Acoustic phonons and negative thermal expansion in MOF-5.

The mechanism of negative thermal expansion (NTE) in MOF-5 has been studied using a rigid unit mode analysis of the phonons responsible for NTE. In addition to confirming the role of the previously proposed optic 'trampoline mode' mechanism [Lock et al., Dalton Trans., 2013, 42, 1933-2320], we have identified a new acoustic mode mechanism that makes a major contribution to the macroscopic NTE of this framework.