Impact of charge doping, oxygen disorder and hydrostatic pressure on thermoelectric and magnetic properties of NdBa0.94La0.06Co2O5+δ

• The behavior of Seebeck coeff. is well described below TMIT by the Heikes formula.
• Thermal conductivity exhibits characteristic decrease at TMIT.
• The TMIT and TN reach maximum values for NdBa0.94La0.06BaCo2O5.5.
• Stabilization of AFM phase by hydrostatic pressure was noticed for δ = 0.5.
• For δ = 0.4–0.6, crystal structure is of “122-type” in Pmmm space group.

Structural, thermoelectric and magnetic properties of NdBa2+1−yLa3+yCo2O5+δ (y = 0.06) were investigated over a wide range of oxygen content 0.26 < δ < 0.90. The single-phase orthorhombic samples with ordered planes of the 5- and 6-coordinated to oxygen cobalt ions were obtained and studied for 0.45 < δ < 0.55. It was found that the Seebeck coefficient in the studied samples is magnetic field insensitive and shows that the doping with cation and oxygen is well described below metal–insulator transition at TMIT by the Heikes formula with the effective charge doping c = −y/2 + δ − 0.5. Thermal conductivity exhibits characteristic decrease at TMIT due to removal of the electronic contribution and is well correlated with suppression of the Seebeck coefficient. The TMIT and the Neel temperature TN reach maximum values for slightly electron doped sample c = −0.03 with a perfect oxygen ordering δ = 0.5 while the Curie temperature TC shows continuous decrease with the increase of δ. Comparison of magnetic ac and dc measurements with those for the GdBaCo2O5+δ (Taskin et al., 2005) and Ca substituted NdBaCo2O5.5 (Kolesnik et al., 2012) systems shows that perfection of the oxygen ordering is more important in controlling the magnetic properties than the overall charge doping. The hydrostatic pressure was observed to stabilize the antiferromagnetic phase for NdBa0.94La0.06Co2O5.5.