Magnetic and Thermoelectric Properties of a Heterogeneous Mixed-Valence System, Yb2Pt3Sn5

We have studied the valence states and thermoelectric properties of Yb2T3Sn5 (T = Pt and Pd) with two inequivalent Yb sites. For Yb2Pt3Sn5, the 4f -hole occupation number nf is estimated to be 0.4 at 300 K from the magnetic susceptibility and LIII-edge absorption spectrum. Intermediate-valence behaviors manifest themselves in a largely negative value of the paramagnetic Curie temperature, � 216 K, a broad peak at 300 K in the magnetic part of the resistivity, and a large minimum in the thermopower, � 38 mV/K at 60 K. The specific heat shows no transition down to 0.6 K. These results are explained by a model in which Yb ions in one site are divalent and those in the other site are in a valence fluctuating state with a Kondo temperature of 200 K. Thus, Yb2Pt3Sn5 is a rare example of the heterogeneous mixed-valence system. An isostructural compound Yb2Pd3Sn5 has a larger unit-cell volume by 2.4%, which allows the Yb ions in both the sites to be divalent.