Structure and dynamics of the negative thermal expansion material Cd(CN)\(_2\) under pressure

We use a combination of variable-temperature / variable-pressure neutron powder diffraction, variable-pressure inelastic neutron scattering, and quantum chemical calculations to interrogate the behaviour of the negative thermal expansion (NTE) material \(^{114}\)Cd(CN)\(_2\) under hydrostatic pressure. We determine the equation of state of the ambient-pressure phase, and discover the so-called `warm hardening' effect whereby the material becomes elastically stiffer as it is heated. We also identify a number of high-pressure phases, and map out the phase behaviour of Cd(CN)\(_2\) over the range \(0\leq p\leq0.5\)\,GPa, \(100\leq T\leq300\)\,K. As expected for an NTE material, the low-energy phonon frequencies are found to soften under pressure, and we determine an effective Gr{\"u}neisen parameter for these modes. Finally, we show that the elastic behaviour of Cd(CN)\(_2\) is sensitive to the local Cd coordination environment, which suggests an interplay between short- (phononic) and long-timescale (cyanide flips) fluctuations in Cd(CN)\(_2\).