Is Geometric Frustration-Induced Disorder a Recipe for High Ionic Conductivity?

Abstract

Ionic conductivity is ubiquitous to many industrially important applications such as fuel cells, batteries, sensors, and catalysis. Tunable conductivity in these systems is therefore key to their commercial viability. Here, we show that geometric frustration can be exploited as a vehicle for conductivity tuning. In particular, we imposed geometric frustration upon a prototypical system, CaF, by ball milling it with BaF, to create nanostructured BaCaF solid solutions and increased its ionic conductivity by over 5 orders of magnitude. By mirroring each experiment with MD simulation, including "simulating synthesis", we reveal that geometric frustration confers, on a system at ambient temperature, structural and dynamical attributes that are typically associated with heating a material above its superionic transition temperature. These include structural disorder, excess volume, pseudovacancy arrays, and collective transport mechanisms; we show that the excess volume correlates with ionic conductivity for the BaCaF system. We also present evidence that geometric frustration-induced conductivity is a general phenomenon, which may help explain the high ionic conductivity in doped fluorite-structured oxides such as ceria and zirconia, with application for solid oxide fuel cells. A review on geometric frustration [ Nature 2015 , 521 , 303 ] remarks that classical crystallography is inadequate to describe systems with correlated disorder, but that correlated disorder has clear crystallographic signatures. Here, we identify two possible crystallographic signatures of geometric frustration: excess volume and correlated "snake-like" ionic transport; the latter infers correlated disorder. In particular, as one ion in the chain moves, all the other (correlated) ions in the chain move simultaneously. Critically, our simulations reveal snake-like chains, over 40 Å in length, which indicates long-range correlation in our disordered systems. Similarly, collective transport in glassy materials is well documented [for example, J. Chem. Phys. 2013 , 138 , 12A538 ]. Possible crystallographic nomenclatures, to be used to describe long-range order in disordered systems, may include, for example, the shape, length, and branching of the "snake" arrays. Such characterizations may ultimately provide insight and differences between long-range order in disordered, amorphous, or liquid states and processes such as ionic conductivity, melting, and crystallization.

Formation and Elimination of Anti-site Defects during Crystallization in Perovskite Ba1–xSrxLiF3

“…The increased ion mobility in the areas showing Sr/Ba intermixing, which can be ascribed to geometric frustration [47] , is in agreement with experiment ( fig. 5), reinforcing the validity of the simulations.…”

Crystal Structural Framework of Lithium Super‐Ionic Conductors

“…As a general phenomenon, frustrated systems for a wide range of physical phenomena are featured with correlated transition between states. [27][28][29]58,59] As reported in previous studies, the dominant diffusion mechanism in SICs is the concerted migration of multiple Li ions, [24][25][26]28] which can be understood as a correlated transition between two Li + configurations. Such correlated transition has a low migration barrier in a disordered sublattice (Figure 1).…”

“…[28] In addition, the significantly increased flexibility of Li-ion positions in large sites leads to an energy degeneracy of the Li sublattice, i.e., a large number of configurations exhibits similar energies, which induces geometrical frustration of the Li sublattice. [27][28][29]58,59] The strong Coulombic interactions among Li ions further complicate the energetics of the Li sublattice. For example, in garnet LLZO (Figure 1), the Li-ion configuration has a known pattern where an occupied 24d site leads to unoccupied nearest-neighbor 96h sites, but occupied next nearest-neighbor 96h sites, as a rearrangement within an enlarged site (i.e., two 96h).…”

Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

“…These values are in good agreement with results from recently performed molecular dynamics studies. 27 For nanocrystalline CaF 2 background effects are predominant F NMR SLR behavior of nanocrystalline Ba 0.5 Ca 0.5 F 2 obtained after t mill = 10 h and t mill = 100 h. While measurements in the kHz range (1/T 1ρ ) are unaffected by t mill , local jump processes, as seen by T 1 NMR, depend on the degree of cation mixing. The activation energies indicated refer to non-corrected rates; background correction to calculate 1/T 1diff changes E a from 0.32 eV to 0.35 eV (100 h) and from 0.44 eV to 0.49 eV with no effect on the ratio of the two activation energies.…”

“…24,25 Recently, it has also attracted groups from theory to model ion conductivities. 26,27 Nanocrystalline Ba 1−x Ca x F 2 (0 < x < 1) can easily be prepared by mechanochemical reaction at low temperatures from the binary fluorides BaF 2 and CaF 2 . 24,25 Joint milling the two fluorides forces the cations to mix at atomic scale while the symmetry of the educts remains the same; both BaF 2 and CaF 2 crystallize with antifluorite structure.…”