New type-I clathrates with ordered Eu distribution

In our search for clathrates containing rare-earth elements we have discovered the three new clathrate-I phases K 6 Eu 2 Ga 10 Ge 36 , K 6 Eu 2 Zn 5 Ge 41 , and K 6 Eu 2 Cd 5 Ge 41 . The Eu atoms occupy only the smaller cages (pentagondodecahedra) leading to a fully ordered arrangement of the guest atoms. X-ray absorption spectroscopy and magnetic susceptibility measurements reveal that at room temperature and above Eu is close to divalent in all compounds. Magnetization measurements at low temperatures, however, may indicate slightly reduced magnetic moments for K 6 Eu 2 Ga 10 Ge 36 and K 6 Eu 2 Zn 5 Ge 41 . The ferromagnetic ordering temperatures of 3.8, 4.0, and 9.3 K for K 6 Eu 2 Ga 10 Ge 36 , K 6 Eu 2 Zn 5 Ge 41 , and K 6 Eu 2 Cd 5 Ge 41 , respectively, are considerably reduced with respect to the 36 K observed for the type-I clathrate β - Eu 8 Ga 16 Ge 30 where all cages are occupied by Eu.