We have investigated the charge ordering phenomenon from the temperature dependence of inverse susceptibility, resistivity, and thermoelectric power (TEP) for Bi1-xSrxMnO3 (BSMO) from 300 K to 700 K. At high temperatures, susceptibility follows Curie-Weiss law. The resistivity data indicate insulating behavior of BSMO. TEP (S(T)) value is negative and weakly temperature-dependent in the high temperature regime. The slope of TEP changes dramatically near the charge ordering temperature (ΤCO), indicating an increase of energy gap due to the charge ordering. In the vicinity of ΤCO, thermal hysteresis is observed in TEP data as well as in the resistivity data, which is consistent with the nature of the martensitic transition of the charge ordering phenomena. From this hysteretic behavior, we estimated ΤCO. As Sr concentration increases, ΤCO shifts to lower temperature from ΤCO ~ 490 K for x = 0.45 to ΤCO ~ 435 K for x = 0.8, and the thermal hysteretic behavior becomes less pronounced. The electrical transport properties have been discussed in terms of carrier localization due to charge ordering transition accompanied by the local lattice distortions.
The lattice effects on the magnetic and transport properties in La0.67-xGdxSr0.33CoO3 series are studied. The introduction of smaller Gd3+ ions leads to an enhanced mismatch between the La-O layer and the CoO2 layer and a decrease of the tolerance factor t. The spin-state of trivalent Co ion transits to low-spin state with the decrease of Co-O bond length. The doping of Gd3+ drives the system from the cluster-glass state to the spin-glass state and progressively decreases the Curie temperature. At high Gd3+ doping content, an interesting negative
magnetoresistance occurs at low temperature.