Local Distortions and Dynamics in Hydrated Y-Doped BaZrO₃

Abstract

Y-doped BaZrO 3 is a promising proton conductor for intermediate temperature solid oxide fuel cells. In this work, a combination of static DFT calculations and DFT based molecular dynamics (DFT-MD) was used to study proton conduction in this material. Geometry optimizations of 100 structures with a 12.5% dopant concentration allowed us to identify a clear correlation between the bending of the metal–oxygen–metal angle and the energies of the simulated cells. Depending on the type of bending, two configurations, designated as inward bending and outward bending, were defined. The results demonstrate that a larger bending decreases the energy and that the lowest energies are observed for structures combining inward bending with protons being close to the dopant atoms. These lowest energy structures are the ones with the strongest hydrogen bonds. DFT-MD simulations in cells with different yttrium distributions provide complementary microscopic information on proton diffusion as they capture the dynamic distortions of the lattice caused by thermal motion. A careful analysis of the proton jumps between different environments confirmed that the inward and outward bending states are relevant for the understanding of proton diffusion. Indeed, intra-octahedral jumps were shown to only occur starting from an outward configuration while the inward configuration seems to favor rotations around the oxygen. On average, in the DFT-MD simulations, the hydrogen bond lengths are shorter for the outward configuration which facilitates the intra-octahedral jumps. Diffusion coefficients and activation energies were also determined and compared to previous theoretical and experimental data, showing a good agreement with previous data measuring local proton motion.

Toward Durable Protonic Ceramic Cells: Hydration-Induced Chemical Expansion Correlates with Symmetry in the Y-Doped BaZrO3–BaCeO3 Solid Solution

“…At an atomistic scale, simulations have represented chemical expansion by point defect-induced strain tensors with locally anisotropic behavior possible even in nominally cubic systems. 23,24 In noncubic systems, anisotropic strains can be observed on a single-crystal or local grain level. 25−27 In such cases, the volumetric chemical expansivity would be the sum of the linear CCEs along the three orthogonal directions.…”

“…Phenomenologically, the linear chemical strain (ε C ) is often found to be proportional to the change in defect concentration (Δ­[(OH) O • ] for the hydration reaction, with units of concentration per formula unit) by the linear coefficient of chemical expansion (CCE), as shown in eq Macroscopically, in randomly oriented polycrystalline materials, the scalar treatment of the CCE is appropriate. At an atomistic scale, simulations have represented chemical expansion by point defect-induced strain tensors with locally anisotropic behavior possible even in nominally cubic systems. , In noncubic systems, anisotropic strains can be observed on a single-crystal or local grain level. − In such cases, the volumetric chemical expansivity would be the sum of the linear CCEs along the three orthogonal directions. The chemical stress associated with all of these chemical strains can be large enough to cause mechanical failure such as cracking and delamination within devices where components are constrained and/or experiencing nonuniform defect concentrations. , This effect can be particularly pronounced in proton conductors, compared to oxygen loss in mixed oxide ion and electronic conductors, as hydration-induced lattice strains of up to 1% have been reported. , Therefore, there is interest in quantifying CCEs for the purposes of device design and modeling as well as in understanding chemical and structural design rules for rationally tailoring CCEs to optimize the components’ chemomechanical response.…”

Oxygen Vacancies Altering the Trapping in the Proton Conduction Landscape of Doped Barium Zirconate

“…While DFT geometry optimizations find zero temperature structures, recent DFT molecular dynamics simulations at proton conduction temperatures (500−1000 K) retain significant octahedral tilting. 30 , the higher energy intraoctahedral move in the study, suggesting that tilting is critical to proton conduction mechanism in the absence of an oxygen vacancy.…”

Recent progress in the experimental and theoretical studies on the barium zirconate proton conductors: A review

“…While considering structural aspects, the defects and local distortions must be included in the discussion in perovskites, as they are directly related to proton mobility [213]. Defect engineering is always mentioned as a suitable approach to manipulate proton conductivity of perovskites such as SrZrO 3 (SZO), CaZrO 3 (CZO), and BZO [207].…”

“…These parameters have facilitated to re-calculate of the primary energy barrier for proton diffusion in BZY that manifests a very consistent experimental value [234]. Local distortion and dynamics were further explored by DFT-based MD simulations [213]. To further visualize the success of DFTbased MD studies to determine the proton diffusion coe cient, one may refer to the temperature-dependent proton diffusion coe cient for BZO (Fig.…”