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 K6Eu2Ga10Ge36, K6Eu2Zn5Ge41, and K6Eu2Cd5Ge41. 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 K6Eu2Ga10Ge36 and K6Eu2Zn5Ge41. The ferromagnetic ordering temperatures of 3.8, 4.0, and 9.3 K for K6Eu2Ga10Ge36, K6Eu2Zn5Ge41, and K6Eu2Cd5Ge41, respectively, are considerably reduced with respect to the 36 K observed for the type-I clathrate β-Eu8Ga16Ge30 where all cages are occupied by Eu.