Modified Single Iteration Synchronous-Transit Approach to Bound Diffusion Barriers for Solid-State Reactions.

Herein, we detail an approach to accelerate the computational screening of materials for properties dictated by the kinetics of solid-state diffusion through reliably and rapidly identifying upper and lower bounds to the transition state (TS) energy through our proposed modified single iteration synchronous-transit (MSIST) approach. While this sacrifices providing detailed information of the explicit TS structure, it requires only 30% of the force evaluations of a full nudged elastic band (NEB) TS search and reduces the computational demand to compute estimated diffusion barriers by ∼70% on average. In all 53 cases in which we explicitly compared our results to those of an NEB calculation, the upper and lower bounds identified using this approach bracketed the TS energy calculated with explicit NEB calculations. We use the applications of diffusion of Na+ in potential sodium-ion battery electrodes and oxygen vacancy diffusion in solid-oxide fuel cell electrodes and redox mediators for solar thermochemical hydrogen production to demonstrate the power of MSIST for analyzing the kinetics of bulk diffusion. For Na+ diffusion through 13 proposed electrode materials in which the average diffusion barrier was 0.28 eV, the average difference between the upper and lower bounds was 0.08 eV. An iterative application of this approach to the three materials with the largest difference between their upper and lower bounds further narrowed the average range of the bounded TS energies to 0.04 eV while still requiring fewer force evaluations than an NEB TS calculation. When applied in a high-throughput manner to study 514 diffusion pathways in 97 different materials, the average difference between the upper and lower bounds was 0.33 eV and the average barrier, as calculated by the average of all upper and lower bounds, was ∼1.7 eV. Because the MSIST approach produces explicit errors, i.e., the difference between the upper and lower bounds energies, even predicted barrier ranges with large errors can be reliably modeled with weighted regression techniques. MSIST enables the analysis of the kinetics of solid-state diffusion across larger sets of materials and can thus efficiently provide data to train statistically learned models of diffusion and to develop physical insights into the diffusion process.

Predicting Spinel Disorder and Its Effect on Oxygen Transport Kinetics in Hercynite.

The iron aluminate spinel hercynite (FeAl2O4) is a promising redox material for solar thermochemical hydrogen production (STCH). Although it has a high H2 production capacity, the kinetics of its oxidation and reduction may be too slow to be practical for STCH. However, our results suggest that Fe-rich hercynite may have substantially faster redox kinetics, which could make hercynite competitive with other materials for STCH. We used density functional theory to investigate the origin of hercynite's slow kinetic behavior and show that it arises from the high activation barrier of 2.46 eV for oxygen vacancy (VO) diffusion in normal hercynite. To model the effect of disorder caused by spinel inversion, we examined 11 of the most common cation arrangements and found a near 1:1 correlation between the diffusion barrier and VO formation energy, both of which decrease by 0.6 eV for each additional nearest-neighbor Fe atom. To examine this trend, we used integrated crystal orbital Hamilton population (ICOHP) analysis to estimate the difference in the metal-oxygen bond strengths of cations neighboring VO and the diffusion transition state. The ICOHP predicted bond strengths correlate to both the diffusion barrier and VO formation energy. We also computed the effect of the charge state of the oxygen vacancy and found that positively charged vacancies are stable at low Fermi energies and have a diffusion barrier of only 0.79 eV, 1.67 eV lower than that of the neutral vacancy, demonstrating that stabilizing these charged vacancies may enable faster oxidation and reduction kinetics in hercynite. We show that uncompensated Fe antisite defects, which are present in Fe-rich hercynite, provide redox flexibility that stabilizes the charged VO and thereby increase the rate of VO diffusion. Finally, we predict that at higher VO concentrations the diffusion barrier depends on the relative positions of the vacancies and decreases when they are next-nearest neighbors.

Physical descriptor for the Gibbs energy of inorganic crystalline solids and temperature-dependent materials chemistry.

The Gibbs energy, G, determines the equilibrium conditions of chemical reactions and materials stability. Despite this fundamental and ubiquitous role, G has been tabulated for only a small fraction of known inorganic compounds, impeding a comprehensive perspective on the effects of temperature and composition on materials stability and synthesizability. Here, we use the SISSO (sure independence screening and sparsifying operator) approach to identify a simple and accurate descriptor to predict G for stoichiometric inorganic compounds with ~50 meV atom-1 (~1 kcal mol-1) resolution, and with minimal computational cost, for temperatures ranging from 300-1800 K. We then apply this descriptor to ~30,000 known materials curated from the Inorganic Crystal Structure Database (ICSD). Using the resulting predicted thermochemical data, we generate thousands of temperature-dependent phase diagrams to provide insights into the effects of temperature and composition on materials synthesizability and stability and to establish the temperature-dependent scale of metastability for inorganic compounds.