Electrochemical and spectroscopic characterisation of barium acid salts of 3,5-disulfophenylphosphonic acid

Barium salts of 3,5-disulfophenylphosphonic acid (H4Y) having formulae Ba2Y, Ba1.8H0.4Y, BaKHY and BaH2Y have been prepared and characterised by X-ray powder diffraction, thermogravimetry, IR spectroscopy, ac conductivity and e.m.f. measurements. Ba2Y is orthorhombic, with a = 8.126, b = 4.691, c = 8.649 Å; Ba1.8H0.4Y and BaKHY have similar cell parameters to these. BaH2Y is also orthorhombic, but with all cell parameters close to double those of the former salts. The anhydrous salts are stable in air up to at least 300°C. The ac conductivity (σ) has been measured at 100°C and controlled relative humidity (RH) in the range 50-80%; at 70% RH, σ increases from ca. 10-8 up to ca. 10-2 S cm-1 according to the sequence: Ba2Y < Ba1.8H0.4Y < BaKHY < BaH2Y. Electromotive force measurements have shown K+ and H+ to be the main charge carriers in BaKHY and BaH2Y, respectively. In both salts protons are localized on the phosphonate group at 53% RH, but IR spectroscopy provides evidence for a degree of proton transfer to water molecules in BaH2Y at 75% RH.