thermodynamic parameters for the intermediate “phases”, which are PHYSICOCHEMISTRY OF LIQUIDS, intensely debated; and the transport properties in the intermediate SOFT MATTER, NANOCHEMISTRY phase have received little consideration, although they allow in principle SYNCHROTRON SOLEIL HIGHLIGHTS 2020 a stringent test of the nucleation theory. In order to test the consistency between the thermodynamic and kinetic SWING BEAMLINE parameters, both the nucleation rate J and the growth rate k of thest crystals must be measured. It requires in turn time-resolved, structural Associated publication data at crystal sizes of only a few structural units, which are out of reach of the techniques used in the seminal studies on non-classical Self-confined nucleation of iron oxide nucleation: potentiometry, cryo-TEM, optical microscopy, or atomic nanoparticles in a nanostructured force microscopy to name a few. amorphous precursor. J. Baumgartner, R. K. Ramamoorthy, SELF-CONFINEMENT EFFECT A. P. Freitas, M. A. Neouze, M.A. Bennet, D. Faivre & D. Carriere. Here we resolved at appropriate nanoscale and millisecond resolutions the crystallization of magnetite (FeO) nanoparticles, a paradigmatic3 4 Nano Letters, 20, 5001 (2020). example of a synthetic, biogenic and geologic iron oxide known to crystallize from disordered precursors. Precursors were mixed in fast References millifluidic and stopped-flow mixers and characterized by synchrotron in [1] Kelton & L. Greer, In "Nucleation in situ X-ray small- and wide-angle scattering (SAXS/WAXS) on the SWING condensed matter: applications in beamline. It enabled us to resolve the developing nanostructures materials and biology"; Pergamon over several length (0.1 – 100 nm), time scales (0.4 - 600 s) and iron materials series; Elsevier: concentrations (12.5 - 50mM). Amsterdam (2010). [2] J. J. D Yoreo et al., Science, 349, From 400 ms up to 15 s of reaction, we evidence that most of the reactants 6247 (2015). are involved into fractal aggregates ofsub-nanometer, aorphom us rnio [3] P. G. Vekilov, Cryst. Growth Des., (oxyhydr)oxide particles of diameters in the 1.3 to 3 nm range. Afterwards, 10, 5007 (2010). these amorphous particles crystallized to magnetite. We showed that the crystal nucleation rate J and the growth rate k retrieved from thest Corresponding authors SAXS/WAXS patterns are inconsistent with the theoretical expectations in David Carrière two usual “non classical” scenarios, namely dissolution-recrystallization, Université Paris-Saclay, CNRS, and crystallization within the dense phase. CEA, NIMBE, LIONS, 91191 Cedex Gif sur Yvette, France On the other hand, the kinetic parameters are consistent with the david.carriere@cea.fr usual two-step crystallization within the dense phase, provided a “self- Damien Faivre confinement” of the reactants is invoked: after they nucleate in the Aix-Marseille Université, amorphous phase, the crystals grow at very high rates until they consume CEA, CNRS, BIAM, away the amorphous nanoparticle; then, the growth rate decreases 13108 Saint Paul lez Durance, France by several orders of magnitude, due to reactant depletion. We expect damien.faivre@cea.fr that such a self-confinement effect is dominant in materials where the amorphous precursor has a lifetime longer than the typical nucleation Captions time. In other words, it provides a synthesis guideline to set the final FIGURE 1: Physical parameters from the scattering sizes of the nanocrystals with the transient amorphous nanostructure. patterns. (a) Total volume fraction of solids (circles), volume fraction of nanocrystals from Bragg peak FIGURE 2 integration (triangles) and SAXS analysis (squares). The blue line shows the maximal crystalline volume fraction expected from cFe. (b) Radius of gyration of the nanocrystals. (c) Number of FeO4/3 structural unit per crystal. (d) Numeric concentration of nanocrystals. The dashed lines are linear regressions used to retrieve the growth rate and nucleation rate. FIGURE 2: Sketch of the non-classical crystallization process. The growth rate of the nanocrystals decrease by several orders of magnitude past the size of the amorphous precursor (arrows). 41