Mesoscopic fast ion conduction in nanometre-scale planar heterostructures

Abstract

Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions. Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals. Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres. The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces. If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed. At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications.

Bio‐Inspired, Smart, Multiscale Interfacial Materials

“…Recently, the novel concept of binary cooperative complementary nanometer-scale interfacial materials was proposed, i.e., materials that exhibit two complementary properties on the nanometer-scale and can switch reversibly between both properties on the ''macroscale'' under certain conditions. [56] Some recent results, [57][58][59][60][61] indicate that this concept is useful for the design of novel interfacial materials. Under certain conditions, these interfaces exhibit unexpected properties, providing a huge potential to be explored theoretically and many potential novel applications.…”

First-principles study of compensation mechanisms in negatively charged LaGaO3/MgAl2O4interfaces

“…[1][2][3][4] The physical origins of this diversity are inherent to the interfacial structures and include symmetry-breaking and epitaxial strain. In turn, the resulting changes in the charge distribution and crystal field can drive atomic relaxations and reconstructions, creating a much larger manifold of possible structures and, consequently, electrical properties compared to parent bulk structures.…”

F anion dynamics in cation-mixed nanocrystalline LaF3: SrF2

“…The spacing of the heterolayer sample was 25 nm. In such a sample, besides any possible mixing effect directly at the interfaces, ion movements in space-charge zones [6,[39][40][41][42] are anticipated to govern ionic conductivity. Worth mentioning, the conductivity of the LaF 3 |SrF 2 hetero-layers shown reflects the longitudinal one, i.e., the values represent ion transport perpendicular to the fluoride layers.…”

“…In this ionic conductor overlapping space-charge zones are assumed to enable fast ion transport even perpendicular to the hetero-interfaces. The most prominent example for this kind of artificial ion conductor is epitaxially grown BaF 2 jCaF 2 [6].…”

“…They constitute an important part in many devices such as sensors [3] and those constructed for electrochemical energy storage [4,5]. Apart from Li and Na ion conductors, also materials with high F anion conductivity have increasingly attracted attention [6], because there is renewed interest in the realization of batteries with fluorine anions as main charge carriers [7][8][9]. Apart from that, binary earth alkaline fluorides, especially CaF 2 and BaF 2 , easily accept rareearth-elements and, thus, allow the preparation of luminescent solid solutions with improved optical properties.…”