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Crystal Growth and Physical Properties of SrCu2As2, SrCu2Sb2 and BaCu2Sb2

We report the growth of single crystals of SrCu2As2, SrCu2Sb2, SrCu2(As{0.84}Sb{0.16})2 and BaCu2Sb2 using the self-flux technique and their structural, magnetic, thermal and transport properties that were investigated by powder x-ray diffraction (XRD), magnetic susceptibility chi, specific heat Cp and electrical resistivity rho measurements versus temperature T from 1.8 to 350 K. Rietveld refinements of XRD patterns for crushed crystals confirm that SrCu2As2 crystallizes in the ThCr2Si2-type structure and SrCu2Sb2 crystallizes in the CaBe2Ge2-type structure. However, BaCu2Sb2 is found to have a large unit cell consisting of three blocks. Here a ThCr2Si2-type block is sandwiched between two CaBe2Ge2-type blocks along the c-axis, as reported, but likely with a monoclinic distortion. The chi data of all these compounds are diamagnetic and reveal nearly T-independent anisotropic behavior. The finite values of the Sommerfeld linear specific heat coefficients gamma and the T dependences of rho reveal metallic character of all four compounds. The electronic and magnetic properties indicate that these compounds are sp metals with Cu in the 3d^{10} electronic configuration corresponding to the oxidation state Cu^{+1}, as previously predicted theoretically for (Sr,Ba)Cu2As2 by D. J. Singh. We find that SrCu2As2 has a collapsed-tetragonal structure. The electronic character of the Cu and the strength of the As-As interlayer bonding are both expected to drastically change between weakly Cu-substituted BaFe2As and pure BaCu2As2.