Tailoring the structure and thermoelectric properties of BaTiO3via Eu2+substitution.
A series of Ba1-xEuxTiO3-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 BaTiO3crystal 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 EuTiO3-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 Ba1-xEuxTiO3-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 Ba1-xEuxTiO3-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, Ba0.1Eu0.9TiO3-δshowed a ZT value of 0.24 at 1123 K.
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Xingxing
X
Xiao
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Marc
M
Widenmeyer
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Wenjie
W
Xie
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Tianhua
T
Zou
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Songhak
S
Yoon
University of Milan, Chemistry Department, Via C. Golgi 19, I-20133 Milano, Italy and CNR-ISTM, Ist. Sci. & Tecnol. Mol., I-20133 Milan, Italy.
Marco
M
Scavini
University of Milan, Chemistry Department, Via C. Golgi 19, I-20133 Milano, Italy.
Stefano
http://orcid.org/0000-0003-0499-4885
S
Checchia
Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany.
Zhicheng
Z
Zhong
Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany.
Philipp
P
Hansmann
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Stefan
S
Kilper
CICECO - Aveiro Institute of Materials, University of Aveiro, Department of Materials and Ceramic Engineering, 3810-193 Aveiro, Portugal.
Andrei
http://orcid.org/0000-0001-5814-9797
A
Kovalevsky
University of Stuttgart, Institute for Materials Science, Heisenbergstr. 3, 70569 Stuttgart, Germany. weidenkaff@imw.uni-stuttgart.de.
Anke
http://orcid.org/0000-0002-7021-1765
A
Weidenkaff