Thermal Conductivity of Sm 3 S 4 System with Mixed Valence Sm Ions

Abstract

The thermal conductivity (κ) and electrical resistivity (ρ) of mixed-valence compound Sm3S4 have been measured in the temperature range 5 to 300 K. The present results and those presented previously [1] for the thermal conductivity between 80 to 850 K are interpreted in terms of the temperature-dependent fluctuating valence of Sm ions. Sm3S4 crystallizes in the cubic Th3P4 structure, and the cations with different valences occupy equivalent lattice sites. Divalent and trivalent Sm ions are randomly distributed in the ratio of 1:2 over all possible crystallographic cation positions (Sm2+ 2Sm3+ 2S2− 4). The behavior of the Sm3S4 lattice thermal conductivity κ ph is extraordinary since valences of Sm ions are fluctuating (Sm3+→Sm2+) with a temperature dependent frequency. In the interval 20 to 50 K (low “hopping” frequencies), κ ph of Sm3S4 varies as κ ph ∼T −1 (it is similar to materials with static distribution of cations with different valences): at 95 to 300 K (average “hopping” frequencies 107 to 1011 Hz), κ ph changes as κ ph ∼T −0.3 (it is similar to materials with defects). Defects in Sm3S4 appear because of local strains in the lattice by the electrons hopping from Sm2+ ions (with big ionic radii) to Sm3+ ions (with small ionic radii) and back (Sm2+→Sm3+), at T>300 K (high “hopping” frequencies), κ ph becomes similar to materials with homogenous mixed valence states [1].