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 Eu(III) compound reveals a host-guest assembly Eu [symbol: see text] Na6Eu8L16 with a substantial quantum yield (32%). A similar Eu(III)-Yb(III) structure displays dual luminescence in the visible (Eu(III)) and NIR (Yb(III)) ranges.