Deterioration of hydrogen-bonded superprotonic conductors belonging to CsHSO4–CsH2PO4–H2O salt system: a single-crystal neutron diffraction investigation

Single-crystal neutron diffraction investigation on Cs4(HSO4)3(H2PO4) and Cs6H(HSO4)3(H2PO4)4 superprotonic crystals, belonging to CsHSO4–CsH2PO4–H2O salt system, is under taken to elucidate the precise hydrogen atom positions in these crystals. The investigation revealed that these crystals are very sensitive to the ambient conditions and can undergo deterioration due to fluctuation in air moisture content. Cs6H(HSO4)3(H2PO4)4 crystals are more stable as compared to Cs4(HSO4)3(H2PO4). Crystal structure of Cs6H(HSO4)3(H2PO4)4 is obtained both before and after deterioration, it is found that the asymmetric O–HO hydrogen bond between the PO4 and SO4 ions of this crystal becomes stronger after deterioration. This led to the shrinkage of the unit cell, and most likely prevented further deterioration. Diabatic state model for hydrogen bonds is used to obtain the energy contour for the O–H···O hydrogen bond of Cs6H(HSO4)3(H2PO4)4 crystal. The influence of this change in the hydrogen bonding on the proton conduction ability of the crystal is discussed.