  In this work, we have thoroughly studied the effects of flux composition and
temperature on the crystal growth of the BaCu2As2 compound. While Pb and CuAs
self-flux produce the well-known {\alpha}-phase ThCr2Si2-type structure (Z=2),
a new polymorphic phase of BaCu2As2 (\b{eta} phase) with a much larger c
lattice parameter (Z=10), which could be considered an intergrowth of the
ThCr2Si2- and CaBe2Ge2-type structures, has been discovered via Sn flux growth.
We have characterized this structure through single-crystal X-ray diffraction,
transmission electron microscopy (TEM), and scanning transmission electron
microscopy (STEM) studies. Furthermore, we compare this new polymorphic
intergrowth structure with the {\alpha}-phase BaCu2As2 (ThCr2Si2 type with Z=2)
and the \b{eta}-phase BaCu2Sb2 (intergrowth of ThCr2Si2 and CaBe2Ge2 types with
Z=6), both with the same space group I4/mmm. Electrical transport studies
reveal p-type carriers and magnetoresistivity up to 22% at 5 K and under a
magnetic field of 7 T. Our work suggests a new route for the discovery of new
polymorphic structures through flux and temperature control during material
synthesis.
