Mixed cerium valence and unusual Ce–Ru bonding in Ce23Ru7Cd4

The rare earth-rich compounds Ce23Ru7Cd4 and Pr23Ru7Cd4 were synthesized from the elements in sealed tantalum ampoules in an induction furnace. Both structures were refined on the basis of diffractometer data: P63mc, Z = 2, a = 988.7(3), c = 2241.6(5) pm, wR2 = 0.0439, 1976 F2 values for Ce23Ru7Cd4 and a = 992.7(2), c = 2236.4(7) pm, wR2 = 0.0466, 2528 F2 values for Pr23Ru7Cd4 with 74 variables per refinement. Striking structural motifs are ruthenium centered trigonal prisms RuCe6 and RuPr6 which are condensed via common edges and corners, building rigid three-dimensional networks. Larger voids within these networks are filled by slightly elongated Cd4 tetrahedra. Five of the nine crystallographically independent cerium sites in Ce23Ru7Cd4 show Ce–Ru distances which are shorter than the Pr–Ru distances in Pr23Ru7Cd4. This strong hint for mixed cerium valence is supported by the magnetic behavior. Pr23Ru7Cd4 shows Curie–Weiss behavior above 50 K with an experimental magnetic moment of 3.62 μB/Pr atom, indicating stable trivalent praseodymium. Complex magnetic ordering sets in at 13 K. Ce23Ru7Cd4 shows a reduced magnetic moment of 2.05 μB/Ce atom. The trivalent cerium atoms show ferro- or ferrimagnetic ordering below TC = 3.6 K.