Abstract
A systematic investigation of cadmium‐doped manganites with compositional formula La~0.67~(Sr~0.33–y~ Cd__~y~__ ) MnO~3~ (y = 0–0.33) has been undertaken mainly to understand their structural as well as their electrical transport behavior. The materials were prepared by the citrate‐gel route and were characterized by X‐ray diffraction (XRD). By analyzing the XRD data using the Rietveld refinement technique, it has been concluded that these materials have rhombohedral structure with R$ \bar 3 $c space group. It has been found that the ferro–paramagnetic transition temperature (T ~C~) values decrease continuously with increasing Cd concentration, while the metal–insulator transition temperatures (T ~P1~), except in the cases of two samples (with y = 0.066 and 0.132), are found to decrease. A qualitative explanation for the observed behavior is offered. With a view to understanding the conduction mechanism, electrical resistivity and thermopower measurements were carried out and the data were analyzed using various theoretical models. It has been concluded that the resistivity (ρ ) data in the ferromagnetic metallic part (T < T ~P1~) is in agreement with ρ (T ) = ρ ~0~ + ρ ~2~T ^2^ + ρ ~4.5~T ^4.5^, indicating the importance of grain/domain boundary effects, electron–electron and two‐magnon scattering processes in the conduction mechanism, while, at high temperature (T > T ~P1~), the paramagnetic insulating regime, the adiabatic small polaron model fits well, thereby indicating that polaron hopping might be responsible for the conduction mechanism. Finally, the low‐temperature thermopower (T < T ~P1~) data were analyzed by considering the spin‐wave fluctuation (∼T ^4^) in addition to the magnon scattering, whereas the high‐temperature (T > T ~P1~) data were analyzed by Mott's equation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)