The Ag...Ag dispersive interaction and exotic physical properties of Ag3Co(CN)6

We report a density functional theory (DFT) study of Ag3Co(CN)6, a material noted for its colossal positive and negative thermal expansion, and its giant negative linear compressibility. Here we explicitly include the dispersive interaction within the DFT calculation, and find that it is essential to reproduce the ground state, the high-pressure phase, and the phonons of this material; and hence essential to understand this material's remarkable physical properties. New exotic properties are predicted. These include heat enhancement of the negative linear compressibility, a large reduction in the coefficient of thermal expansion on compression with change of sign of the mode Grueneisen parameters under pressure, and large softening of the material on heating. Our results suggest that these are associated with the weak Ag...Ag dispersive interactions acting with an efficient hinging mechanism in the framework structure.