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Séminaire 25.01.2013 à 14h

par Mathieu Salanne - 22 janvier 2013

Ben Morgan, du Department of Materials de l’université d’Oxford (Royaume-Uni), présentera un séminaire le 25 janvier 2013 à 14h00 dans la bibliothèque du laboratoire PECSA (7e étage, bâtiment F, porte 754) intitulé :

Exploring Nanoionic Behaviour in Heterostructured Solid-State Electrolytes by Atomistic Simulation


The field of nanoionics concerns itself with the behaviour of ionic charge carriers in materials characterised by nanometer length scales [1,2]. The conduction behaviour of nanoionic materials can strongly deviate from that of analogous bulk materials. For example, the room-temperature ionic conductivity of CaF2/BaF2 heterostructures can be increased by up to three orders of magnitude by reducing the spacing between CaF2/BaF2 interfaces to a few nanometers [3]. Nanocrystalline AgI ; which has been found to form as unusual 7H and 9R polytypes, equivalent to isochemical heterostructures with alternating wurtzite and zinc-blende domains ; exhibits up to ×104 enhancements in ionic conductivity relative to the bulk wurtzite phase [4]. These extreme conductivity enhancements suggest a potential for the development of high performance solid-state electrolytes, for use in technologies where ionic conduction is critical, such as fuel cells, or rechargeable batteries.

Nanoionic behaviour is commonly interpreted using a continuum thermodynamic model that predicts the spontaneous formation of space charges ; locally non-stoichiometric regions with increased point defect concentrations ; at surfaces and internal interfaces [5]. The massive conductivity enhancement of nanoscale CaF2/BaF2 heterostructures has been interpreted as an increasing contribution from space charge regions at the internal heterostructure interfaces with decreasing interfacial separation. In AgI nanocrystals, space charge formation has similarly been predicted at wurtzite/zinc-blende stacking faults. Here these stacking faults are separated by only a few interatomic separations, and the 7H and 9R polytypes are therefore expected to display intrinsic Ag+ disorder throughout their entirety.

Since the thermodynamic analysis that leads to the prediction of space charge formation starts with a continuum model, it is not known to what extent the discrete nature of the crystalline lattice of specific heterostructures plays a role in determining the defect distribution and associated ionic conductivity. We have used molecular dynamics simulations to explore the relationship between structure and ionic transport in these heterostructures. I will present results for a model fluorite-structured CaF2/BaF2 analogue, and for a range of AgI polytypes, and will discuss how these can be used to examine the predictions of the continuum space charge model, and to provide insight into the factors controlling the “nanoionic” conductivity seen in experimental samples.

[1] J. Maier, Nat. Mater. 4, 805 (2005).

[2] J. Maier, Phys. Chem. Chem. Phys. 11, 3011 (2009).

[3] N. Sata, K. Eberman, K. Eberl, and J. Maier, Nature 408, 946 (2000).

[4] Y. Guo, J. Lee, and J. Maier, Adv. Mater. 17, 2815 (2005).

[5] J. Maier, Prog. Solid State Chem. 23, 171 (1995).