Adiabatic small polaron-hopping conduction in Ln0.85Ca0.15MnO3 (Ln = Nd, Pr and Sm) perovskites

Polycrystalline bulk ferromgnetic insulating (FMI) Ln(0.85)Ca(0.15)MnO(3) (Ln = Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>theta(D)/2, theta(D) being the Debye temperature) dc conductivity (sigma(dc)) of these samples is due to adiabatic small polaron-hopping conduction. Greaves' and Mott's variable range hopping (VRH) conduction mechanisms are not Suitable to explain the sigma(dc) data at low temperature (T