = 7.6003(10) Å, b = 8.3085(11) Å, c = 6.1412(8) Å, T = 153 K), GdCu_0.33Te₂ (a = 7.5670(15) Å, b = 8.2110(16) Å, c = 6.0893(12) Å, T = 107 K), and DyCu_0.32Te₂ (a = 7.5278(13) Å, b = 8.1269(14) Å, c = 6.0546(11) Å, T = 107 K) crystallize with four formula units in space group D(2h)11-Pbcm of the orthorhombic system. In each, the rare-earth (Ln) atom is coordinated by a bicapped trigonal prism of Te atoms and the Cu atom is coordinated by a tetrahedron of Te atoms. Infinite linear Te(¹⁺(x))^- chains run parallel to c, with Te-Te distances decreasing from 3.1558(5) Å in LaCu_0.40Te₂ to 3.0273(3) Å in DyCu_0.32Te₂. Both the thermopower and conductivity data in the c direction show LaCu_0.40Te₂ to be a semiconductor at all temperatures, and NdCu_0.37Te₂, SmCu_0.34Te₂, and GdCu_0.33Te₂ to be semiconductors above 150-200 K. The thermopower data for these three compounds exhibit very high peaks of approximately 900 μV/K in the vicinity of 150 K, followed by a rapid decrease at lower temperatures. This behavior deviates from the trend expected for semiconductors. Huckel calculations predict that the Te(¹⁺(x))^- chains in LnCu(x)Te₂ should show metallic properties. Possible reasons for this discrepancy between theory and experiment involve distortions of the Te chains or disorder of the Cu atoms. GdCu_0.33Te₂ is paramagnetic with μ(eff) = 7.74(3) μ(B), typical for Gd³⁺.