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The call for material systems with enhanced mass transport properties is central in the development of next-generation fuel cells, batteries and solid state energy devices in general. While two-dimensional doping by artificial heterostructuring or nanoscaling has shown great potential for overcoming kinetic limitations of ion diffusion, the length scale of interface effects requires the development of advanced tools for capturing and quantifying local phenomena in greater detail. In the present paper, an in-depth study of grain boundary oxygen conduction in Sr-doped lanthanum manganite films is presented by means of novel isotope-exchange atom probe tomography. Local pathways for fast mass transport are directly mapped by two-dimensional reconstructions and line profiles of the oxygen isotope concentration. Accurate finite element modelling is employed to retrieve the local kinetic parameters, highlighting an enhancement of two orders of magnitude for both the diffusivity and surface exchange rate with respect to the bulk (D*gb=2.1×1014 cm2 s1 and k*gb=4.3×1010 cm s1, respectively, for grain boundaries at 550 °C). Co-acquired reconstruction of the cationic distribution reveals strong inhomogeneities (dopant de-mixing) across the grain boundaries and in the sub-surface region, leading to local Sr accumulation. The findings provide unequivocal quantitative assessment of fast grain boundary oxygen diffusion in lanthanum strontium manganite, giving further insights into local stoichiometry deviations and promoting isotope exchange-atom probe tomography as a powerful tool for the study of local interface effects with high resolution. Different models for the explanation of the phenomena are critically discussed on the basis of the experimental findings.

Crystal Geyser (Utah, USA) is a CO2-rich low-temperature geyser that is studied as a natural analog for CO2 leakage from carbon capture and storage (CCS) sites. In order to better constrain the biogeochemical processes influencing CaCO3 precipitation at geological CO2 escape sites, we characterized fast-forming iron-rich calcium carbonate pisoids and travertines precipitating from the fluids expelled by the geyser. The pisoids, located within a few meters from the vent, are composed of concentric layers of aragonite and calcite. Calcite layers contain abundant ferrihydrite shrubs in which iron is encasing bacterial forms. The aragonite layers contain less abundant and finely dispersed iron, present either as iron-oxide microspherules or iron adsorbed to organic matter dispersed within the carbonate matrix. We propose that carbonate polymorphism in the pisoids is mostly controlled by local fluctuations of the iron redox state of the fluids from which they form, caused by episodic blooms of iron-oxidizing bacteria. Indeed, the waters expelled by Crystal Geyser contain >200 µM dissolved iron (Fe2+), a known inhibitor of calcite growth. The calcite layers of the pisoids may record episodes of intense microbial iron oxidation, consistent with observations of iron-oxide rich biofilms thriving in the rimstone pools around the geyser and previous metagenomic analyses showing abundant neutrophilic, microaerophilic iron-oxidizing bacteria in vent water. In turn, aragonite layers of the pisoids likely precipitate from Fe2+-rich waters, registering periods of less intense iron oxidation. Separately, CaCO3 polymorphism in the travertines, where calcite and aragonite precipitate concurrently, is not controlled by iron dynamics, but may be locally influenced by the presence of microbial biofilms. This study documents for the first time an influence of microbial iron oxidation on CaCO3 polymorphism in the environment, and informs our understanding of carbonate formation at CO2 leakage sites and in CCS contexts.