Lattice dynamics and thermal expansion behavior in the metal cyanides MCN (M = Cu, Ag, Au): Neutron inelastic scattering and first-principles calculations

We reportmeasurement of temperature dependence of phonon spectra in quasi-one-dimensionalmetal cyanides MCN (M = Cu, Ag, Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon spectra as well as to understand the anomalous anisotropic 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 the "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 in the a-b plane in all the compounds. The calculated Born-effective charges show that AuCN has a covalent nature of bonding, which results in least distortion as well as the least number of unstable modes among the three cyanides. This result is well correlated with the fact that the coefficient of negative thermal expansion along the c axis in AuCN is the smallest.