Origin of the colossal positive and negative thermal expansion in Ag${}_3$[Co(CN)${}_6$]: an {\it ab initio} Density Functional Theory study

DFT calculations have been used to provide insights into the origin of the colossal positive and negative thermal expansion in Ag${}_3$[Co(CN)${}_6]$. The results confirm that the positive expansion within the trigonal basal plane and the negative expansion in the orthogonal direction are coupled due to the existence of a network defined by nearly-rigid bonds within the chains of Co–C–N–Ag–N–C–Co linkages. The origin of the colossal values of the coefficients of thermal expansion arise from an extremely shallow energy surface that allows a flexing of the structure with small energy cost. The thermal expansion can be achieved with a modest value of the overall Gr"{u}neisen parameter. The energy surface is so shallow that we need to incorporate a small empirical dispersive interaction to give ground-state lattice parameters that match experimental values at low temperature. We compare the results with DFT calculations on two isostructural systems: H${}_3$[Co(CN)${}_6$], which is known to have much smaller values of the coefficients of thermal expansion, and Au${}_3$[Co(CN)${}_6$], which has not yet been synthesised but which is predicted by our calculations to be another candidate material for showing colossal positive and negative thermal expansion.