Clusters containing nine lanthanide and six sodium ions are crystallized with the help of a tridentate ligand bearing a phosphonic acid group; the structure of the EuIII compound reveals a host-guest assembly Eu⊂Na6Eu8L16 with a substantial quantum yield (32%). A similar EuIII-YbIII structure displays dual luminescence in the visible (EuIII) and NIR (YbIII) ranges.