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.

Fig. 1. Negative X-ray expansion: the evolution of a series of diffraction patterns (λ = 0.82507 Å) for a sample of Cd(CN)₂, held at (A and B) 200 K and (C and D) 100 K, as a function of X-ray exposure (blue to red). (A) At 200 K Cd(CN)₂ adopts cubic Pn3̄m symmetry and increasing irradiation results in contraction of the unit cell, as seen in (B) the (110) peak moves to progressively larger values of 2θ. (D) With minimal X-ray exposure at 100 K (blue) Cd(CN)₂ has undergone a symmetry-lowering phase transition. On increasing exposure these additional peaks coalesce and those of cubic Cd(CN)₂ are restored. X-ray exposure reverses the phase transition. (E) Cd(CN)₂ crystallises with Pn3̄m symmetry where tetrahedrally coordinated Cd centres (red/pink to highlight interpenetration frameworks) are connected by disordered cyanide ions (C/N grey spheres).

Fig. 2. The relative changes in the lattice parameters (, where ₀ is the relevant lattice parameter with minimal X-ray exposure at 110 K) of Cd(CN)₂ and KMn[Ag(CN)₂]₃ as a function of X-ray exposure from blue to red (∼200 seconds) with λ = 0.82507 Å. Whilst the lattice parameter of the former is seen to shrink as a function as a function of exposure at all temperatures, those of the latter show no change, as seen by the overlapping symbols, eliminating the possibility of local heating as a mechanism for the negative X-ray expansion in Cd(CN)₂.

Fig. 3. Diffraction patterns (λ = 0.82484 Å) collected for a sample of Cd(CN)₂ at 300 K (black) and in the same position at 100 K (red) and at 100 K having been translated (blue = minimal prior exposure). The nature of the phase transition is clearly affected by irradiation prior to cooling.

Fig. 4. (A) The cubic lattice parameter, a, of cadmium cyanide collected on heating between 150 K and 750 K for minimal X-ray irradiation, shown as black circles (errors are smaller than the circles). Previously reported data are included for comparison to illustrate the concomitant effects of thermal expansion and X-ray expansion, shown as red triangles (ref. 26), black squares (ref. 30) and blue diamonds (ref. 31) The NXE-minimised thermal expansion coefficient is α_V = −55(2) MK⁻¹. (B) The Ashby plot of ref. 48 comparing the magnitudes and temperature ranges of NTE for different families of materials, where the NTE capacity χ_α = ΔV/V is maximised in the top right corner. The new value of α_V reported here places Cd(CN)₂ alongside CaNbF₆ as the material with the largest NTE capacity.