Lattice Dynamics and Thermal Expansion Behavior in Metal Cyanides, MCN (M=Cu, Ag, Au): Neutron Inelastic Scattering and First Principles Calculations

TL;DR

This study combines neutron inelastic scattering and first-principles calculations to analyze phonon spectra and thermal expansion in metal cyanides MCN (M=Cu, Ag, Au), revealing mode-specific contributions to their anomalous thermal behaviors.

Contribution

It provides a detailed phonon mode analysis linking lattice dynamics to thermal expansion differences in MCN compounds, highlighting the role of chain-sliding modes.

Findings

Chain-sliding modes cause negative thermal expansion in Cu and Ag compounds.

Transverse modes lead to positive thermal expansion along a and b axes.

Differences in bonding influence elastic constants and phonon behavior.

Abstract

We report measurement of temperature dependence of phonon spectra in quasi one-dimensional metal cyanides MCN (M=Cu, Ag and Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon-spectra as well as to understand the anamolous thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along c axis in the Cu and Ag compounds, while the same modes contribute to positive thermal expansion in the Au compound. Several low energy transverse modes lead to positive thermal expansion along a and b axis in all the compounds. The calculated elastic constants and Born effective charges are correlated with the difference in nature of bonding among these metal cyanides.