Rare earth valence and paramagnetic properties of the skutterudites Ce1−zYbzFe4Sb12, BayFe4Sb12 and LayFe4Sb12

We report on the rare earth valence and paramagnetic properties of the double filled skutterudites Ce1−yYbyFe4Sb12, BayFe4Sb12 and LayFe4Sb12. The susceptibility of all these compounds are well described by a Curie-Weiss law above 150 K. BayFe4Sb12 and LayFe4Sb12 have, respectively, an effective moment of 3.0 and 2.9 μB  (f.u.)−1 and a paramagnetic temperature (θp) of 29 K and −70 K. In the Ce1−zYbzFe4Sb12 series, the paramagnetic temperature steadily increases from -80 K in CeFe4Sb12 to +55 K in Yb0.93Fe4Sb12, underlining a change in the character of the dominant interactions, from antiferromagnetic-to-ferromagnetic when one goes from trivalent cerium to nearly divalent ytterbium (v=2.2). The effective paramagnetic moment has been separated into three contributions, arising, respectively, from cerium, ytterbium and [Fe4Sb12]. The weak contribution of ytterbium to paramagnetism and the great similarity between the thermal variations of susceptibility in Yb0.93Fe4Sb12 and in BayFe4Sb12 seriously challenge previous scenarios for the low-temperature properties of Yb0.93Fe4Sb12. The nearly constant contribution of [Fe4Sb12] (μ[Fe4Sb12]) across the series (3.1 μB (f.u.)−1 in CeFe4Sb12 and 3.2 μB (f.u.)−1 in Yb0.93Fe4Sb12) contradicts the prediction of the “ionic model” and demonstrates the itinerant character of [Fe4Sb12] electrons. When substituting Fe by Ni in CeyFe4−xNixSb12, μ[Fe4Sb12] and θp are both affected, reflecting a change in the electronic band structure.