Compositional dependence of anomalous thermal expansion in perovskite-like ABX3 formates

Abstract

The compositional dependence of thermal expansion behaviour in 19 different perovskite-like metal–organic frameworks (MOFs) of composition [A^I][M^II(HCOO)₃] (A = alkylammonium cation; M = octahedrally-coordinated divalent metal) is studied using variable-temperature X-ray powder diffraction measurements. While all systems show essentially the same type of thermomechanical response—irrespective of their particular structural details—the magnitude of this response is shown to be a function of A^I and M^II cation radii, as well as the molecular anisotropy of A^I. Flexibility is maximised for large M^II and small A^I, while the shape of A^I has implications for the direction of framework hingeing.