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

Abstract

Cyanide based framework compounds are known to show large negative thermal expansion behaviour.Here we report the phonon and anomalous lattice behavior of two metal cyanide framework compounds i.e. KMnAg 3 (CN) 6 and KNiAu 3 (CN) 6 . We have studied the role of van der Waals dispersion and magnetic interactions on 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.

Understanding Large Negative Thermal Expansion of NdFe(CN)6 through the Electronic Structure and Lattice Dynamics

“…Only rarely do some materials show a negative thermal expansion (NTE), and these play an important role since they make possible the realization of materials with zero thermal expansion (ZTE) or even better of materials with tailored thermal expansion . Accordingly, the NTE phenomenon has received great attention and rapid development, such that many NTE materials and mechanisms have been discovered, as reported in recent reviews. − Based on the NTE mechanism, the NTE materials can be grouped into two categories: − electron-driven NTE materials when the NTE is due to magnetic volume effect or charge transfer − and phonon-driven NTE when the presence of low-frequency phonon vibrations gives rise to NTE, ,,,, such as in some oxides, − fluorides, , cyanides, − and MOFs . Open-framework structure materials are excellent candidates for exhibiting phonon-driven NTE, whose origin is often related to the transverse thermal vibration by bridging atoms or the tilting and rotations of atomic polyhedral units.…”