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

TL;DR

This study investigates the behavior of the negative thermal expansion material Cd(CN)$_2$ under pressure using neutron diffraction, scattering, and quantum calculations, revealing phase behavior, elastic properties, and phonon softening.

Contribution

It provides new insights into the pressure-dependent phase transitions, elastic stiffness, and phonon dynamics of Cd(CN)$_2$, highlighting the interplay between local coordination and fluctuations.

Findings

Cd(CN)$_2$ exhibits phase transitions under pressure.

The material shows 'warm hardening' with increased elastic stiffness upon heating.

Phonon modes soften under pressure, affecting thermal expansion behavior.

Abstract

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)$_2$.