Hydrogen insertion and protonic conductivity in lead phosphate Pb3(PO4)2

The influence of hydrogen insertion on the conductivity of lead phosphate Pb3(PO4)2 is reported. Transmission infrared (IR) spectra of single crystals of Pb3(PO4)2 after annealing in water-moistened nitrogen or air show a broad band at 3250 cm−1 and a doublet at 2750–2800 cm−1, which are characteristic of the stretching vibrations of two different types of hydrogen bonding formed on insertion of hydrogen. Annealing in wet nitrogen increases the conductivity of lead phosphate crystals by factor 20–40. The monoclinic phase of lead phosphate, which exists below 453 K displays 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 are similar (Ha=0.64 eV, Hc=0.58 eV). In the trigonal phase above 453 K, the activation enthalpy of Ha=0.24 eV is typical of a superionic conductor. The quasi-two-dimensional character of the dc conductivity of Pb3(PO4)2 is 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 is discussed on the basis of its structural isomorphism with the latter acid salts.