We report the results of thermoelectric power (TEP) of a Cr doped ${\mathrm{La}}_{0.5}{\mathrm{Pb}}_{0.5}{\mathrm{Mn}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ $(x=0\mathrm{}–0.45)$ system measured both in the presence and the absence of magnetic field $(B=1.5\mathrm{}\mathrm{T}).\mathrm{}$ The small field dependence of the Seebeck coefficient is observed around the metal-semiconductor transition (MST) temperature ${(T}_p)$ of the samples. The field dependence of TEP is stronger in the undoped sample. It is noticed from the TEP data that the small polaron hopping conduction mechanism is valid for all these samples (for $T>\mathrm{}{T}_p).\mathrm{}$ The polaron radius $r_p$ is found to decrease with increase of magnetic field. Low-temperature (below $T_p)$ field-dependent and field-independent TEP data can be fitted with ${S=S}_0{+S}_{3/2}{T}^{3/2}{+S}_4{T}^4$ suggesting that the electron-magnon scattering strongly affects the low-temperature (ferromagnetic phase) TEP data of the manganites. Activation energy gradually increases with increasing Cr concentration both in the presence and the absence of magnetic field. Field-dependent thermopower also indicates the importance of spin fluctuations affecting the phonon scattering. Power factor ${(S}^2/\rho )$ estimated from the Seebeck coefficient (S) and resistivity $\left(\rho \right)$ (at zero and 1.5 T field) showed a minimum around the MST temperature similar to the field-dependent thermal conductivity data.

• Received 21 May 2001

DOI:https://doi.org/10.1103/PhysRevB.64.104428