Toward Rational Design of Fast Ion Conductors: Molecular Dynamics Modeling of Interfaces of Nanoscale Planar Heterostruc
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Increased ionic conductivity at nanoscale planar interfaces of the CaF2|BaF2 system was successfully modeled using molecular dynamics simulations. A criterion was established to construct simulation cells containing any arbitrarily lattice-mismatched interfaces while permitting periodic boundary condition. The relative (to the bulk) ionic conductivity increase at the 111 (CaF2)|111 (BaF2) interface was qualitatively reproduced. Higher conductivity, by a factor of 7.6, was predicted for the 001 (CaF2)|001 (BaF2) interface. A crystalline nanocomposite of the CaF2|BaF2 system, in which the [001] morphology is encouraged and crystallite dimensions are approximately 4 nm, was proposed to give ionic conductivity approaching that predicted for the 001 (CaF2)|001 (BaF2) interface. I. INTRODUCTION
Ionic conductors are of importance in fuel cell technology, sensors, and high-temperature batteries and displays.1 Experimental and theoretical efforts have been devoted to lowering the super ionic conduction temperatures, which are usually over 1000 K.2,3 On the other hand, there is an increasing need for solid electrolyte in an all-solid-state batteries.4 Such demands have driven research to find ionic conductors that conduct at or near room temperature. It was discovered a few decades ago that ionic conductivity could be increased remarkably at solid heterojunctions.5 More recently, Sata and coworkers demonstrated dramatic increases in ionic conductivity parallel to the interfaces of the CaF2|BaF2 planar heterostructures when individual layer thickness is than 50 nm. At such thickness, single layers lose their individuality. The discovery opened the door for super ionic conductors using nanocomposites. It also raised some fundamental questions. For instance, in the search for super ionic conductors, it is advantageous to know, even before the actual synthesis, how much increase in ionic conductivity may be achieved for a heterostructure with a given composition. Also, the smaller the dimensions of individual phase in the heterostructure, the greater the potential interface area, and hence the greater the ionic conductivity. Therefore, it is essential to determine the limiting factors that dictate the smallest dimensions of individual phases attainable while not disrupting the overall crystalline (not solid solution) nature of the phases. Classic computer simulation has been successfully applied to the study of ionic conductivities of bulk fluorite materials.3,6 Recently, there have also been attempts to 1686
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J. Mater. Res., Vol. 17, No. 7, Jul 2002 Downloaded: 13 Mar 2015
study the interface morphologies7 and interactions at the atomic scale at hetero-interfaces.8 However, lattice mismatch has so far not been considered. Indeed, it is a challenge to simulate a lattice mismatched heterostructure while still maintaining the periodical boundary condition (PBC). The ability to use the PBC is crucial to model efficiently in nanoscale the repeatedly alternating planar heterostructures that give drama
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