Low-temperature antiferromagnetic moments at the 4a site in Ce 3 Pd 20 Ge 6

We have performed powder neutron diffraction experiments to examine the magnetic ordering in Ce 3 Pd 20 Ge 6 , which exhibits ferroquadrupolar ordering at T Q 1 ≈1.2 K and antiferromagnetic ordering at T N 2 = 0.75 K. In the series of isostructural cubic rare-earth (R) compounds R 3 Pd 20 X 6 (X = Si, Ge; space group Fm m ), it is common that at the Néel temperature T N 1 the magnetic moments at the 8c site (simple cubic sublattice) order in an antiferromagnetic sequence with propagation vector k 1 = [1,1,1] and at T N 2 the magnetic moments at the 4a site (face-centred cubic sublattice) order with a propagation vector k 2 = [0,0,1]. In Ce 3 Pd 20 Ge 6 the Néel temperature T N 1 is replaced by T Q 1 and at the 8c site a possible ordered cerium moment is found to be reduced to an undetectable small value down to the lowest measured temperature T = 0.05 K. In contrast, at the 4a site the cerium moments order below T N 2 with the expected antiferromagnetic structure. The moments align perpendicular to the propagation vector k 2 = [0,0,1] with a saturation value µ 2 (4a) = (1.1±0.1) µ B /Ce that is reasonably large for the crystal field ground-state quartet Γ 8 . Our results provide indirect experimental evidence that only the quadrupolar moments of cerium at the 8c site are involved in the phase transition at T Q 1 .