Gravar-mail: Tetra­kis(μ-naphthalene-1-acetato-1:2κ(2) O:O′)bis­(naphthalene-1-acetato)-1κ(2) O,O′;2κ(2) O,O′-bis­(1,10-phenanthroline)-1κ(2) N,N′;2κ(2) N,N′-europium(III)samarium(III)