Low-temperature antiferromagnetic moments at the 4a site in Ce3Pd20Ge6

We have performed powder neutron diffraction experiments to examine the magnetic ordering in Ce3Pd20Ge6, which exhibits ferroquadrupolar ordering at TQ1≈1.2 K and antiferromagnetic ordering at TN2 = 0.75 K. In the series of isostructural cubic rare-earth (R) compounds R3Pd20X6 (X = Si, Ge; space group Fmm), it is common that at the Neel temperature TN1 the magnetic moments at the 8c site (simple cubic sublattice) order in an antiferromagnetic sequence with propagation vector k1 = [1,1,1] and at TN2 the magnetic moments at the 4a site (face-centred cubic sublattice) order with a propagation vector k2 = [0,0,1]. In Ce3Pd20Ge6 the Neel temperature TN1 is replaced by TQ1 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 TN2 with the expected antiferromagnetic structure. The moments align perpendicular to the propagation vector k2 = [0,0,1] with a saturation value µ2(4a) = (1.1±0.1) µB/Ce that is reasonably large for the crystal field ground-state quartet Γ8. Our results provide indirect experimental evidence that only the quadrupolar moments of cerium at the 8c site are involved in the phase transition at TQ1.