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

Journal article

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.

Authors: Fairbank, V; Thompson, A; Cooper, R; Goodwin, A

• Publication status: Published
• Journal: Physical Review B
• Volume: 86
• Issue: 10
• Publication date: 2012-06-03
• DOI: 10.1103/PhysRevB.86.104113
• EISSN: 1550-235X
• ISSN: 1098-0121
• Language: English
• Keywords: cond-mat.mtrl-sci