Comprehensive Modeling of Ion Conduction of Nanosized CaF₂/BaF₂ Multilayer Heterostructures

Abstract

Molecular beam epitaxy‐grown CaF2/BaF2 heterolayers are a demonstration of the potential of nanoionics. It has been shown that ion conductivities both parallel and perpendicular to the interfaces increase with decrease in interfacial spacing. This size effect was attributed to the thermodynamically necessary redistribution of the mobile fluoride ions (N. Sata, K. Eberl, K. Eberman, J. Maier, Nature 2000, 408, 946; X. X. Guo, I. Matei, J.‐S. Lee, J. Maier, Appl. Phys. Lett. 2007, 91, 103102). On this basis, the striking phenomenon of an upward bending in the effective parallel conductivity as a function of inverse interfacial spacing ${\sigma}_{m}^{||} ({{\rm 1}/ \ell})$ for low temperatures (T ≤ 593 K) has been satisfactorily explained by application of a modified Mott–Schottky model for BaF2 (X.X. Guo, I. Matei, J. Jamnik, J.‐S. Lee, J. Maier, Phys. Rev. B 2007, 76, 125429). This model was further confirmed by measurements perpendicular to the interfaces that offer complementary information on the more resistive parts. Here a successful comprehensive modeling of parallel and perpendicular conductivities for the whole parameter range, namely for interfacial spacings ranging from 6 to 200 nm and investigated temperatures ranging from 455 to 833 K, is presented. The model is based on literature data for carrier mobilities and Frenkel reaction constants and the assumption of a pronounced F− redistribution. Given the fact that an impurity content that was experimentally supported is taken into account and apart from minor assumptions concerning profile homogeneity, the only fit parameter is the space charge potential. In particular, it is worth mentioning that in BaF2 the low temperature Mott–Schottky space charge zone which is determined by impurities changes over, at high temperatures, into a Gouy–Chapman situation owing to increased thermal disorder. (The situation in CaF2 is of Gouy–Chapman type at all temperatures.)

Electronic nature of the enhanced conductivity in YSZ-STO multilayers deposited by PLD

“…This effect was related to the variation of the charge carriers density, or their mobilities, indicating the importance of the interfaces in enhancing charge transport phenomena beyond trivial size effects of increasing surface-tovolume ratio in nanocrystalline material. Epitaxial heterostructures prepared by sequential deposition of CaF 2 /BaF 2 have also shown enhancement of the F -ionic conductivity along the interfaces, by several orders of magnitude, upon reducing the thickness to a few nanometers [7,8]. This effect was attributed to the variation of the density of majority charge carriers in the space charge region generated at the interfaces between both materials.…”

From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2–BaF2 System

“…[2][3][4][5] These efforts have been comple-mented by a series of modeling studies that have resulted in a very detailed picture of nanoscale heterolayer systems. [6,7] An alternative approach to such interface-dominated materials has involved studying the ionic conductivity of clusterassembled nanocrystalline CaF 2 and BaF 2 homo-and heterosystems that were prepared by inert-gas condensation or by high-energy ball-milling. [8][9][10][11][12][13] In such systems, the introduction of interfacial defects can be understood in terms of a heterogeneous Frenkel reaction [Eq.…”

State-of-the-Art Thin Film Electrolytes for Solid Oxide Fuel Cells

“…Thus, the ionic conductivity of nanoscale materials can be further enhanced by developing multilayer thin films with nanoscale interfaces [202][203][204]. It has been observed that, nanoscale hetero-structures of CaF 2 and BaF 2 exhibit significantly higher ionic conductivity along the interfacial direction at moderate temperatures due to the redistribution of mobile fluoride ions in the overlapped space-charge regions of CaF 2 and BaF 2 [18,205,206]. These results paved the way to develop multilayer thin film electrolytes to enhance the oxygen ionic conductivity by introducing hetero-structured interfaces [59,61,[207][208][209][210][211][212][213][214].…”