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Negative X-Ray Expansion in Cadmium Cyanide

preprint
submitted on 22.12.2020, 12:11 and posted on 23.12.2020, 08:50 by Chloe Coates, Claire A. Murray, Hanna Boström, Emily Reynolds, Andrew Goodwin
Cadmium cyanide, Cd(CN)2, is a flexible coordination polymer best studied for its strong and isotropic negative thermal expansion (NTE) effect. Here we show that this NTE is actually X-ray exposure dependent: Cd(CN)2 contracts not only on heating but also on irradiation by X-rays. This behaviour contrasts that observed in other beam-sensitive materials, for which X-ray exposure drives lattice expansion. We call this effect ‘negative X-ray expansion’ (NXE) and suggest its origin involves an interaction between X-rays and cyanide ‘flips’; in particular, we rule out local heating as a possible mechanism. Irradiation also affects the nature of a low-temperature phase transition. Our analysis resolves discrepancies in NTE coefficients reported previously on the basis of X-ray diffraction measurements, and we establish the ‘true’ NTE behaviour of Cd(CN)2 across the temperature range 150–750 K. The interplay between irradiation and mechanical response in Cd(CN)2 highlights the potential for exploiting X-ray exposure in the design of functional materials.

Funding

Leverhulme Trust

DEFect NETwork materials science and engineering

European Commission

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The Complexity Revolution: Exploiting Unconventional Order in Next-Generation Materials Design

European Research Council

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History

Email Address of Submitting Author

andrew.goodwin@chem.ac.uk

Institution

University of Oxford

Country

United Kingdom

ORCID For Submitting Author

0000-0001-9231-3749

Declaration of Conflict of Interest

No conflict of interest.

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