The Seebeck effect and the thermal diffusivity of La0.67Ca0.33MnO3−δ were investigated in the temperature range 35<T<300 K and in particular in the regime of the high magnetoresistance and maximum resistivity ρmax. We find a sign change of the thermoelectric power S(T) and a step in heat conductivity near ρmax which we attribute to a temperature-induced metal–insulator transition at about 80 K, suggesting that the large giant magnetoresistance found in these substances is due to a field-induced metal–insulator transition.

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The antiferromagnetic BaCo0.9Ni0.1S2−y and the paramagnetic CuIr2S4 Mott systems show an inverse conductivity behavior, i.e., the metallic phase occurs at low or high temperatures, respectively, which also suggests inverse behavior in other transport properties. In this contribution we present magnetization data, conductivity, thermal conductivity, and Seebeck effect in the temperature range 4.2 K<T<350 K and in particular through the metal-insulator transition at TMI. We recover the anomalies in χ(T),σ(T), which are discussed using Jahn-Teller and anti-Jahn-Teller models; also, we compare the anomalies in σ(T), S(T), and of the heat conductivity κ(T) at TMI. In particular, for BaCo0.9Ni0.1S1.95, we find a step in κ of almost one order of magnitude (times 6) and a large pressure shift, ⩽−24 K/kbar, which makes this material a potential pressure activated heat switch, while CuIr2S4 shows a small step of κ (times 1.2) and a pressure shift of +2.8 K/kbar.

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