  Two stoichiometric metal-organic frameworks containing Eu$^{3+}$ cations are
probed as candidates for photon-based quantum information storage. Synthesis
procedures for growing 0.2 mm, rod-shaped Eu(HCOO)$_3$ and 1-3 mm, rhombohedral
Eu(HCOO)$_{3}\cdot$(HCONH$_2$)$_2$ single crystals are presented with visible
precipitation as soon as 1 h into heating for Eu(HCOO)$_3$ and 24 h for
Eu(HCOO)$_{3}\cdot$(HCONH$_2$)$_2$. Room temperature and 1.4 K
photoluminescence measurements of the $^5$D$_{0}\rightarrow {^7}$F$_J$
transitions of Eu$^{3+}$ are analyzed for both compounds. Comparisons of peak
width and intensity are discussed along with the notable first report for both
of the $^5$D$_{0}\rightarrow {^7}$F$_0$ transition, the hyperfine structure of
which has potential use in quantum memory applications. The air instability of
Eu(HCOO)$_{3}\cdot$(HCONH$_2$)$_2$ and the transformation of its
photoluminescence properties are discussed.
