Origin of the colossal positive and negative thermal expansion in Ag3[Co(CN)6]: an 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 Ag3[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 Grüneisen 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: H3[Co(CN)6], which is known to have much smaller values of the coefficients of thermal expansion, and Au3[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. PACS numbers: 61.43.Fs, 62.20.Dc, 63.50.+x