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Posted: Jun 04, 2015

A coherent look at crystalline defects

(Nanowerk News) A ‘lensless’ X-ray microscope has captured, in stunning detail, the first three-dimensional images of crystalline defects during crystal growth of a mineral, calcite.
The ‘lensless’ microscope allowing this 3D imaging was pioneered by the group of Ian Robinson at the London Centre for Nanotechnology (LCN) at UCL, and reveals important information regarding the disruption of the perfect order that defines a crystal. The findings are published in the journal Nature Materials ("Three-dimensional imaging of dislocation propagation during crystal growth and dissolution"). A related article has recently published in the journal PNAS ("Imaging transient melting of a nanocrystal using an X-ray laser").
Jesse Clark, previously from the LCN now at Stanford University and a lead author of the paper said: “Dislocations are atomic-scale crystal defects that play significant roles in the definition of crystal properties. They have therefore been the subject of intense study, and much effort has been made to visualise their structures in 3D with atomic-scale resolution. While significant advances have been, other techniques cannot provide 3D imaging of dislocations while the crystal is subject to an external stimulus such as growth and dissolution, or mechanical work.”
“Only by visualising the entire network of dislocations with a crystal, and how it responds to processes such as growth and dissolution can we fully understand crystallization mechanisms.”
Scientists found they could identify the defects within the crystal and follow their evolution as the crystal grew. They were able to correlate directly – in 3D - pronounced growth of the crystal with the locations of the defects.
This represents the first time that crystal defects (dislocations) have been imaged in 3D to high resolution using X-rays. In a single experiment this powerful technique provides 3D visualization of the crystal morphology, the propagation of the dislocation network and the strain-fields associated with these dislocations within an evolving crystal.
The team carried out the experiments at the Diamond Light Source in Oxfordshire using a very bright source of X-rays – a synchrotron. The X-rays are directed onto individual calcite crystals, each 100 times smaller than the width of a human hair, before the scattered x-rays are recorded on a detector – much like the ones found in modern smartphone cameras.
Unlike conventional microscopy, which uses a lens to form an image, the team of scientists instead used a ‘virtual lens’ – an algorithm on a computer to form the images. This allows much easier imaging of processes such as crystal growth due to the uncomplicated experimental setup.
Johannes Ihli, from the University of Leeds and a lead author of the study, said: "We have shown directly that the lattice deformation surrounding a defect governs the internal energy of a finite crystal, which in turn alters the activation barrier for growth and dissolution. The network of dislocations within a crystal therefore dictates among other factors the mechanism by which it grows and dissolves.
“This work is of immediate relevance to the behaviour of calcite in the environment, and to the formation and properties of bio-minerals such as seashells. These investigations also provide unique and general insight into fundamental mechanism''.
Source: London Centre for Nanotechnology
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