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 Cd2+ 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.
@article{arxiv.1206.0437,
title = {Charge-ice dynamics in the negative thermal expansion material Cd(CN)$_2$},
author = {Vanessa E. Fairbank and Amber L. Thompson and Richard I. Cooper and Andrew L. Goodwin},
journal= {arXiv preprint arXiv:1206.0437},
year = {2015}
}