Local Multimodal Electro‐Chemical‐Structural Characterization of Solid‐Electrolyte Grain Boundaries

Abstract

Typical models of polycrystalline ionic materials treat the grain boundary properties as single valued, without consideration of the full range of values that define the macroscopically measured average. Here a unique experimental platform suitable for local multimodal characterization of individual grain boundaries in bicrystal fibers is reported. A variation of three orders of magnitude in the grain boundary conductivity of ceria is observed, as measured across six individual bicrystals by both alternating current impedance spectroscopy and direct current (D.C.) linear sweep voltammetry. Nonlinear behavior of the D.C. measurements is consistent with resistance due to a space charge effect. Time‐of‐flight secondary ion beam spectroscopy reveals a correlation between grain boundary resistance and the concentration of impurities Si, Al, and Ca segregated at the grain boundaries, although the bulk concentrations of these impurities are negligible. Electron backscatter diffraction analysis of the crystal orientations suggests a correlation between the misorientation across the grain boundaries and grain boundary resistance. These correlations point towards a grain boundary resistance that arises from impurity‐generated space charge effects and variations in impurity concentration and hence resistivity driven by the energetics of impurity segregation to grain boundaries of differing surface energies.

Unveiling Alternating Current Electronic Properties at Ferroelectric Domain Walls

“…One particularly promising but so far unexplored possibility is to investigate the impedance characteristics of individual isolated domain walls in nanostructured samples, analogous to so-called bicrystals (see Figure 4c). [153] By performing measurements on samples containing individual ferroelectric domain walls, an unprecedented frequency range could be covered within a single experiment, enabling a systematic analysis of features in the dielectric spectra and their relation to, e.g., the domain wall orientation and charge state.…”

“…b) The application of AFM-based techniques, such as NIM, [102,103] SIM, [104] and AC-cAFM, [23] enables spectroscopy measurements at ferroelectric domain walls with nanoscale spatial resolution. c) Analogous to bicrystals, [153] which were used in studies on grain-boundary-related effects in polycrystalline ceramics, the investigation of nanostructured ferroelectrics with only one domain wall represents a powerful approach for future frequency-dependent investigations.…”

“…This facilitated impedance characterization of the grain boundary from 1 mHz to 1 MHz at temperatures from 300 to 500 °C. [153] Although FIB-based extraction of domain walls in ferroelectric materials has been demonstrated, [119,209] the impedance characteristics of the microscopic crystals, as proposed in Figure 4c, have so far not been analyzed.…”

Dislocation-Mediated Conductivity in Oxides: Progress, Challenges, and Opportunities

“…Similarly, assessments of variability of transport parameters, including local conductivity, electrical width, core charge, and activation energy, can complement such structural and chemical studies. A combination of ac and dc electrical measurements, SIMS, and EBSD imaging has recently been applied to observe variability in grain boundaries of CeO 2 bicrystal fibers and has provided insight into the chemical origin of the variability in core charge . Similarly, Xu et al .…”

“…A combination of ac and dc electrical measurements, SIMS, and EBSD imaging has recently been applied to observe variability in grain boundaries of CeO 2 bicrystal fibers and has provided insight into the chemical origin of the variability in core charge. 61 Similarly, Xu et al combined atom probe tomography with electron holography to provide insight into the diversity of space charge potentials among grain boundaries in polycrystalline (Ce,Sm)O 2 . 62 Analysis of transport anisotropy can be performed by creating ordered arrays of aligned dislocations and performing macroscopic transport measurements parallel and perpendicular to the alignment direction.…”