PROTON CONDUCTIVITY OF ZIRCONIUM CARBOXY NORMAL-ALKYL PHOSPHONATES WITH AN ALPHA-LAYERED STRUCTURE

Alpha-layered zirconium carboxy n-alkyl phosphonates, Zr(O3P(CH2)nCOOH)2 with n = 1-5 have been prepared and characterized by X-ray diffraction, TG-DTA, DSC and ac-conductivity measurements. The interlayer distance of the phosphonates increases almost linearly with increasing n from 11.1 to 18.7 angstrom. Compounds with n = 2, 4, 5 undergo a reversible phase transition at 165, 95 and 215-degrees-C, associated with a weak endothermic effect and with a discontinous increase of the interlayer distance of 1.0, 1.3 and 0.8 angstrom respectively. Structural considerations indicated a different arrangement of the terminal-COOH groups in the interlayer region as a consequence of the odd or even number of methylene groups in the alkyl chain. ac-conductivity measurements, performed on pressed pellets by the impedance method, in fact showed that compounds with n = 2 and 4 have a conductivity much lower than that of compounds with n = 1, 3, 5. The conductivity of these latter is 2 x 10(-9), 1.6 x 10(-8) and 1.4 x 10(-7) S cm-1 at 200-degrees-C, with activation energies of 77, 104 and 132 kJ mol-1 respectively. Finally the high frequency limit capacitance of the pellets made up of compounds with n = 2 and 4 was found to change significantly at the phase transition temperature indicating a change in dielectric properties of these compounds.