Crystal growth and physical properties of SrCu2As2, SrCu2Sb2, and BaCu2Sb2

Abstract

We report the growth of single crystals of SrCu${}_{2}$As${}_{2}$, SrCu${}_{2}$Sb${}_{2}$, SrCu${}_{2}$(As${}_{0.84}$Sb${}_{0.16}$)${}_{2}$, and BaCu${}_{2}$Sb${}_{2}$ using the self-flux technique and their structural, magnetic, thermal, and transport properties that were investigated by powder x-ray diffraction (XRD), magnetic susceptibility $\ensuremath{\chi}$, specific heat ${C}_{\mathrm{p}}$, and electrical resistivity $\ensuremath{\rho}$ measurements versus temperature $T$ from 1.8 to 350 K. Rietveld refinements of XRD patterns for crushed crystals confirm that SrCu${}_{2}$As${}_{2}$ crystallizes in the ThCr${}_{2}$Si${}_{2}$-type body-centered tetragonal structure (space group $I4/mmm$) and SrCu${}_{2}$Sb${}_{2}$ crystallizes in the CaBe${}_{2}$Ge${}_{2}$-type primitive tetragonal structure (space group $P4/nmm$). However, as reported previously, BaCu${}_{2}$Sb${}_{2}$ is found to have a large unit cell consisting of three blocks. Here a ThCr${}_{2}$Si${}_{2}$-type block is sandwiched between two CaBe${}_{2}$Ge${}_{2}$-type blocks along the $c$ axis with an overall symmetry of $I4/mmm$, as reported, but likely with a monoclinic distortion. The $\ensuremath{\chi}$ data of all these compounds are diamagnetic and reveal nearly $T$-independent anisotropic behavior. The $\ensuremath{\chi}$ of SrCu${}_{2}$As${}_{2}$ is found to be larger in the $ab$ plane than along the $c$ axis, as also previously reported for pure and doped BaFe${}_{2}$As${}_{2}$, whereas the $\ensuremath{\chi}$ values of SrCu${}_{2}$Sb${}_{2}$ and BaCu${}_{2}$Sb${}_{2}$ are larger along the $c$ axis. This difference in anisotropy appears to arise from the differences between the crystal structures. The finite values of the Sommerfeld linear specific heat coefficients $\ensuremath{\gamma}$ and the $T$ dependences of $\ensuremath{\rho}$ reveal metallic character of all four compounds. The electronic and magnetic properties indicate that these compounds are $sp$ metals with Cu in the nonmagnetic $3{d}^{10}$ electronic configuration corresponding to the oxidation state Cu${}^{+1}$, as previously predicted theoretically for SrCu${}_{2}$As${}_{2}$ by Singh [Phys. Rev. B 79, 153102 (2009)]. We present a brief review of theoretical and experimental work on the doping character of transition metals for Fe in BaFe${}_{2}$As${}_{2}$. The As--As covalent interlayer bond distances in the collapsed-tetragonal (Ca,Sr,Ba)Cu${}_{2}$As${}_{2}$ compounds are much shorter than the nonbonding As--As distances in BaFe${}_{2}$As${}_{2}$. Thus, 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 BaFe${}_{2}$As${}_{2}$ and pure BaCu${}_{2}$As${}_{2}$, perhaps via a first-order lattice instability such as a miscibility gap in the Ba(Fe${}_{1\ensuremath{-}x}$Cu${}_{x}{)}_{2}$As${}_{2}$ system.