Materials containing rare-earth cations are promising quantum information storage (QIS) candidates. Their 4f -4f optical transitions are well-shielded by filled 5s and 5p orbitals, and they have long nuclear spin and optical coherence times. Often for QIS applications with Eu 3+ -containing compounds, low Eu 3+ concentrations are introduced as dopants in host materials. But this introduces local strain, increasing Eu 3+ site inhomogeneity. This inhomogeneity broadens transition linewidths and prevents the resolution of hyperfine transi-tions within the narrow 5 D 0 → 7 F 0 optical transition. In contrast, stoichiometric EuCl 3 •6H 2 O, with Eu 3+ on homogeneous crystallographic sites, has a dramatically higher optical center density and an extremely narrow inhomogeneous linewidth that allows for hyperfine transition resolution. Narrow linewidths, though, ultimately depend on the synthesis route and defect chemistry, and these connections are underdeveloped for stoichiometric systems. New stoichiometric candidates with well-characterized defects will help correlate imperfections with the crucial inhomogeneous linewidth.