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Nucleation plays a critical role in many physical and biological phenomena ranging from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases 1-3. However, nucleation is a challenging process to study in experiments especially in the early stage when several atoms/molecules start to form a new phase from its parent phase. Over the years, a number of experimental and computational methods have been used to investigate nucleation processes 4-17 , but it remains unachievable to experimentally determine the 3D atomic structure and dynamics of early stage nuclei. Here, we develop 4D atomic electron tomography (AET) to study early stage nucleation at atomic resolution. Using FePt nanoparticles as a model system, we reveal that early stage nuclei are irregularly shaped, each has a core of one to a few atoms 2 with the maximum order parameter, and the order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter distribution and its gradient. These experimental observations are corroborated by molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. Our experimental and molecular dynamics results differ from classical nucleation theory (CNT) 1,2,18 , indicating a theory beyond CNT is needed to describe early stage nucleation at the atomic scale. Looking forward, we anticipate that 4D AET opens the door to study many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry such as phase transition, atomic diffusion, grain boundary dynamics, interface motion, defect dynamics and surface reconstruction with 4D atomic resolution. AET is a powerful method to determine the 3D atomic structure of materials without the assumption of crystallinity 19 and has been applied to study dislocations, stacking faults, grain boundaries, atomic displacement, strain tensor, chemical order/disorder and point defects with unprecedented 3D detail 20-26. But all of these studies were of static structures. To probe the 4D atomic structure of early stage nucleation, we have tracked the same nuclei at different times and applied AET to determine their 3D atomic coordinates and species at each time (Methods). We used FePt nanoparticles as a model system because binary alloys have been widely used to study phase transitions 2 and FePt is a very promising material for next generation magnetic recording media 25,27. As-synthesized FePt nanoparticles form a chemically disordered face-centred cubic (fcc) structure (A1 phase) 27. With annealing, the A1 phase Author contributions J.M. conceived and directed the project; F.S. and H.Z. prepared the samples; J.Z.,

The Haber-Bosch industrial process for synthesis of ammonia (NH) from hydrogen and nitrogen produces the millions of tons of ammonia gas annually needed to produce nitrates for fertilizers required to feed the earth's growing populations. This process has been optimized extensively, but it still uses enormous amounts of energy (2% of the world's supply), making it essential to dramatically improve its efficiency. To provide guidelines to accelerate this improvement, we used quantum mechanics to predict reaction mechanisms and kinetics for NH synthesis on Fe(111)-the best Fe single crystal surface for NH synthesis. We predicted the free energies of all reaction barriers for all steps in the mechanism and built these results into a kinetic Monte Carlo model for predicting steady state catalytic rates to compare with single-crystal experiments at 673 K and 20 atm. We find excellent agreement with a predicted turnover frequency (TOF) of 17.7 s per 2 × 2 site (5.3 × 10 mol/cm/sec) compared to TOF = 10 s per site from experiment.

Boron carbide (B 4 C) is very hard, but its applications are hindered by stress-induced amorphous band formation. To explain this behavior, we used density function theory (Perdew-Burke-Ernzerhof flavor) to examine the response to shear along 11 plausible slip systems. We found that the ð0111Þ=h1101i slip system has the lowest shear strength (consistent with previous experimental studies) and that this slip leads to a unique plastic deformation before failure in which a boron-carbon bond between neighboring icosahedral clusters breaks to form a carbon lone pair (Lewis base) on the C within the icosahedron. Further shear then leads this Lewis base C to form a new bond with the Lewis acidic B in the middle of a CBC chain. This then initiates destruction of this icosahedron. The result is the amorphous structure observed experimentally. We suggest how this insight could be used to strengthen B 4 C. DOI: 10.1103/PhysRevLett.113.095501 PACS numbers: 61.50.Ks, 62.20.M-, 64.70.-p, 82.40.Fp Boron carbide (B 4 C) exhibits such novel properties as high melting temperature, high thermal stability, high hardness, low density, and high abrasion resistance [1][2][3][4][5][6][7][8][9][10][11]. The combination of these properties makes it widely useful in refractory applications, as abrasive powders, in body armor, and as a neutron radiation absorbent [1][2][3][4]. However, the brittle failure under impact exhibited by B 4 C prevents the wide insertion of boron carbide into extended engineering applications. Although B 4 C has a high Hugoniot elastic limit (HEL) of 17-20 GPa, approximately twice that of normal ceramics, it fractures easily just above the HEL at high impact velocities and pressures [12][13][14][15]. Chen et al. reported the observation of shock-induced local amorphization bands that might be responsible for the low fracture toughness of B 4 C [6]. In addition, amorphization bands had been observed in nanoindentation and scratch experiments, where the loading rate is much lower than for dynamical shock loading [16][17][18][19]. Particularly, recent nanoindentation experiments revealed amorphous shear bands along the (0111) plane [19]. Indeed, the same amorphous shear band has been observed experimentally in boron suboxide (B 6 O) [20]. An attempt to understand this through Gibbs free-energy calculations based on density functional theory suggested that the ðB 12 ÞCCC structure provides a possible source of failure of boron carbide just above the HEL [7]. However, despite these extensive experimental and theoretical efforts, the atomistic mechanism underlying pressure-induced amorphization and phase transitions of boron carbide is still not known [3].Stressed materials normally dissipate the accumulating elastic energy through plastic deformation that is mediated by dislocation slip and deformation twinning in metallic systems. Unlike these general deformation mechanisms in conventional metallic materials, we find that boron carbide deforms by atomic scale amorphous band formation, a particular mode of deforma...