Negative X-ray expansion in cadmium cyanide

Journal article

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 expo- sure 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 re- sponse in Cd(CN)2 highlights the potential for exploiting X-ray exposure in the design of functional materials.

Authors: Coates, CS; Murray, C; Boström, H; Reynolds, EM; Goodwin, A

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Royal Society of Chemistry
• Journal: Materials Horizons
• Volume: 8
• Issue: 5
• Pages: 1446-1453
• Publication date: 2021-02-12
• Acceptance date: 2021-02-12
• DOI: 10.1039/d0mh01989e
• EISSN: 2051-6355
• ISSN: 2051-6347
• Language: English
• Keywords: FFR