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Materials

Polarized X-Rays Map Bonds Within Solids

Birefringence imaging reveals bond orientations without requiring perfectly ordered materials

by Jyllian Kemsley
June 2, 2014 | APPEARED IN VOLUME 92, ISSUE 22

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Credit: Kenneth D.M. Harris/Cardiff U
A portion of the ordered thiourea (top and bottom) and 1-bromoadamantane (middle) material examined by X-ray birefringence imaging.
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Credit: Kenneth D.M. Harris/Cardiff U
A portion of the ordered thiourea (top and bottom) and 1-bromoadamantane (middle) material examined by X-ray birefringence imaging.

An X-ray version of a polarized light microscope provides a new way to selectively identify and map bonds within a material (Science 2014, DOI: 10.1126/science.1253537). The approach doesn’t require a perfectly ordered crystal to work and instead can help illuminate regions of a material that might differ from each other. Called X-ray birefringence imaging (XBI), the technique exploits the fact that the refractive index of a birefringent material depends on its orientation relative to the direction of linearly polarized radiation. XBI uses polarized X-rays at an energy near the edge of an absorption band of specific atoms to yield structural information about the bonding environment around those atoms, as demonstrated by a group led by Kenneth D. M. Harris of Cardiff University, in Wales. The researchers tested XBI on inclusion complex materials composed of bromine-containing guest molecules in a thiourea host, with X-rays tuned to an absorption edge of bromine to pick up the orientation of C–Br bonds. They were able to determine the orientation of 1-bromoadamantane molecules in one material. In another, they identified areas of different atomic alignment within a single crystal.

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