Phonons and anomalous lattice behaviour in KMnAg₃(CN)₆ and KNiAu₃(CN)₆: inelastic neutron scattering and first-principles calculations

Many oxides, fluorides and cyanides framework compounds are found to exhibit negative thermal expansion (NTE) behaviour. K(NiAu₃(CN)₆ and KMnAg₃(CN)₆ are isostructural cyanide framework compounds which shows a large NTE and negative linear compressibility (NLC) along hexagonal c-axis. We have applied inelastic neutron scattering to measure temperature dependent phonon spectra and ab-initio lattice dynamics simulations to investigate the mechanism responsible for NTE and NLC behaviour in these compounds. The calculated thermal expansion coefficient along a- and c- axis agrees very well with the experimental data. The negative thermal expansion is found to arise from the low energy optic phonon modes involving transverse vibrations of Ag/Au atoms connecting (Mn/Ni)(CN)₆ octahedral units