Insulator-to-Proton-Conductor Transition in a Dense Metal–Organic Framework

Metal–organic
frameworks (MOFs) are prone to exhibit phase
transitions under stimuli such as changes in pressure, temperature,
or gas sorption because of their flexible and responsive structures.
Here we report that a dense MOF, ((CH3)2NH2)2­[Li2Zr­(C2O4)4], exhibits an abrupt increase in proton conductivity
from <10–9 to 3.9 × 10–5 S/cm at 17 °C (activation energy, 0.64 eV) upon exposure to
humidity. The conductivities were determined using single crystals,
and the structures were analyzed by X-ray diffraction and X-ray pair
distribution function analysis. The initial anhydrous structure transforms
to another dense structure via topotactic hydration (H2O/Zr = 0.5), wherein one-fourth of the Li ions are irreversibly rearranged
and coordinated by water molecules. This structure further transforms
into a third crystalline structure by water uptake (H2O/Zr
= 4.0). The abrupt increase in conductivity is reversible and is associated
with the latter reversible structure transformation. The H2O molecules coordinated to Li ions, which are formed in the first
step of the transformation, are considered to be the proton source,
and the absorbed water molecules, which are formed in the second step,
are considered to be proton carriers.