  Infinite-layer (IL) nickelates are an emerging class of superconductors,
where the Ni$^{1+}$ valence state in a square planar NiO$_2$ coordination can
only be reached via topotactic reduction of the perovskite phase. However, this
topotactic soft chemistry with hydrogenous reagents is still at a stage of
rapid development, and there is a number of open issues, especially considering
the possibility of hydrogen incorporation. Here we study the time dependence of
the topotactic transformation of LaNiO$_3$ to LaNiO$_2$ for powder samples with
x-ray diffraction and gas extraction techniques. While the hydrogen content of
the powder increases with time, neutron diffraction shows no negative
scattering of hydrogen in the LaNiO$_2$ crystal lattice. The extra hydrogen
appears to be confined to grain boundaries or secondary-phase precipitates. The
average crystal structure, and possibly also the physical properties, of the
primary LaNiO$_2$ phase are therefore not noticeably affected by hydrogen
residues created by the topotactic transformation.

Strongly Correlated Electrons;Materials Science;Superconductivity