Polaron hopping conduction and thermoelectric power in LaMnO3+δ

Abstract

Two different phases of LaMnO3+δ [one showing a metal–insulator transition (MIT), referred to as LaMn–C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn–S followed a nonadiabatic hopping conduction mechanism while LaMn–C and the Pb doped samples viz. La1−xPbxMnO3 (x=0.05–0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(EF), Debye temperature (θD), electron–phonon interaction constant (γP), etc. of LaMn–S showed appreciable differences from those of LaMn–C and La1−xPbxMnO3, which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons.

Evidence of non‐adiabatic small polaron hopping conduction in Bi0.1A0.9MnO3 (A = Ca, Sr, Pb)

“…At this point we can say that both the two models (Eqs. (1) and (3)) fit the conductivity data above θ D /2 [20]. We cannot, however, distinguish which of these models is better for the present semiconducting system.…”

“…where Table 1 are smaller than those of La-Pb-Mn-O or La-Ca-Mn-O systems [20,23] showing CMR behavior.…”

“…Comparing these values of J and ϕ shown above, it is observed that for all the three samples J < ϕ, which confirms that the non-adiabatic hopping condition is strictly satisfied. Here we should mention that in the La-PbMn-O systems [20] (with different concentrations of Pb), showing semiconductor to metal transitions, a changeover from adiabatic (high-temperature phase above the metal-insulator transition temperature) to non-adiabatic (low-temperature phase) hopping conduction mechanism was reported. But in the present Bi-(Sr, Pb, Ca)-Mn-O systems showing no semiconductor to metallic transition, such changeover is not detected and the system follows non-adiabatic hopping conduction throughout the range of temperature (above θ D /2).…”

“…4), the activation energy E p has been calculated as shown in Table 1. The values of θ D /2 are estimated from the temperature where deviation from linearity occurs in the high-temperature region, as shown in other perovskite manganites [20,21]. From Table 1 one finds that the activation energy E p for BSM and BPM with lower conductivity are much higher than that of BCM.…”

Magnetotransport of La0.5Ba0.5MnO3

“…They are comparable to those reported for similar manganites. 28,[52][53][54] Notice the specific B power law dependence for the derived T 0 and R W parameters. For the three dimensional case, T 0 is related to the density of states at the Fermi level N͑E F ͒ and to the localization length L as follows: 28,55 …”