  Nanomaterials bring to expression unique electronic properties that promote
advanced functionality and technologies. Albeit, nanoscale growth presents
paramount challenges for synthesis limiting the diversity in structures and
compositions. Here, we demonstrate solid-state topotactic exchange that
converts Wurtzite InAs nanowires into Zintl phase Eu$_3$In$_2$As$_4$ nanowires.
In situ evaporation of Eu and As over InAs nanowire cores in molecular beam
epitaxy results in mutual exchange of Eu from the shell and In from the core. A
continuous Eu$_3$In$_2$As$_4$ shell thereby grows that gradually consumes the
InAs core and converts it into a single phase Eu$_3$In$_2$As$_4$ nanowire.
Topotaxy, which facilitates the mutual exchange, is supported by the
substructure of the As matrix which is similar across the Wurtzite InAs and
Zintl Eu$_3$In$_2$As$_4$. We provide initial evidence of an antiferromagnetic
transition at T$_N$ $\sim$ 6.5 K in the Zintl phase Eu$_3$In$_2$As$_4$
nanowires. Ab initio calculation confirms the antiferromagnetic state and
classifies Eu$_3$In$_2$As$_4$ as a $C_2 T$ axion insulator hosting both chiral
hinge modes and unpinned Dirac surface states. The topotactic mutual-exchange
growth of Zintl Eu$_3$In$_2$As$_4$ nanowires thus enables the exploration of
intricate magneto-topological states of nanomaterials. Moreover, it may open
the path for topotactic mutual-exchange synthesis of nanowires made of other
exotic compounds.
