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International audience; Inorganic molten salts are known as fluxes for the synthesis of novel bulk inorganic compounds and of mesostructures and nanostructures with crystal habits different from those observed in more conventional solvents. However, they have not demonstrated the ability to provide mesostructures and nanostructures of complex metal oxides that are currently unreported at the mesoscale and nanoscale. In this report, we show the first occurrence of Sr4Mn3O10 at the mesoscale, as platelets synthesized in molten strontium hydroxide at 600 °C with basal faces of few hundreds of nanometers and thicknesses ranging from 20 to 100 nm. We address carefully the atom-scale structure by transmission electron microscopy, including electron energy loss spectroscopy and electron tomography. We then propose that the final morphology is driven by the surface charge of each facet through surface energy. The reactivity of these platelets is then addressed, highlighting cation leaching when in contact with acidic water, which results in crystalline–amorphous core–shell platelets that are active electrocatalysts towards the oxygen reduction reaction.