  Topotactic transformations between related crystal structures are a powerful
emerging route for the synthesis of novel quantum materials. Whereas most such
"soft chemistry" experiments have been carried out on polycrystalline powders
or thin films, the topotactic modification of single crystals -- the gold
standard for physical property measurements on quantum materials -- has proven
remarkably difficult. Here, we report the topotactic reduction of
La$_{1-x}$Ca$_{x}$NiO$_3$ single-crystals to La$_{1-x}$Ca$_{x}$NiO$_{2+\delta}$
using CaH$_2$ as the reducing agent. The transformation from the
three-dimensional perovskite to the quasi-two-dimensional infinite-layer phase
was thoroughly characterized by X-ray diffraction, electron microscopy, Raman
spectroscopy, magnetometry, and electrical transport measurements. Our work
demonstrates that the infinite-layer structure can be realized as a bulk phase
in crystals with micrometer-sized single-domains. The electronic properties of
these specimens resemble those of epitaxial thin films rather than powders with
similar compositions.
