Seebeck coefficient in polycrystallineLa0.7Sr0.3−xAgxMnO3 pellets: analysis in terms of a phase separation model

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The temperature dependent resistivity and thermoelectric power of monovalent (Ag) dopedLa0.7Sr0.3−xAgxMnO3 polycrystallinepellets (x = 0.05,0.10,0.20,0.25) between 20 and 450 K are reported. Ag substitution enhances the conductivity of this system. The Curietemperature (TC) also increases from 303 to 364 K with increasing Ag content. In the paramagnetic region(T>TC), theelectrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region(T<TC), the resistivity data show a nearly perfect fit for all the samples to an expression containing theresidual resistivity, a two-magnon scattering term and a term associated with small-polaronmetallic conduction, which involves a relaxation time due to a soft optical phonon mode. Thesmall-polaron hopping mechanism is found to fit well to the thermoelectric power data forT>TC and in the high temperature limit for the thermoelectric power isprimarily defined by the spin contribution. At low temperatures(T<TC) in the ferromagnetic region, the Seebeck coefficient(S) is well explained with an expression of the type:SFM = S0+S3/2T3/2+S4T4, and confirms the dominant role of electron–magnon scattering. Both resistivity andthermopower data over the entire temperature range are further examined in the light of atwo-phase model based on an effective medium approximation. Thermopower data onLa0.7Sr0.3−xAgxMnO3, computed using measured resistivity and the expression derived using theeffective medium approach, give a fairly good description of the observedthermal variation of thermoelectric power in monovalent (Ag) dopedLa0.7Sr0.3−xAgxMnO3.