Negative X-ray expansion in cadmium cyanide.

ABSTRACT: 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.

SUBMITTER: Coates CS 

PROVIDER: S-EPMC8111741 | biostudies-literature | 2021 May

REPOSITORIES: biostudies-literature