title: 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, CaF2, by ball milling it with BaF2, to create nanostructured Ba1-xCaxF2solid 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 Ba1-xCaxF2system. 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.
authors.affiliation: Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover , Callinstrasse 3-3a, D-30167 Hannover, Germany.|School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Andre
authors.identifier: 0000-0002-2393-5453
authors.initials: A
authors.lastname: Düvel
authors.affiliation: Institute of Physical Chemistry and Electrochemistry, Leibniz Universität Hannover , Callinstrasse 3-3a, D-30167 Hannover, Germany.
authors.forename: Paul
authors.initials: P
authors.lastname: Heitjans
authors.affiliation: General Physics Institute of Russian Academy of Sciences , 119991 Moscow, Russia.
authors.forename: Pavel
authors.initials: P
authors.lastname: Fedorov
authors.affiliation: Department of Chemistry, Humboldt-Universität zu Berlin , Brook-Taylor-Str. 2, D-12489, Berlin, Germany.
authors.forename: Gudrun
authors.initials: G
authors.lastname: Scholz
authors.affiliation: Diamond Light Source, Harwell Science and Innovation Campus , Didcot, Oxfordshire OX11 0DE, U.K.
authors.forename: Giannantonio
authors.initials: G
authors.lastname: Cibin
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Alan V
authors.initials: AV
authors.lastname: Chadwick
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: David M
authors.initials: DM
authors.lastname: Pickup
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Silvia
authors.initials: S
authors.lastname: Ramos
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Lewis W L
authors.initials: LWL
authors.lastname: Sayle
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Emma K L
authors.initials: EKL
authors.lastname: Sayle
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Thi X T
authors.initials: TXT
authors.lastname: Sayle
authors.affiliation: School of Physical Sciences, University of Kent , Canterbury, Kent CT2 7NH, U.K.
authors.forename: Dean C
authors.identifier: 0000-0001-7227-9010
authors.initials: DC
authors.lastname: Sayle