Tailoring the structure and thermoelectric properties of BaTiO₃via Eu²⁺ substitution

A series of Ba_1-xEu_xTiO_3-δ (0.1 ≤ x ≤ 0.9) phases with ∼40 nm particle size were synthesized via a Pechini method followed by annealing and sintering under a reducing atmosphere. The effects of Eu²⁺ substitution on the BaTiO₃ crystal structure and the thermoelectric transport properties were systematically investigated. According to synchrotron X-ray diffraction data only cubic perovskite structures were observed. On the local scale below about 20 Å (equal to ∼5 unit cells) deviations from the cubic structure model (Pm3[combining macron]m) were detected by evaluation of the pair distribution function (PDF). These deviations cannot be explained by a simple symmetry breaking model like in EuTiO_3-δ. The best fit was achieved in the space group Amm2 allowing for a movement of Ti and Ba/Eu along 〈110〉 of the parent unit cell as observed for BaTiO₃. Density functional calculations delivered an insight into the electronic structure of Ba_1-xEu_xTiO_3-δ. From the obtained density of states a significant reduction of the band gap by the presence of filled Eu²⁺ 4f states at the top of the valence band was observed. The physical property measurements revealed that barium-europium titanates exhibit n-type semiconducting behavior and at high temperature the electrical conductivity strongly depended on the Eu²⁺ content. Activation energies calculated from the electrical conductivity and Seebeck coefficient data indicate that at high temperatures (800 K < T < 1123 K) the conduction mechanism of Ba_1-xEu_xTiO_3-δ (0.1 ≤ x ≤ 0.9) is a polaron hopping when 0 < x ≤ 0.6 and is a thermally activated process when 0.6 < x < 1. Besides, the thermal conductivity increases with increasing Eu²⁺ concentration. Due to a remarkable improvement of the power factor, Ba_0.1Eu_0.9TiO_3-δ showed a ZT value of 0.24 at 1123 K.