Coates Chloe S, Murray Claire A, Boström Hanna L B, Reynolds Emily M, Goodwin Andrew L
Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK.
Mater Horiz. 2021 May 1;8(5):1446-1453. doi: 10.1039/d0mh01989e. Epub 2021 Feb 23.
Cadmium cyanide, Cd(CN), 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) 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) across the temperature range 150-750 K. The interplay between irradiation and mechanical response in Cd(CN) highlights the potential for exploiting X-ray exposure in the design of functional materials.
氰化镉（Cd(CN)₂）是一种柔性配位聚合物，因其强烈且各向同性的负热膨胀（NTE）效应而得到广泛研究。在此我们表明，这种NTE实际上依赖于X射线照射：Cd(CN)₂不仅在加热时收缩，在X射线照射下也会收缩。这种行为与在其他对光束敏感的材料中观察到的情况相反，在那些材料中，X射线照射会导致晶格膨胀。我们将这种效应称为“负X射线膨胀”（NXE），并认为其起源涉及X射线与氰化物“翻转”之间的相互作用；特别是，我们排除了局部加热作为一种可能机制。照射还会影响低温相变的性质。我们的分析解决了先前基于X射线衍射测量报告的NTE系数中的差异，并确定了Cd(CN)₂在150 - 750 K温度范围内的“真实”NTE行为。Cd(CN)₂中照射与力学响应之间的相互作用突出了在功能材料设计中利用X射线照射的潜力。
