The effect of ionic defect interactions on the hydration of yttrium-doped barium zirconate.

Hydrated acceptor-doped barium zirconate is a well-investigated proton conductor. In the analysis of most experimental studies, an ideal defect model is applied to fit the measured hydration data and obtain corresponding enthalpies and entropies. However, the data show a distinct deviation from ideal behaviour and thus defect interactions cannot be neglected. In the present contribution, the thermodynamics of water uptake into the yttrium-doped bulk material are investigated on the microscopic level with regards to ionic defect interactions. Metropolis Monte Carlo simulations using interaction models from first-principles energy calculations are applied to obtain an estimation of the free energy of interaction. The present results indicate that the ionic defect interactions are the primary reason for the non-ideality observed in experiments with varying yttrium fraction, proton fraction, and temperature. The activity coefficient quotients for the mass action law are obtained, which connect the ideal and real model and are of relevance to data evaluation and theoretical calculations.

MOCASSIN: Metropolis and kinetic Monte Carlo for solid electrolytes.

The combination of density functional theory and Monte Carlo simulations is a powerful approach for the atomistic modeling of defect transport in solid electrolytes. The present contribution introduces the MOCASSIN software (Monte Carlo for Solid State Ionics) for kinetic and Metropolis Monte Carlo simulations of crystalline materials. MOCASSIN combines model building, visualization, and simulation, aiming to provide accessible MC for end users. Developed for the investigation of solid electrolytes, MOCASSIN is ideal for screening common variation parameters, such as temperature and doping fraction. The input effort is minimized using space groups for processing symmetry. The graphical interface for model building allows complex model input, including multiple mobile species, multiple migration paths, small polaron hopping, vehicle movements, multiple complex migration mechanisms, and custom interaction clusters. The software is provided free of charge for noncommercial usage.

Publisher Correction: Nanoscale percolation in doped BaZrO3 for high proton mobility.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Nanoscale percolation in doped BaZrO3 for high proton mobility.

Acceptor-doped barium zirconate is a promising proton-conducting oxide for various applications, for example, electrolysers, fuel cells or methane-conversion cells. Despite many experimental and theoretical investigations there is, however, only a limited understanding as to how to connect the complex microscopic proton motion and the macroscopic proton conductivity for the full range of acceptor levels, from diluted acceptors to concentrated solid solutions. Here we show that a combination of density functional theory calculations and kinetic Monte Carlo simulations enables this connection. At low concentrations, acceptors trap protons, which results in a decrease of the average proton mobility. With increasing concentration, however, acceptors form nanoscale percolation pathways with low proton migration energies, which leads to a strong increase of the proton mobility and conductivity. Comparing our simulated proton conductivities with experimental values for yttrium-doped barium zirconate yields excellent agreement. We then predict that ordered dopant structures would not only strongly enhance the proton conductivities, but would also enable one- or two-dimensional proton conduction in barium zirconate. Finally, we show how the properties of other dopants influence the proton conductivity.