Structure and thermoelectric properties of EuTi(O,N)3 ± δ

Abstract

After partial substitution of nitrogen for oxygen in EuTiO 3 , the crystal structure, thermoelectric properties, morphology, and electronic structure of the products were analyzed and compared with pristine EuTiO 3. The space group of EuTi(O,N) 3 6 d was orthorhombic Pnma due to the tilt and rotation of the anion octahedra, compared to cubic Pm 3m of EuTiO 3 (at room temperature). The thermoelectric properties of oxynitride polycrystalline bodies sintered in three different ways were investigated in the temperature range of 300 K < T < 950 K. The Seebeck coefficients (S) of the oxynitrides were lower compared with the oxide, and the electrical resistivities (q) were increased about one order of magnitude. The activation energies (E A) indicated a larger band gap of EuTi(O,N) 3 6 d when compared to the pristine EuTiO 3 ($1.3 eV compared to 0.98 eV). A morphological characterization by transmission electron microscopy and scanning electron microscopy illustrated intrinsic nanopores within the individual particles and weak grain-interconnections indicating poor intergrain electron transport. Ab initio calculations of the electronic structures confirmed a larger band gap of the distorted crystal structure of the oxynitride and showed a decrease of the density of states at the Fermi level, explaining the reduction of the measured S. V

Tailoring the structure and thermoelectric properties of BaTiO3via Eu2+ substitution

“…The results revealed that BaTiO3 has in accordance with literature reports 6,63 an energy gap of 3.2 eV between filled oxygen 2p states and empty Ti 3d states. Similar as previously reported 6,24,63,64 for EuTiO3 an energy gap of 0.9 eV between filled Eu 4f (leading to a magnetic moment of 7B) and empty Ti 3d states was observed. For Ba1-xEuxTiO3 (x > 0) narrow and sharp Eu 2+ 4f states were present in the original band gap of BaTiO3.…”

“…The small polaron hopping nature of BaTiO3 was already clarified by Ihrig et al 68 . Whereas, when x > 0.6, EA is not larger than ES implying the dominating conducting mechanism of Ba1-xEuxTiO3- system changes from polaron hopping conduction into thermally activated band conduction, which is consistent with EuTiO3 24 .…”

“…Their reduced forms BaTiO3- and EuTiO3- show n-type semiconducting behavior at high temperatures 23,24 . Sofo and Mahan 25 pointed out that semiconductors with a band gap of 10 kBTo are good thermoelectrics, kB being the Boltzmann constant and To the operating temperature.…”

“…Similar to most of the ATiO3 perovskites (e.g., SrTiO3, BaTiO3), the charge transfer gap of EuTiO3 separates a valence band of filled oxygen 2p states and an empty conduction band of titanium 3d states 28,29 . However, a key difference of EuTiO3 is the presence of a narrow and sharp Eu 2+ 4f band localized at the top of the valence band 8,30 , which is beneficial to a large Seebeck coefficient 24 .…”

Influence of tungsten substitution and oxygen deficiency on the thermoelectric properties of CaMnO3−δ

“…Further details on experiment, quantitative analysis and expected precision are given elsewhere. 41 The tungsten concentration is normalized to the total content of calcium, manganese, and tungsten. Furthermore, the samples were characterized by Rutherford backscattering spectrometry (RBS).…”