Back-bonding driven negative thermal expansion along the one dimensional Hg[sbnd]C[tbnd]N linkage in [HgCN](NO3) probed by Raman spectroscopy

Abstract

In this paper, the mechanistic aspect for anisotropic negative thermal expansion (NTE) in [Hg(CN)](NO3) is correlated to back-bonding along a one dimensional HgCN network using temperature dependent Raman spectroscopy. Signatures of back-bonding are contraction of the HgC bond (~299 cm−1), expansion of the CN bond (~2249 cm−1) and bending of the HgCN bond. These are clearly observed in the Raman spectra, correlating to a continuous increase in the HgCN bending frequency (~346 cm−1) with an increase in temperature. This bending of the HgCN bond may be responsible for an expansion of the Hg lattice along the ab-plane and a reorientation of the NO3− ions, as seen by the change in frequency variation of the 1637 cm−1 mode and the reduction of its intensity with the increase in temperature. All these effects are in agreement with the reported single crystal X-ray data. Thus, the present model of back-bonding electron transfer from the metal d-orbital to the π*-orbital of the cyanide ligand and the effect of temperature changes on it can be explored to understand the NTE behaviour in metal cyanide systems and more broadly in multifunctional materials.