  Structural phase transitions accompanied by incommensurate and commensurate
charge density wave (CDW) modulations of unconventional nature have been
reported in BaNi$_2$As$_2$, a nonmagnetic cousin of the parent compound of
Fe-based superconductors, BaFe$_2$As$_2$. The strong dependence of the
structural and CDW transitions of BaNi$_2$As$_2$ on isoelectronic substitutions
alongside original dynamical lattice effects suggests strong tunability of the
electronic phase of the system through structural effects. Here, we present a
comprehensive synchrotron x-ray diffraction and first-principles calculation
study of the evolution of the crystal structure and lattice instabilities of
BaNi$_2$As$_2$ as a function of temperature and hydrostatic pressure (up to 12
GPa). We report a cascade of pressure-induced structural phase transitions and
electronic instabilities up to 10 GPa, above which all CDW superstructures
disappear. We reveal that the stable high-pressure phase consists of planar Ni
zigzag chains, from which the surrounding As atoms have been pushed away. This
yields a strong reduction of the interlayer As-As distance (along the original
c axis), akin to what is observed in the collapsed tetragonal structure of
other pnictides, albeit here with a monoclinic structure. The discovery of
polymorphs in the pressure-temperature phase diagram of BaNi$_2$As$_2$
emphasizes the importance of the relative Ni-Ni and Ni-As bond lengths in
controlling the electronic ground state of this compound and increases our
understanding of viable electronic phases under extreme conditions.
