Silver hexacyanocobaltate, Ag3[Co(CN)6], is a covalent framework material that exhibits “colossal” positive and negative thermal expansion. This means that, when heated, its framework lattice expands along some directions at a rate that is about ten times greater than the normal behaviour for solid materials. At the same time the framework actually shrinks along another set of directions, and does so just as quickly.

In fact, the uniaxial negative thermal expansion (NTE) effect is about 14 times stronger than in ZrW2O8, offering an effective means of countering the usual positive thermal expansion effects of “normal” engineering materials.

Its behaviour under pressure is just as bizarre. Using hydrostatic conditions, where the same force is applied equally in all directions, increasing pressure causes the framework to reduce its volume by actually expanding along the NTE directions as it is compressed.

Colossal positive and negative thermal expansion in the framework material Ag3[Co(CN)6]. Science 319, 794 (2008)

Large negative linear compressibility of Ag3[Co(CN)6]. Proc Natl Acad Sci USA 105, 18708 (2008)

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