The influence of H insertion on the conductivity of lead phosphate was reported. The transmission infra-red spectra of single crystals, after annealing in water-moistened N or air, showed a broad band at 3250/cm and a doublet at 2750 to 2800/cm, which were characteristic of the stretching vibrations of 2 different types of H bonding formed on insertion of H. Annealing in wet N increased the conductivity of lead phosphate crystals by a factor of 20 to 40. The monoclinic phase of lead phosphate, which exists below 453K displayed a strong anisotropy of the conductivity with that in the a-direction being a factor 100 greater than that in the c-direction, although the values of activation enthalpies were similar (Ha = 0.64eV, Hc = 0.58eV). In the trigonal phase above 453K, the activation enthalpy of Ha = 0.24eV was typical of a superionic conductor. The quasi-two-dimensional character of the direct-current conductivity of Pb3(PO4)2 was similar to that observed in well-known proton conductors of the M3H(AO4)2 family where M = K, Rb, NH4, Cs; A = S, Se. The mechanism of proton transport in lead phosphate was explained on the basis of its structural isomorphism with the latter acid salts.

Hydrogen Insertion and Protonic Conductivity in Lead Phosphate Pb3(PO4)2. A.I.Baranov, V.M.Duda, D.J.Jones, J.Rozière, V.V.Sinitsyn, R.C.T.Slade: Solid State Ionics, 2001, 145[1-4], 241-7