^{1}, K. Z. Rushchanskii

^{2}, A. Maegli

^{1}, S. Yoon

^{1}, S. Populoh

^{1}, A. Shkabko

^{1,3}, S. Pokrant

^{1}, M. Ležaić

^{2}, R. Waser

^{3}and A. Weidenkaff

^{1}

### Abstract

After partial substitution of nitrogen for oxygen in EuTiO3, the crystal structure, thermoelectric properties, morphology, and electronic structure of the products were analyzed and compared with pristine EuTiO3. The space group of EuTi(O,N)3 ± δ was orthorhombic Pnma due to the tilt and rotation of the anion octahedra, compared to cubic of EuTiO3 (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 (ρ) were increased about one order of magnitude. The activation energies (E A) indicated a larger band gap of EuTi(O,N)3 ± δ when compared to the pristine EuTiO3 (∼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.

Financial support from SNF (Swiss National Science Foundation) within the National Centre of Competence in Research (NCCR) “MaNEP – Materials with Novel Electronic Properties” is highly acknowledged. Neutron diffraction was performed at the Swiss Spallation Neutron Source SINQ, at the Paul Scherrer Institut, Villigen, Switzerland (proposal-number 20100537). The authors thank Dr. Denis Sheptyakov for helpful discussion and support during the neutron measurement campaign. This work was also supported by the Young Investigators Group Program of the Helmholtz Association, Germany (contract VH-NG-409), the Jülich Supercomputer Centre, and the Empa Electron Microscopy Center.

I. INTRODUCTION

II. EXPERIMENTAL METHODS

III. CALCULATIONS

IV. RESULTS AND DISCUSSION

V. CONCLUSIONS

### Key Topics

- Crystal structure
- 15.0
- Powders
- 15.0
- Activation energies
- 14.0
- Band gap
- 14.0
- Electrical resistivity
- 13.0

## Figures

Powder neutron diffraction pattern (neutron wavelength: 1.1545 Å) and Rietveld refinement plot of EuTi(O,N)3 ± δ obtained at room temperature. The difference plot of observed and calculated diffraction profiles is shown beneath and the Bragg positions are given by short vertical tick markers ( a EuTi(O,N)3 ± δ , b TiN, c V).

Powder neutron diffraction pattern (neutron wavelength: 1.1545 Å) and Rietveld refinement plot of EuTi(O,N)3 ± δ obtained at room temperature. The difference plot of observed and calculated diffraction profiles is shown beneath and the Bragg positions are given by short vertical tick markers ( a EuTi(O,N)3 ± δ , b TiN, c V).

(a) Cubic structure of EuTiO3 and (b) orthorhombic structure of EuTi(O,N)3± δ due to the rotation and tilt of the anion octahedra (view along the [001] direction).

(a) Cubic structure of EuTiO3 and (b) orthorhombic structure of EuTi(O,N)3± δ due to the rotation and tilt of the anion octahedra (view along the [001] direction).

(a) Main electron diffraction pattern of EuTi(O,N)3 ± δ (white labels) and secondary pattern indicating a symmetry breaking (yellow labels). (b) High resolution micrograph of the same oriented grain.

(a) Main electron diffraction pattern of EuTi(O,N)3 ± δ (white labels) and secondary pattern indicating a symmetry breaking (yellow labels). (b) High resolution micrograph of the same oriented grain.

(a) Deconvoluted Ti 2p XPS spectrum of EuTi(O,N)3 ± δ . (b) Comparison of Ti 2p XPS spectra of EuTiO3 and EuTi(O,N)3 ± δ normalized to the maximum.

(a) Deconvoluted Ti 2p XPS spectrum of EuTi(O,N)3 ± δ . (b) Comparison of Ti 2p XPS spectra of EuTiO3 and EuTi(O,N)3 ± δ normalized to the maximum.

(a) Eu 4d XPS multiplet of EuTi(O,N)3 ± δ deconvoluted. (b) Eu 4d XPS spectrum of EuTi(O,N)3 ± δ and EuTiO3 normalized to the maximum.

