Mesoscopic fast ion conduction in nanometre-scale planar heterostructures

Ion conduction is of prime importance for solid-state reactions in ionic sy stems, and for devices such as high-temperature batteries and fuel cells, c hemical filters and sensors(1,2). 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 spac e-charge regions(3-11). Heterojunctions in two-phase systems should be part icularly efficient at improving ionic conduction(3,4), and a qualitatively different conductivity behaviour is expected when interface spacing is comp arable to or smaller than the width of the space-charge regions in comparat ively large crystals(12-15). Here we report the preparation, by molecular-b eam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proporti onally with interface density-for interfacial spacing greater than 50 nanom etres. The results are in excellent agreement with semi-infinite space-char ge calculations(3), assuming a redistribution of fluoride ions at the inter faces. If the spacing is reduced further, the boundary zones overlap and th e predicted mesoscopic size effect(3,12) is observed. At this point, the si ngle layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results sho uld lead to fundamental insight into ionic contact processes and to tailore d ionic conductors of potential relevance for medium-temperature applicatio ns.