VALENCE CHANGE AND PHYSICAL-PROPERTIES IN SEMICONDUCTING MIXED-VALENT RARE-EARTH COMPOUNDS

Ce3Pt3Sb4 is a mixed-valent rare-earth semiconductor, the ground state properties of which can be explained by the band theory based on an itinerant electron model for the 4f electrons. On the other hand, the isostructural compound Ce3Au3Sb4 is also a semiconductor with a resistivity of 22 OMEGA cm at 0.13 K, but magnetic susceptibility and inelastic neutron scattering show that 4f electrons are well defined to be localized 4f1 state. In the latter, no clear anomaly in the thermomagnetic curve down to 0.5 K has been observed. However, there exists a broad peak at 1.5 K in the specific heat (C/T approximately 2500 mJ/mole K2) and the peak, which is symmetrical against log T, shifts towards the higher temperature side under the influence of a magnetic field. In this sense, Ce3Au3Sb4 is a good candidate for the study of the Kondo effect in an insulator.