Miscibility gap between BaMn$_2$As$_2$ and BaFe$_2$As$_2$*

${\rm BaMn_2As_2}$ and ${\rm BaFe_2As_2}$ both crystallize at room temperature in the same tetragonal ThCr$_2$Si$_2$-type structure but with divergent unit cell volumes of 234.12 and 204.38~\AA$^3$, respectively, suggesting that the Mn$^{+2}$ is in a high-spin state while Fe$^{+2}$ is in a low-spin state. The physical properties of the two compounds are therefore also highly divergent; \emph{e.g.}, ${\rm BaMn_2As_2}$ is an insulating local moment antiferromagnet with a high N\'eel temperature $T_{\rm N} = 625$~K whereas BaFe$_2$As$_2$ is a metallic itinerant antiferromagnet with a much lower $T_{\rm N} = 137$~K\@.\footnote{D. C. Johnston, Adv. Phys. {\bf 59}, 803--1061 (2010).} We have discovered a miscibility gap in the pseudobinary phase diagram between these two isostructural compounds, probably arising from their divergent chemistry. Our investigations of the miscibility gap and of the structural, magnetic, electronic transport and thermal properties of various compositions in this system will be discussed.