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

Abstract

We have studied the valence states and thermoelectric properties of Yb 2 T 3 Sn 5 (T = Pt and Pd) with two inequivalent Yb sites. For Yb 2 Pt 3 Sn 5 , the 4 f -hole occupation number n f is estimated to be 0.4 at 300 K from the magnetic susceptibility and L III -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 µV/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, Yb 2 Pt 3 Sn 5 is a rare example of the heterogeneous mixed-valence system. An isostructural compound Yb 2 Pd 3 Sn 5 has a larger unit-cell volume by 2.4%, which allows the Yb ions in both the sites to be divalent.