Complex transport and magnetism in inhomogeneous mixed valence Ce3Ir4Ge13

We report the discovery of Ce3Ir4Ge13, a Remeika phase with a complex array of structural, electronic, and magnetic properties. Our single crystal x-ray diffraction measurements show that Ce3Ir4Ge13 forms in the tetragonally distorted I4(1)/amd space group, which has three distinct rare earth sites. The electrical resistivity is almost temperature independent over three decades in temperature, from 0.4 K to 400 K, while the Hall coefficient measurements are consistent with a low-carrier semimetal. Magnetic susceptibility measurements reveal an effective moment, mu(exp)(eff) = 1.87 mu(B)/Ce, that falls significantly short of the expected value for Ce3+, suggesting that two of the rare earth sites are occupied by magnetic Ce3+ while the third is occupied by nonmagnetic Ce4+. Upon cooling, Ce3Ir4Ge13 first enters a short range magnetically ordered state below T-SRO = 10 K, marked by a deviation from Curie-Weiss behavior in susceptibility and a broad field-independent heat capacity anomaly. At lower temperatures, we observe a second, sharper peak in the heat capacity at T* = 1.7 K, concurrent with a splitting of the field-cooled and zero-field-cooled susceptibilities. A small resistivity drop at T* suggests a loss of spin disorder scattering consistent with a magnetic ordering or spin freezing transition. Ce3Ir4Ge13 is therefore a rare example of an inhomogeneous mixed valence compound with a complex array of thermodynamic and transport properties.