Several observations about the proposed compounds should be made before discussing synthesis attempts. Two iodide compounds were predicted to be stable, Cs 3 Eu 2 I 9 and Na 3 EuI 6 , but we did not attempt to synthesize them because there are no known Eu 3+ iodides. EuI 3 has not been realized experimentally as its hydrated phases decompose directly into EuI 2 , and it has an estimated formation enthalpy ∼0.6 eV/atom higher than that of EuI 2 . Also, while Na 3 M I 6 has been synthesized for M =Sm, Gd, Tb, and Dy, Na 3 EuI 6 is conspicuously absent. Second, the quaternary fluoride Cs 2 NaEuF 6 has a non-polar predicted europium site symmetry that should preclude observation of the 5 D 0 → 7 F 0 transition; however, the composition's actual structure, if Figure : Three sets of pseudopotentials are tested on the Eu 2 O 3 -Al 2 O 3 system. We assume that the binary oxides are stable. Blue compounds are on the calculated hull while red ones are not. Calculations using the set with 4f 6 electrons in the Eu pseudopotential core best represent experimental observations. Table : Compounds predicted to be stable (E hull = 0) are listed along with their formation energy relative to the convex hull if it were constructed without them (E rel ). Also listed are the predicted symmetry and separation of their Eu 3+ cations. Site symmetries followed by "(x)" are, in theory, not compatible with observing the 5 D 0 → 7 F 0 transition.