Lattice effects on the transport properties of (R,Sr)(3)Mn2O7 (R = La, Eu,and Pr)

The resistivity, magnetoresistance, and thermoelectric power (TEP) of R-1.4 (Sr1-yCay)(1.6)Mn2O7 and R(Sr1-yCay)(2)Mn2O7 (R = La, Eu, Pr) have been stu died to investigate the lattice effects (when the Mn4+ concentration is 30 and 50 %) on the transport properties. The semiconducting behavior in the r esistivity curve can be explained with the nearest-neighbor small polaron h opping. A changeover from the adiabatic to the nonadiabatic regime in the h opping mechanism is observed when the ratio of Mn4+/(Mn3++Mn4+) changes fro m 30 to 50 %. The behavior of the magnetoresistance in R-1.4(Sr1-yCay)(1.6) Mn2O7 is different from that in R(Sr1-yCay)(2)Mn2O7 which might be due to t he presence of a more complex magnetic interaction dominated by superexchan ge in R(Sr1-yCay)(2)Mn2O7. The TEP data below the metal-insulator transitio n of R-1.4(Sr1-yCay)(1.6)Mn2O7 are the combinations of the phonon drag and the diffusion thermopower but in RSr2Mn2O7 some other terms may be present.