Single crystal growth and physical properties of SrCu$_2$As$_2$, SrCu$_2$Sb$_2$ and BaCu$_2$Sb$_2$*

We present the physical properties of self-flux-grown single crystals of SrCu$_2$As$_2$, SrCu$_2$Sb$_2$, SrCu$_2$(As$_{0.84}$Sb$_{0.16}$)$_2$ and BaCu$_2$Sb$_2$ investigated by magnetic susceptibility $\chi $, specific heat $C_{\rm p}$ and electrical resistivity $\rho$ vs. temperature $T$ measurements. Contrasting structures occur for SrCu$_2$As$_2$ (ThCr$_2$Si$_2$-type), SrCu$_2$Sb$_2$ (CaBe$_2$Ge$_2$-type) and BaCu$_2$Sb$_2$ (a distorted intergrowth of ThCr$_2$Si$_2$-type and CaBe$_2$Ge$_2$-type unit cells). The $\chi(T) $ data for all these compounds exhibit weakly anisotropic diamagnetic behaviors. For $1.8 \leq T \leq 300$~K, the $\rho(T)$ data show metallic character and are well-described by the Bloch-Gr\"{u}neisen model, and the $C_{\rm p}(T)$ data are well-fitted by metallic $\gamma T$ plus Debye lattice contributions. From the low-$T$ $C_{\rm p}(T)$ data, we infer Sommerfeld coefficients $\gamma$ = 2.2--3.9 mJ/mol\,K$^2$ and Debye temperatures $\Theta_{\rm D}$ = 204--246 K\@. The electronic properties indicate that these compounds are $sp$ metals containing Cu in a nonmagnetic $3d^{10}$ electronic configuration.\footnote{D. J. Singh, Phys. Rev. B {\bf 79}, 153102 (2009).}