Probing Structural Changes in a Phosphonate-based Metal–Organic Framework Exhibiting Reversible Dehydration

A one-step hydrothermal
synthesis with small amines and 1,3,5-benzenetriphosphonic
acid was used to prepare single crystals of isostructural anionic
metal–organic frameworks (MOF): Zn2.5(H)0.4–0.5(C6H3O9P3)­(H2O)1.9–2(NH4)0.5–0.6 and Zn2.5(H)0.75(C6H3O9P3)­(H2O)2(CH3NH3)0.25. The ammonium ions are exchangeable
with lithium ions. The MOF exhibits reversible dehydration, and the
process was studied by two complementary methods: solid state NMR
and in situ X-ray diffraction. These experiments revealed three different
phases. The crystal structures of all phases have been determined,
showing loss in volume of the structure due to a phase change. The
ammonium ions remain in the structure and are forced to occupy the
larger pores due to a reduction in free volume. The change in positions
of the guest molecules in the framework has an effect on the potential
conductivity properties of the materials. Changes in framework and
guest molecules due to negative expansion have an effect on other
physical and chemical properties and need to be explored.