  The recent observation of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$
calls for further investigation and optimization of the synthesis of this
metastable infinite-layer nickelate structure. Here, we present our current
understanding of important aspects of the growth of the parent perovskite
compound via pulsed laser deposition on SrTiO$_{3}$ (001) substrates, and the
subsequent topotactic reduction. We find that to achieve single-crystalline,
single-phase superconducting Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$, it is essential
that the precursor perovskite Nd$_{0.8}$Sr$_{0.2}$NiO$_{3}$ thin film is
stabilized with high crystallinity and no impurity phases; in particular, a
Ruddlesden-Popper-type secondary phase is often observed. We have further
investigated the evolution of the soft-chemistry topotactic reduction
conditions to realize full transformation to the infinite-layer structure with
no film decomposition or formation of other phases. We find that capping the
nickelate film with a subsequent SrTiO$_{3}$ layer provides an epitaxial
template to the top region of the nickelate film, much like the substrate.
Thus, for currently optimized growth conditions, we can stabilize
superconducting single-phase Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$ (001) epitaxial thin
films up to ~ 10 nm.
