Observation of a new ordered phase in the kondo semiconductor CeOs₄Sb₁₂

We report the results of high-resolution dilatometry and dc magnetization measurements of a small single crystal of CeOs₄Sb₁₂, which exhibits an anomalous ordered state. Both linear thermal expansion coefficient and magnetization results reveal the presence of a new phase (phase A) transition at T_A = 0.87 K, slightly below the antiferromagnetic (AFM) transition temperature T_N = 1.05 K. In addition, the magnetostriction and magnetization results provide unambiguous thermodynamic evidence of the presence of a field-induced phase transition to a high-field ordered phase (phase B), as previously suggested by neutron scattering experiments. The resulting magnetic field-temperature (H-T) phase diagram of CeOs₄Sb₁₂ consists of three ordered phases: A, B, and AFM phases. From a thermodynamic point of view, it is likely that the AFM and high-field ordered states coexist within the A phase, and that the B phase is of nonmagnetic origin.