R-005

Andrejs Cesnokovs (a)

andrejs.cesnokovs@cfi.lu.lv

Denis Gryaznov (a), Rotraut Merkle (b), Eugene A. Kotomin (a,b)

(a) Institute of Solid State Physics, University of Latvia, Kengaraga 8, Riga, Latvia 

(b) Max Planck Institute for Solid State Research, Heisenbergstrasse 1, Stuttgart, Germany

Proton migration in triple conductors: insights from DFT calculations

Triple-conducting” perovskites are relevant as material for air electrodes in protonic ceramic electrochemical cells. However, uncoupling proton conductivity from competing electronic/oxygen-ion contributions remains experimentally challenging. Thus, DFT calculations represent an important complementary approach to investigate proton transport. BaFeO3 is a typical representative for triple-conducting perovskites. Due to a negative charge transfer effect it behaves like a half-metal and, thus, requires advanced calculation methods. We utilize the DFT+U approach (U=4.0 eV on Fe 3d electrons) to treat the strong correlation effects in BaFeO3. [1,2] The transition states of proton transfer are investigated using the climbing-image nudged elastic band method. The impact of dopants of different size (Ga3+, Sc3+, In3+ and Y3+) on the energetics of proton transfer is also investigated.

First, we carefully investigate the protonation sites, i.e. the oxygen positions to which the proton forms ~1 Å long covalent bond. The role of local distortions and/or oxygen vacancies in undoped BaFeO3-δ is discussed. We analyse key parameters associated with proton addition to the system: structural (such as covalent O–H bond length, hydrogen bond O...H length, interatomic O...O distances etc.) and electronic (for instance, the O 2p band centre). A composite descriptor comprising the O 2p band centre and the hydrogen bond length correlates very well with the protonation site preference, yielding chemical insights from groundstate DFT calculations. For doped BaFeO3, the system energy depends on the proton-dopant distance. The smaller dopants Ga3+ and Sc3+ show a stabilization of the proton in the 1st coordination shell, similar to many dopants in BaZrO3. In contrast, for larger In3+ and Y3+ ions the proton is repelled from the 1st coordination shell, because strong local distortions hinder hydrogen bond-assisted proton stabilization.

Second, we calculate migration energies for proton transfer. Owing to the lattice distortions the barriers show a rather broad distribution in undoped BaFeO3-δ, and the average barrier decreases with increasing oxygen deficiency from 0.22 eV to 0.18 eV.[1] The proton transfer proceeds in two stages, similar as in BaZrO3. For doped BaFeO3, the largest barrier of 0.32 eV is found for the very oversized Y-dopant. In contrast, Ga3+, which stabilizes the proton in 1st and 2nd shell, leads to smaller barriers as low as 0.11 eV. [3]

[1] M.F. Hoedl, A. Chesnokov, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Mater. Chem. A 11, 6336 (2023)

[2] M.F. Hoedl, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Phys. Chem. C 124, 11780 (2020)

[3] A. Chesnokov, D. Gryaznov, E. A. Kotomin, J. Maier, R. Merkle, Solid State Ionics 421, 116788 (2025)