Kinetic Monte Carlo Simulations for Solid State Ionics: Case Studies with the MOCASSIN Program

Kinetic Monte Carlo simulations are a useful tool to predict and analyze the ionic conductivity in crystalline materials. We present here the basic functionalities and capabilities of our recently published Monte Carlo software for solid state ionics called MOCASSIN, exemplified by simulations of several model systems and real materials. We address the simulation of tracer correlation factors for various structures, the correlation in systems with complex migration mechanisms like interstitialcy or vehicle transport, and the impact of defect interactions on ionic conductivity. Simulations of real materials include a review of oxygen vacancy migration in doped ceria, oxygen interstitial migration in La-rich melilites, and proton conduction in acceptor doped fully hydrated barium zirconate. The results reveal the impact of defect interactions on the ionic conductivity and the importance of the defect distribution. Combinations of these effects can lead to unexpected transport behavior in solid state ionic materials, especially for multiple mobile species. Kinetic Monte Carlo simulations are therefore useful to interpret experimental data which shows unexpected behavior regarding the dependence on temperature and composition.

Effect of disordered imidazole substructure on proton dynamics in imidazolium malonic acid salt

The influence of a disorder in cation substructure on proton conductivity of imidazolium malonate (Im-MAL) is studied. Imidazolium in salts with dicarboxylic acids have been found to have a well ordered hydrogen-bond network and only in Im-MAL [Pogorzelec-Glaser et al. (2006). Mater. Sci.-Pol. (2006), 24, 245–252] were two types of cation observed: ordered Im-I and disordered Im-II. Im-I is involved in hydrogen bonds with malonic acid molecules, whereas Im-II is disordered between two symmetrically equivalent positions with occupancy of 0.5. NMR studies by Mizuno et al. [Hyperfine Interact. (2015), 230, 95–100] showed an 180° flip of ordered Im-I and calculated contribution of Im-I flipping to proton conductivity of Im-MAL. Ławniczak et al. [Solid State Ionics (2017), 306, 25] reported that temperature variation of the proton conductivity by impedance spectroscopy yielded the conductivity value higher than that calculated by Mizuno for Im-I. Moreover these detailed structure studies at 240 K and 280 K excluded any phase transition. Repeated X-ray studies from 14 K to 360 K show a continuous increase in anisotropic displacement factors. The half-occupied hydrogen bonds linking the Im-II nitrogen atoms with hydroxyl oxygen atoms may be considered as electric dipoles and the interbond proton transfer as dipolar switching. It assumed here a coherent switching at low temperatures and a decrease of the coupling at higher temperatures with the disappearance at cross-over temperature at 318 K. The possible proton pathway in the crystal structure is determined and the contribution of the proton dynamics of Im-II to phonon-assisted proton diffusion in the ordered substructure is estimated.

Maintaining pronounced proton transportation of solid oxides prepared with a sintering additive

Proton-conducting electrolytes (PCEs) have led to significant advances in the fields of solid-state ionics, energy conversion and high-temperature electrochemistry, providing the basis of various solid oxide devices that demonstrate outstanding...