Magnetic, thermal, and electronic properties of iron-antimony filled skutterudites MFe4Sb12 (M = Na, K, Ca, Sr, Ba, La, Yb)

The magnetic susceptibility, field-dependent heat capacity, and electrical transport properties (resistivity and Hall effect) are investigated for filled skutterudites MFe4Sb12 with M = Na, K, Ca, Sr, Ba, Yb, and La. The specific heat c(p)(T) reveals linear terms gamma ranging from 100 to 200 mJ mol(-1) K-2. An Einstein term contributing to c(p)(T) is analyzed and discussed in connection with thermal vibrations of the cations M. The Einstein temperatures Theta(E)= 70-105 K are compared with values derived from atomic displacement parameters obtained by x-ray diffraction and other methods. Deviations of the amplitude and frequency of the Einstein term from the expected values are discussed. The paramagnetic susceptibility and the magnetic ground state varies systematically with the cation charge. While NaFe4Sb12 and KFe4Sb12 are nearly half-metallic weak itinerant ferromagnets with T-C approximate to 80 K, CaFe4Sb12, SrFe4Sb12, BaFe4Sb12, and YbFe4Sb12 are nearly ferromagnetic metals with a high Sommerfeld-Wilson ratio and strong ferromagnetic spin fluctuations. LaFe4Sb12 is paramagnetic with predominantly antiferromagnetic fluctuations. The decrease of the low-temperature specific heat with magnetic field is investigated. A large positive magnetoresistance is observed for both ferromagnetic and nearly ferromagnetic compounds, while that of the La compound is small. Size and temperature dependence of the Hall coefficient points to a compensation of hole and electron carriers. The temperature dependence of the resistivity displays different characteristic power laws and an anomaly around 80 K. These observations can be explained by details of the electronic structure. A systematic variation of the properties with charge transfer from the cation M to the [Fe4Sb12] polyanion is recognized.