abstract
A series of Ba1-xEuxTiO3-delta (0.1 <= x <= 0.9) phases with similar to 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 angstrom (equal to similar to 5 unit cells) deviations from the cubic structure model (Pm% (3) over barm) were detected by evaluation of the pair distribution function (PDF). These deviations cannot be explained by a simple symmetry breaking model like in EuTiO3-delta. 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-delta. 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-delta (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-delta showed a ZT value of 0.24 at 1123 K.
keywords
BARIUM-TITANATE; PIEZOELECTRIC PROPERTIES; ELECTRONIC-STRUCTURE; TRANSPORT-PROPERTIES; OPTICAL-PROPERTIES; THIN-FILMS; SRTIO3; FERROELECTRICITY; CE1-XGDXO2-X/2; PERFORMANCE
subject category
Chemistry; Physics
authors
Xiao, XX; Widenmeyer, M; Xie, WJ; Zou, TH; Yoon, S; Scavini, M; Checchia, S; Zhong, ZC; Hansmann, P; Kilper, S; Kovalevsky, A; Weidenkaff, A
our authors
Andrei Kavaleuski
Principal Researcheracknowledgements
This research was supported by Deutsche Forschungsgemeinschaft through the DFG Priority Program SPP 1386 (Grant WE 2803/2-2). The authors are grateful to the MPI-FKF Nanostructuring Lab (Prof. Dr Jurgen Weis) for providing the opportunity to use the SEM facility and to the European Synchrotron Radiation Facility (ESRF) for the provision of beam time. They are also greatly indebted to Dr Mauro Coduri for kind assistance in using the ID22 beamline. Dr A. V. Kovalevsky acknowledges the support of FCT (projects UID/CTM/50011/2013 and IF/00302/2012). Finally, we would like to thank Dr Angelika Veziridis for intensive discussion on the manuscript.