Vibrational and Thermal Properties of Ag₃[Co(CN)₆] from First-Principles Calculations and Infrared Spectroscopy

We have investigated the phonon modes of Ag 3 [Co(CN) 6 ] using first-principles based methods and high-pressure infrared spectroscopy to understand the origin of its colossal thermal expansion. Grüneisen parameters of its zone-center vibrational modes were obtained from the calculations and compared to the experimental ones. We found that optical phonon modes below 100 cm –1 mainly contribute to its negative thermal expansion, and they also have a significant mixing with acoustic branches. These modes have been assigned to translational motion of the Co–CN–Ag–NC–Co linkages. We also identify a pressure-induced softening of zone-center phonon modes that could account for the experimentally observed phase transition in terms of a first-order displacive instability from the trigonal P3̅1 m space group to the acentric Cm space group.