Three-dimensional imaging of dislocation propagation during crystal growth and dissolution (1501.02853v1)

Abstract: Atomic level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials. Their effects are important and wide-reaching, and range from increased chemical reactivity to enhanced mechanical properties to vastly increased rates of crystal growth. Dislocations have therefore been widely studied using traditional techniques such as X-ray diffraction (XRD) and optical imaging. More recently, advances in microscopy have allowed their direct visualization. Atomic force microscopy (AFM) has enabled the 2D study of single dislocations while transmission electron microscopy (TEM), which was initially limited to 2D projections of thin specimens, can now visualize strain fields in 3D with near atomic resolution. However, these techniques can- not offer in situ, 3D imaging of the formation or movement of dislocations during dynamic processes such as crystal growth and dissolution. Here, we describe how Bragg Coherent Diffraction Imaging (BCDI) can be used to visualize in 3D the entire network of dislocations present within an individual crystal. Using calcite (CaCO3) single crystals, we also use BCDI to monitor the propagation of the dislocation network during repeated growth and dissolution cycles, and show how this is intimately linked to the growth and dissolution mechanisms. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli.

Three-dimensional Imaging of Dislocation Propagation During Crystal Growth and Dissolution

This paper presents an advanced paper employing Bragg Coherent Diffraction Imaging (BCDI) to achieve three-dimensional visualization of dislocation networks within calcite (CaCO₃) crystals throughout growth and dissolution cycles. Dislocations, which are atomic-level defects, play crucial roles in defining macroscopic properties such as chemical reactivity, mechanical strength, and the kinetics of crystal growth and dissolution. Historically, dislocation studies have been conducted using methods like X-ray diffraction (XRD) and transmission electron microscopy (TEM), which have limitations in providing in situ, three-dimensional images during dynamic processes. BCDI overcomes these limitations by enabling the three-dimensional imaging of strain and dislocation evolution at nanoscales without the need for sample preparation, thereby offering insights into the dynamic behavior of crystals in response to environmental stimuli.

In this research, calcite was chosen for investigation due to its well-studied nature and clearly defined rhombohedral morphology, which makes it a suitable model for understanding the relationship between dislocation network organization and crystal growth/dissolution mechanisms. BCDI utilizes coherent X-rays to obtain diffraction patterns sensitive to the density and internal strain of crystals. From these diffraction patterns, real-space images are reconstructed highlighting electron density and lattice strain, which reveal the presence and behavior of dislocations.

Key observations reported in the paper include the non-uniform distribution of defects contributing to differential growth rates across crystal surfaces and preferential directions of dissolution marked by enhanced etch-pit formation at defect sites. The investigation highlights the persistence of screw dislocations characterized by their spiral lattice displacement and hollow cores throughout growth and dissolution processes. This behavior is supported by simulations comparing spiral phase displacement in experimentally reconstructed data to simulated atomic resolution screw dislocations.

The analytical approach elucidates how dislocation dynamics such as increase during growth and annihilation during dissolution are intimately linked to changes in strain distribution primarily affecting regions near the crystal surface. The role of dislocation distribution is critical in dictating crystal morphology, revealing that defect-rich regions grow faster and dissolve more readily than more stable core areas. The ability to map these defects from three-dimensional perspectives marks a considerable advancement in characterizing dynamic nano-scale mechanisms in crystallization.

The implications of this research are substantial, offering a 3D characterization tool that can be harnessed across diverse fields such as material science, geology, and biomineralization to paper crystal response to external pressures such as mechanical forces or temperature variations. Future applications might encompass studying dislocation evolution under varied environmental conditions or in different crystal systems, further elucidating nanoscale mechanical and chemical processes. Thus, BCDI represents a significant step forward in understanding dislocation-mediated properties of materials, promoting insights into optimizing material performance based on crystalline defect management.