  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.
