Finally, we attempted to synthesize Cs 5 EuO 4 by heating a stoichiometric mixture of Cs 2 CO 3 and Eu 2 O 3 in an alumina crucible. After 48 h at 1000 • C, only Eu 2 O 3 remained. We believe that Cs 2 CO 3 decomposed to Cs 2 O, which then evaporated since it was well above its melting point.