Nonadiabatic Hopping Conduction in Sr1+xLa1-xFeO4 (0x0.20) below 300 K

We measured bulk conductivities using complex-plane impedance analyses and frequency dependencies of dielectric properties in Sr1+xLa1-xFeO4 (0x0.20) with the four-probe dc conductivities. The temperature dependence of bulk conductivity in every specimen exhibits the nonadiabatic hopping conduction; i.e., =A0 exp[-(WH+WO/2)/kBT]/T3/2, WH and WO being the hopping energy and the energy required to create a free hopping polaron. In every specimen, a dielectric relaxation process shows up with an activation energy nearly equal to that of the bulk conduction. The electron transfer integrals, J, estimated in the dielectric relaxations meet the nonadiabatic requirements; i.e., J< 4WH and J<(4WHkBT)1/4(OL)1/2, where OL is the frequency of the longitudinal optical mode. Speculation on the pre-exponential factor of the conductivity, A0, indicates that the holes in valence bands created by electron excitation predominantly contribute to electrical transport rather than the doped holes introduced by Sr-substitution for La. The experimental results are explained self-consistently by nonadiabatic hopping conduction.