Synergistic effect of lattice, electronic and magnetic modulations on the thermoelectric behaviour of Cr-substituted La0.65Bi0.20Sr0.15CoO3

Abstract

In this work, we report a detailed investigation on the electrical, magnetic, and thermal transport properties of a series of polycrystalline samples of Cr_x-substituted La_0.65Bi_0.20Sr_0.15CoO₃ (x = 0, 0.10, 0.15, 0.20 and 0.25) in view of thermoelectric applications. The optimized composition of x = 0.25 has been arrived at by studying the temperature and field dependent properties of the samples, where both Co and Cr have mixed valencies that give rise to interesting magnetic ordering, effective at low temperatures. A p-type nature is observed for all compositions from Seebeck and Hall coefficient measurements. An in-depth analysis of electrical and thermal transport provides good understanding for the various conduction mechanisms (i.e., small polaron hopping, variable range hopping and phonon drag) involved at different temperature regimes. The Seebeck coefficient increases with Cr-substitution (S_max ∼ 660 μV K⁻¹ for Cr-25 at 115 K) while electrical conductivity decreases. This decrease in electrical conductivity is due to a decrease in the mobility and charge carrier concentration (∼1.27 × 10²¹ cm⁻³ for Cr-25). The value of thermal conductivity is found to be strongly suppressed (6-fold) in Cr-doped compositions (∼0.46 W m⁻¹ K⁻¹ for Cr-25). Another important feature is the field dependent Seebeck, electrical and thermal conductivity measurements which show the negative magnetoresistance of the Cr-25 and help to improve the electrical conductivity and hence thermoelectric efficiency at low temperatures. A field induced increment in thermoelectric efficiency, i.e. ΔZT ∼ 500%, is observed for 9 Tesla at 50 K. This provides an effective scheme for magnetically tuned thermoelectricity at low temperatures.