(a) Eu 4d XPS multiplet of EuTi(O,N)3 ± δ deconvoluted. (b) Eu 4d XPS spectrum of EuTi(O,N)3 ± δ and EuTiO3 normalized to the maximum.

Absolute value of the Seebeck coefficient (S) of the oxide and the three oxynitride pellets measured in the temperature range of 390 K–950 K.

Absolute value of the Seebeck coefficient (S) of the oxide and the three oxynitride pellets measured in the temperature range of 390 K–950 K.

(a) Electrical resistivity (ρ) of the oxide and oxynitride pellets measured in the temperature range of 300 K–950 K (error bars within the data points). (b) ln(ρ) vs. 1/T and calculated activation energies in the low temperature (extrinsic) and high temperature (intrinsic) regimes.

(a) Electrical resistivity (ρ) of the oxide and oxynitride pellets measured in the temperature range of 300 K–950 K (error bars within the data points). (b) ln(ρ) vs. 1/T and calculated activation energies in the low temperature (extrinsic) and high temperature (intrinsic) regimes.

Lattice thermal conductivity (κ lat) of the oxynitride and the oxide in the temperature range of 330 K–850 K.

Lattice thermal conductivity (κ lat) of the oxynitride and the oxide in the temperature range of 330 K–850 K.

TEM micrographs of (a) EuTi(O,N)3 ± δ and (c) EuTiO3 powder particles, and SEM (SE) micrographs of (b) EuTi(O,N)3± δ (I) and (d) EuTiO3 sintered pellets.

TEM micrographs of (a) EuTi(O,N)3 ± δ and (c) EuTiO3 powder particles, and SEM (SE) micrographs of (b) EuTi(O,N)3± δ (I) and (d) EuTiO3 sintered pellets.

Partial electronic density of states of (a) pristine EuTiO3 and (b) nitrogen substituted EuTi(O,N)3 ± δ. The zero level of energy was chosen as the top of the valence band.

Partial electronic density of states of (a) pristine EuTiO3 and (b) nitrogen substituted EuTi(O,N)3 ± δ. The zero level of energy was chosen as the top of the valence band.

(a) Electronic charge density distribution of the top of the VB (energy range: −1 to 0 eV) (b) and section of Eu-N bonds in the energy region −4 to −2.64 eV. These numbers correspond to the energy scale of Fig. 10(b) . The isosurfaces of the charge density plots were 0.01 and 0.05 e−/Å3 for figures (a) and (b), respectively. The contours at the sections are plotted with color code blue-green-red covering the interval from 0 to 0.2 e−/Å3 for figure (a) and from 0 to 1 e−/Å3 for figure (b).

(a) Electronic charge density distribution of the top of the VB (energy range: −1 to 0 eV) (b) and section of Eu-N bonds in the energy region −4 to −2.64 eV. These numbers correspond to the energy scale of Fig. 10(b) . The isosurfaces of the charge density plots were 0.01 and 0.05 e−/Å3 for figures (a) and (b), respectively. The contours at the sections are plotted with color code blue-green-red covering the interval from 0 to 0.2 e−/Å3 for figure (a) and from 0 to 1 e−/Å3 for figure (b).

Comparison of the total density of states of EuTi(O,N)3 ± δ and EuTiO3. The zero level of energy was chosen as the top of the valence band.

Comparison of the total density of states of EuTi(O,N)3 ± δ and EuTiO3. The zero level of energy was chosen as the top of the valence band.

## Tables

Crystallographic parameters and structural refinements.

Crystallographic parameters and structural refinements.

Crystal structural parameters of EuTi(O,N)3 ± δ determined from the Rietveld refinement of the powder neutron diffraction pattern.

Crystal structural parameters of EuTi(O,N)3 ± δ determined from the Rietveld refinement of the powder neutron diffraction pattern.

Oxygen and nitrogen stoichiometry determined by XPS quantification of the O 1s and N 1s peaks.

Oxygen and nitrogen stoichiometry determined by XPS quantification of the O 1s and N 1s peaks.

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