Impact of charge doping, oxygen disorder and hydrostatic pressure on thermoelectric and magnetic properties of NdBa_0.94La_0.06Co₂O_5+δ

Highlights: • The behavior of Seebeck coeff. is well described below T_MIT by the Heikes formula. • Thermal conductivity exhibits characteristic decrease at T_MIT. • The T_MIT and T_N reach maximum values for NdBa_0.94La_0.06BaCo₂O_5.5. • 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. - Abstract: Structural, thermoelectric and magnetic properties of NdBa²⁺_1−yLa³⁺_yCo₂O_5+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 T_MIT by the Heikes formula with the effective charge doping c = −y/2 + δ − 0.5. Thermal conductivity exhibits characteristic decrease at T_MIT due to removal of the electronic contribution and is well correlated with suppression of the Seebeck coefficient. The T_MIT and the Neel temperature T_N reach maximum values for slightly electron doped sample c = −0.03 with a perfect oxygen ordering δ = 0.5 while the Curie temperature T_C shows continuous decrease with the increase of δ. Comparison of magnetic ac and dc measurements with those for the GdBaCo₂O_5+δ (Taskin et al., 2005) and Ca substituted NdBaCo₂O_5.5 (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 NdBa_0.94La_0.06Co₂O_5.5