COLOSSAL NEGATIVE THERMAL EXPANSION AND NEGATIVE LINEAR COMPRESSIBILITY IN Ag3Co(CN)6

   

Figure showing an exaggerated view of thermal expansion within Ag3Co(CN)6 between the low-temperature and high-temperature material (left and right-hand figures, respectively).  Cobalt octahedra (red) are linked together via a cyanide-silver-cyanide chain with the silver ions forming argentophilic bonds perpendicular to the c-axis at low temperature. .

 

Ag3Co(CN)6 possesses anomalously large and anisotropic thermal expansion with negative and positive  linear expansivities of magnitudes greater than 100MK-1 parallel and perpendicular to the c-axis, respectively.  This is due to the breaking of weak argentophilic bonding within silver ion layers perpendicular to the c-axis as temperature is increased; the size of these layers increases significantly and the hinged structure induces a corresponding spectacular collapse along the c-axis [1-3].  Since the overall volume is lowest at low temperature application of hydrostatic pressure 'drives' this process in reverse giving rise to an increase in size along the c-axis and a large negative linear compressibility [4].  Materials with a similar structure but without the silver-ion network do not produce such a large effect; H3Co(CN)6 for example displays much more modest mechanical behaviour [3,5].

 

 

  

References 

 

1. Colossal positive and negative thermal expansion in the framework material Ag3[Co(CN)6] A L Goodwin, M Calleja, M J Conterio, M T Dove, J S O Evans, D A Keen, L Peters and M G Tucker Science (2008) 319 794-797
2. Local structure in Ag3[Co(CN)6]: Colossal thermal expansion, rigid unit modes and argentophilic interactions

M J Conterio, A L Goodwin, M G Tucker, D A Keen, M T Dove, L Peters and J S O Evans

J. Phys.: Condensed Matter (2008) 20 255225
3. Argentophilicity-dependent colossal thermal expansion in extended Prussian blue analogues A L Goodwin, D A Keen, M G Tucker, M T Dove, L Peters and J S O Evans J. Am. Chem. Soc. (2008) 130 9660-9661
4. Large negative linear compressibility of Ag3[Co(CN)6] A L Goodwin, D A Keen and M G Tucker Proc. Natl. Acad. Sci. (USA) (2008) 105 18708-13
5. Hydrogen-bonding transition and isotope-dependent negative thermal expansion in H3Co(CN)6 D A Keen, M T Dove, J S O Evans, A L Goodwin, L Peters and M G Tucker J. Phys. Condensed Matter (2010) 22 404202

 

 

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