Charge-ice dynamics in the negative thermal expansion material Cd(CN)$_2$

TL;DR

This study reveals low-energy vibrational modes involving off-centering of Cd$^{2+}$ ions in Cd(CN)$_2$, suggesting a new mechanism for its negative thermal expansion linked to ice-rule-like local correlations.

Contribution

It provides experimental evidence for a novel NTE mechanism involving off-centering vibrational modes and local correlations akin to ice-rule dynamics, differing from previous rotation-based models.

Findings

Identification of low-energy vibrational modes involving Cd off-centering

Observation of structured diffuse scattering indicating local correlations

Monte Carlo simulations linking dynamics to ice-rule behavior

Abstract

We use variable-temperature (150--300\,K) single-crystal X-ray diffraction to re-examine the interplay between structure and dynamics in the ambient phase of the isotropic negative thermal expansion (NTE) material Cd(CN)$_2$. We find strong experimental evidence for the existence of low-energy vibrational modes that involve off-centering of Cd$^{2+}$ ions. These modes have the effect of increasing network packing density---suggesting a mechanism for NTE that is different to the generally-accepted picture of correlated Cd(C/N)$_4$ rotation modes. Strong local correlations in the displacement directions of neighbouring cadmium centres are evident in the existence of highly-structured diffuse scattering in the experimental X-ray diffraction patterns. Monte Carlo simulations suggest these patterns might be interpreted in terms of a basic set of `ice-rules' that establish a mapping between the dynamics of Cd(CN)$_2$ and proton ordering in cubic ice VII.