Local structure in Ag$_3$[Co(CN)$_6$]: Colossal thermal expansion, rigid unit modes and argentophilic interactions

TL;DR

This study investigates the local structural dynamics of Ag$_3$[Co(CN)$_6$], revealing that colossal thermal expansion is driven by flexible linkages and rigid unit modes, with significant argentophilic interactions influencing thermodynamic behavior.

Contribution

The paper combines neutron scattering and RMC analysis to elucidate the role of rigid unit modes and argentophilic interactions in thermal expansion of Ag$_3$[Co(CN)$_6$], providing new insights into its structural dynamics.

Findings

Large thermal expansion occurs with minimal distortion of coordination polyhedra.

Rigid unit modes dominate low-energy phonon spectra below 150 K.

Thermal variation affects Ag...Ag interactions, influencing thermodynamic properties.

Abstract

Local structure in the colossal thermal expansion material Ag$_3$[Co(CN)$_6$] is studied here using a combination of neutron total scattering and reverse Monte Carlo (RMC) analysis. We show that the large thermal variations in cell dimensions occur with minimal distortion of the [Co(CN)$_6$] coordination polyhedra, but involve significant flexing of the Co--CN--Ag--NC--Co linkages. We find real-space evidence in our RMC configurations for the importance of low-energy rigid unit modes (RUMs), particularly at temperatures below 150 K. Using a reciprocal-space analysis we present the phonon density of states at 300 K and show that the lowest-frequency region is dominated by RUMs and related modes. We also show that thermal variation in the energies of Ag$...$Ag interactions is evident in both the Ag partial pair distribution function and in the Ag partial phonon density of states. These findings are discussed in relation to the thermodynamic properties of the material.