Negative and Zero Linear Compressibility in MCN (M = Ag, Au, Cu): A First-Principles Study

Abstract

Negative linear compressibility (NLC) is the counterintuitive phenomenon in which a crystallographic axis expands under hydrostatic pressure. A related phenomenon, zero linear compressibility (ZLC), occurs when an axis shows no length change under pressure. Both responses are rare and arise in materials with highly anisotropic mechanical properties. Motivated by recent reports of anomalous behavior in metal cyanides - including negative thermal expansion and NLC - we use first-principles calculations to investigate the potential of the MCN family (M = Ag, Au, Cu) to exhibit ZLC or NLC. Our simulations reveal three main findings. First, we predict the existence of previously unreported phases: the P6mm phase for AgCN and CuCN, and the R3m phase for AuCN. Second, all six members of the MCN family studied exhibit extreme elastic anisotropy, which indeed leads to ZLC or NLC in each case. Third, we identify a mechanism for these responses distinct from previously reported ones. The mechanism arises from the unique "bamboo forest" geometry of the material: weakly interacting rigid rods whose sparse packing allows pressure to be accommodated through changes in packing density rather than rod compression. This mechanism enables the anomalous response to persist over a wide pressure range, contrasting favorably with other materials that exhibit similar behavior. Our findings expand the relatively small family of materials known to exhibit ZLC or NLC and provide deeper insight into the microscopic origin of these unusual mechanical responses.