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

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

2008-03-04

arxiv.org/abs/0803.0480v1

Journal of Physics: Condensed Matter 20, 255226 (2008)

Physics

Condensed Matter

Materials Science

7 figures, format for Journal of Physics: Condensed Matter

Scientific paper

10.1088/0953-8984/20/25/255226

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.

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