Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1-xCrxO3

The conductivity and magnetoresistance of La0.5Pb0.5Mn1-xCrxO3 (0.0 less than or equal tox less than or equal to0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158-276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1-xCrxO3 showing no (MIT) for x greater than or equal to0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (T-p) shifts towards lower temperature with increasing x while magnetic field shifts T-p to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (rho) curve (below T-p) is well fitted with rho (T)=rho (0)+rho T-2.5(2.5) indicating the importance of electron-magnon interaction (second term). We have successfully fitted the high temperature (T > theta (D)/2, theta (D) is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the samples showing MIT while nonadiabatic hopping conduction mechanism is obeyed by the samples showing no MIT. The lower temperature (between T-p and theta (D)/2) conductivity data of all the samples can be well fitted to the variable range hopping (VRH) model similar to the case of many semiconducting transition metal oxides. Temperature dependent Seebeck coefficient data also support the small polaron hopping conduction mechanism above T-p. (C) 2001 American Institute of Physics.