Effect of the Al-doping on the electrical and thermoelectric response of TbMnO₃ polycrystalline samples: Evidence of polaronic transport

High-quality, polycrystalline Tb_1−xAl_xMnO₃ (x = 0–0.2) samples were prepared by means of standard solid state reaction. The incorporation of the Al ions into the TbMnO₃ lattice is verified by X-ray diffraction measurements. The zero-field electrical conductivity, thermopower, and thermal conductivity of these samples are carefully measured and analyzed. The pristine compound shows low electrical conductivity, high and positive thermopower values (∼300 μV/K at 300 K) in the whole temperature range measured, and rather low thermal conductivity (∼5 W m⁻¹K⁻¹ at 300 K). Al substitution leads to a dramatic reduction in resistivity and stabilization of the ferromagnetic phase at low temperatures. The low value of the electrical conductivity constitutes a major obstacle for thermoelectric applications of this material. Nevertheless, the incorporation of Al ions at the Tb site provokes a slight reduction in the value of the thermopower and a slight increase in the thermal conductivity. For the tested samples dκ/dT > 0 (the behavior of an amorphous solid) is verified in the whole temperature range (2–390 K). Nevertheless, dκ/dT > 0 here should be related not to quenched structural disorder but rather to unusually large dynamic lattice distortions accompanying charge transport. The lower values of κ at low-temperatures (as compared to those reported for single crystals) can be linked to the high concentration of grain boundaries. The variation in both the resistivity and thermopower with temperature is well described by the polaronic model, which, nevertheless, is not linked to the stabilization of a ferromagnetic phase in the compound. Although the values of the figure of merit ended up being too small to thermoelectric applications, it is noted that the addition of Al ions improve the values of this parameters by more than one order of magnitude. The improvement of the low electrical conductivity upon Al addition is behind this technical achievement. - Graphical abstract: Temperature dependent resistivity for Tb_1−xAl_xMnO₃ samples with Al content x = 0, 0.05, 0.1, and 0.2. Inset: variation of ln(ρ/T) versus 1000/T for the samples in the main panel. - Highlights: • High-quality, polycrystalline Tb_1−xAl_xMnO₃ (x = 0–0.2) samples are synthesized by physical route. • The electrical and thermoelectric properties of the samples are carefully measured. • Al-doping improves largely the electrical conductivity. • Electrical and thermoelectrical data are well described by the polaronic model. • The values of the figure of merit are low, but comparable to those found for other perovskite-type materials.