Abstract
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. Griineisen 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 trigona1P-3-1m space group to the acentric Cm space group.