Combinatorial Materials Discovery Approach for Proton Incorporation in BaZr_mFe_nY_(1-(m+n))O_3-d Epitaxial Thin Films

Hydrogen incorporation in solids can provide pathways for controlling electrochemical transformations important for carbon-neutral energy and modulating transport characteristics in materials essential for brain-inspired computing. Owing to its small mass, the migration of hydrogenic species (H⁺: proton) is very distinct from the typical oxygen ion conduction in a general oxide system. The definitive features that determine the charge-transfer reaction rates and transport dynamics remain to be categorized and quantified to enable predictive materials design. In the present work, for the high throughput H⁺ uptake studies, we have employed a combinatorial materials discovery approach that utilizes pulsed laser deposition epitaxial thin-film growth, optical transmission relaxation measurements, X-ray scattering, and spectroscopy characterizations in conjunction with theoretical calculations to understand the mechanism of H⁺ incorporation in complex oxides via hydration (or hydrogenation). Several phase and electronic changes are observed in the factors influencing the long-range crystal structure, the charge state of transition metals, and the local coordination environment within the perovskite crystal framework.


We demonstrate that the partial substitution of Zr⁴⁺ by Fe³⁺ and Y³⁺ transition metals (TMs) in BaZr_mFe_nY_(1-(m+n))O_3-d (BZFYO) perovskite system introduces more oxygen vacancies which are favorable for the H⁺ uptake through hydration reaction. Using crystal structure studies as an indirect signature for protonation and H⁺ surface exchange constant (k_H+) analysis for kinetics, we have found that when the Fe content exceeds 30% of the total TM cations, an optimal H⁺ uptake and surface exchange reaction rate can be achieved in BZFYO thin film. H⁺ incorporation via gas-solid interface reactions results in surface roughening, lattice expansion, crystallinity degradation, and slight oxidation of the Fe species observed by means of low-angle X-ray reflectivity, specular X-ray diffraction, and X-ray absorption near edge spectroscopy measurements. Protonation-induced lattice expansion observed in both long-range and short-range structural studies of BaZr_0.4Fe_0.4Y_0.2O_3-d / LaAlO₃(001) thin film is explained by considering the difference in ionic sizes of oxygenic species (O_L: lattice oxygen > OH^-: hydroxyl > O_V: oxygen vacancies) before and after the hydration reaction. Our theoretical calculations suggest the preference of O_V near the Fe³⁺ and Y³⁺ sites in the BaZr_0.4Fe_0.4Y_0.2O_3-d model system. This local microenvironment of O_V further dominates the feasibility of hydration reactions, which is indicated by the expansion of crystal lattices and redox of Fe sites (Fe^(3-d)+ à Fe^(3+d)+). Our findings will help generate a general insight into the governing mechanisms and physical descriptors of hydrogen intercalation and migration in perovskite materials.