Just as we expect materials to expand on increasing temperature, so too is it our intuition that materials should shrink under pressure. The opposite effect — expansion when pressure is applied — is termed negative linear compressibility (NLC) and has only recently been observed experimentally to any significant extent. The material in question, Ag3[Co(CN)6], behaves much like a collapsible wine-rack in that its volume reduces by actually expanding strongly in one direction. But despite having magnitude, NLC in Ag3[Co(CN)6] does not have range: the framework collapses to a new phase at just 0.19 GPa.
So what can be done, and does it even matter?
By including well ordered stacks of monovalent cations within the same basic framework structure, we have now shown how to prevent shear-induced collapse whilst still retaining the structural motif responsible for strong NLC. K+ ions can sit within the gaps of the structure and stop framework collapse — a tactic akin to placing wine bottles into the molecular “wine-rack” to stop it folding up! The result is the material KMn[Ag(CN)2]3, which we have found to exhibit the most extreme persistent NLC effect ever discovered (extending beyond 2 GPa).
Any potential applications of NLC — from transcontinental fibre optic cables junctions to bullet proof armour — require materials that perform over as large a pressure range as possible. Consequently, KMn[Ag(CN)2]3 and its structural relatives are now the first realistic candidates to exploit this unusual phenomenon in next-generation engineering materials.