  Here we use low-temperature and variable-temperature scanning tunneling
microscopy to study the pnictide superconductor, Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$.
In the low-temperature phase (triclinic phase) of BaNi$_2$As$_2$, we observe
the unidirectional charge density wave (CDW) with $Q$ = 1/3 on both the Ba and
NiAs surfaces. On the NiAs surface of the triclinic BaNi$_2$As$_2$, there are
structural-modulation-induced chain-like superstructures with distinct
periodicities. In the high-temperature phase (tetragonal phase) of
BaNi$_2$As$_2$, the NiAs surface appears as the periodic 1 by 2 superstructure.
Interestingly, in the triclinic phase of Ba$_{0.5}$Sr$_{0.5}$Ni$_2$As$_2$, the
unidirectional CDW is suppressed on both the Ba/Sr and NiAs surfaces, and the
Sr substitution stabilizes the periodic 1 by 2 superstructure on the NiAs
surface, which enhance the superconductivity in
Ba$_{0.5}$Sr$_{0.5}$Ni$_2$As$_2$. Our results provide important microscopic
insights for the interplay among the unidirectional CDW, structural modulation,
and superconductivity in this class of pnictide superconductors.
