Exploring the valence transition in CeOs4Sb12 in high magnetic fields*

We present measurements of resistivity, magnetostriction, and PDO conductivity on the skutterudite CeOs₄Sb₁₂ in static and pulsed magnetic fields up to 30/60 T, respectively. The temperature and field dependencies allow us to map out the boundaries of the low temperature phase (L phase) in the H-T-phase diagram which - contrary to previous results - shows an unusual dome-like appearance extending to 45 T. The L phase is characterized by semiconducting behaviour with a small Fermi surface whereas the metallic high-temperature, high-field H phase exhibits a large Fermi surface. Magnetostriction results suggest that the L to H transition is accompanied by a valence transition. Additional features in transport and PDO within the L phase point towards further alteration of the band structure with applied field. We discuss the phase diagram of CeOs₄Sb₁₂ in the context of hybridization of 4f electrons and compare it to other skutterudites.