Deciphering chemical order/disorder and material properties at the single-atom level

Abstract

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Tuning crystal structure and magnetic property of dispersible FePt intermetallic nanoparticles

“…The as-prepared FePt NPs have a chemically disordered face centered cubic (fcc) structure (A1 phase), in which Fe and Pt atoms occupy randomly in fcc lattice. When annealing at temperature higher than 500°C, FePt undergoes a disorder-order transition, resulting in the formation of chemically ordered intermetallics with distinct crystal structures compared to the A1 phase [10,11]. According to FePt alloy equilibrium phase diagram, chemically ordered FePt intermetallics can form AuCu-type tetragonal (L1 0 -FePt) and AuCu 3 -type cubic (L1 2 -FePt 3 or L1 2 -Fe 3 Pt) structures depending on the chemical composition of the alloy [12].…”

Three-Dimensional Point Defect Imaging by Large-angle Illumination STEM

“…There are several approaches towards achieving high-resolution 3D imaging and spectroscopy, including electron tomography, hollow-cone illumination STEM and confocal STEM. Though electron tomography has achieved excellent 3D atom-by-atom imaging in some cases [3], it is not well-suited to typical plate-shape thin specimens. Since the successful development of higher-order aberration correctors [2], it has become possible to increase the illumination angle (α) up to 70 mrad.…”

“…Since the successful development of higher-order aberration correctors [2], it has become possible to increase the illumination angle (α) up to 70 mrad. Therefore, optical depth sectioning with large-angle illumination (LAI) STEM [3] may allow us to solve three-dimensional materials problems. However, two technical difficulties must be overcome to perform atomic-scale optical depth sectioning via LAI-STEM [4].…”

Atomic Electron Tomography: Probing 3D Structure and Material Properties at the Single-Atom Level

“…Using atomic electron tomography (AET) [1,[9][10][11][12][13][14], we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an FePt nanoparticle to correlate the 3D atomic structure with material properties at the single-atom level [15]. We identify rich structural variety and chemical order/disorder including 3D atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects.…”