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 th an or equal tox less than or equal to0.45) measured at 0.0 and 1.5 T magnet ic field have been reported. All the oxide samples except x=0.45, showed me tal insulator transition (MIT) between 158-276 K, depending on x. In contra st 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 w ith 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 resi stivity (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 te rm). We have successfully fitted the high temperature (T > theta (D)/2, the ta (D) is Debye temperature) conductivity data, both in presence and in abs ence of magnetic field, with small polaron hopping conduction mechanism. Ad iabatic small polaron hopping conduction mechanism is followed by the sampl es 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 variab le range hopping (VRH) model similar to the case of many semiconducting tra nsition metal oxides. Temperature dependent Seebeck coefficient data also s upport the small polaron hopping conduction mechanism above T-p. (C) 2001 A merican Institute of Physics.