Charge-ice dynamics in the negative thermal expansion material Cd(CN)2
V E Fairbank, A L Thompson, R I Cooper and A L Goodwin
Phys Rev B 86, 104113 (2012)
The final authors’ version of this article is available open access via. the Oxford Research Archive.
We use variable-temperature (150–300 K) single-crystal x-ray diffraction to reexamine 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 in this material. Strong local correlations in the displacement directions of neighboring cadmium centers are evident in the existence of highly structured diffuse scattering in the experimental x-ray diffraction patterns. Monte Carlo simulations suggest that 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.