Negative linear compressibility in common materials

Abstract

Negative linear compressibility (NLC) is still considered an exotic property, only observed in a few obscure crystals. The vast majority of materials compress axially in all directions when loaded in hydrostatic compression. However a few materials have been observed which expand in one or two directions under hydrostatic compression. At present, the list of materials demonstrating this unusual behaviour is confined to a small number of relatively rare crystal phases, biological materials and designed structures, and the lack of widespread availability hinders promising technological applications. Using improved representations of elastic properties, this study revisits existing databases of elastic constants and identifies several crystals missed by previous reviews. More importantly, several common materials –drawn polymers, certain types of paper and wood, and carbon fibre laminates– are found to display NLC. We show that NLC in these materials originates from the misalignment of polymers/fibres. Using a beam model we propose that maximum NLC is obtained for misalignment of 26º. The existence of such widely available materials increases significantly the prospects for applications of NLC.