PASCal: a principal axis strain calculator for thermal expansion and compressibility determination

This article describes a web‐based tool (PASCal; principal axis strain calculator; http://pascal.chem.ox.ac.uk) designed to simplify the determination of principal coefficients of thermal expansion and compressibilities from variable‐temperature and variable‐pressure lattice parameter data. In a series of three case studies, PASCal is used to reanalyse previously published lattice parameter data and show that additional scientific insight is obtainable in each case. First, the two‐dimensional metal–organic framework [Cu2(OH)(C8H3O7S)(H2O)]·2H2O is found to exhibit the strongest area negative thermal expansion (NTE) effect yet observed; second, the widely used explosive HMX exhibits much stronger mechanical anisotropy than had previously been anticipated, including uniaxial NTE driven by thermal changes in molecular conformation; and third, the high‐pressure form of the mineral malayaite is shown to exhibit a strong negative linear compressibility effect that arises from correlated tilting of SnO6 and SiO4 coordination polyhedra.