Phonons and Anomalous Lattice Behavior in KMnAg3(CN)6 and KNiAu3(CN)6: Inelastic Neutron Scattering and First-Principles Calculations

Cyanide-based framework compounds are known to show large negative thermal expansion behavior. Here, we report the phonon and anomalous lattice behavior of two metal-cyanide framework compounds, i.e., KMnAg3(CN)(6) and KNiAu3(CN)(6). We have studied the role of van der Waals dispersion and magnetic interactions in structural stability of these compounds. The behavior of these compounds under isotropic compression shows the presence of negative linear compressibility. The calculated phonon spectra, validated by inelastic neutron scattering measurements and elastic constants, are used to study the negative thermal expansion behavior, which is found to arise from low-energy phonon modes involving the folding of A-NC-B-CN-A linkage about B atoms (A = Mn, Ni and B = Ag, Au).