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

DOI: 10.1021/jacs.7b00502 DOI Help

Authors: Andre Duvel (Gottfried Wilhelm Leibniz University Hannover; University of Kent) , Paul Heitjans (Gottfried Wilhelm Leibniz University Hannover) , Pavel Fedorov (General Physics Institute of Russian Academy of Science) , Gudrun Scholz (Humboldt-Universität zu Berlin) , Giannantonio Cibin (Diamond Light Source) , Alan V. Chadwick (University of Kent) , David M. Pickup (University of Kent) , Silvia Ramos (University of Kent) , Lewis W.l. Sayle (University of Kent) , Emma K Sayle (University of Kent) , Thi X. T. Sayle (University of Kent) , Dean C. Sayle (University of Kent)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society

State: Published (Approved)
Published: March 2017
Diamond Proposal Number(s): 8912

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-xCaxF2 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, pseudo vacancy arrays and collective transport mechanisms; we show that the excess volume correlates with ionic conductivity for the Ba1-xCaxF2 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, 512, 303] remarks that ‘classical crystallography is inadequate to describe systems with correlated disorder, but that geometric frustration has clear crystallographic signatures’. Here, we identify two possible crystallographic signatures: 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, 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.

Subject Areas: Chemistry, Physics

Instruments: B18-Core EXAFS