Interfacial Water Molecules as Agents for Phase Change Control and Proton Conductivity Enhancement in the Ammonium...

This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P4₁2₁2), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D₂O instead of H₂O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.