Hydrogen insertion and protonic conductivity in lead phosphate Pb-3(PO4)(2)

The influence of hydrogen insertion on the conductivity of lead phosphate P b-3(PO4)(2) is reported, Transmission infrared (IR) spectra of single cryst als of Pb-3(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 c haracteristic of the stretching vibrations of two different types of hydrog en bonding formed on insertion of hydrogen. Annealing in wet nitrogen incre ases the conductivity of lead phosphate crystals by factor 20-40. The monoc linic phase of lead phosphate, which exists below 453 K displays a strong a nisotropy of the conductivity with that in the a-direction being a factor 1 00 greater than that in the c-direction, although the values of activation enthalpies are similar (H-u = 0.64 eV, H-c = 0.58 eV). In the trigonal phas e above 453 K, the activation enthalpy of H-a = 0.24 eV is typical of a sup erionic conductor, The quasi-two-dimensional character of the de conductivi ty of Pb-3(PO4)(2) is similar to that observed in well-known proton conduct ors of the M3H(AO(4))(2) family where M = K, Rb, NH4, Cs; A = S, Se. The me chanism of proton transport in lead phosphate is discussed on the basis of its structural isomorphism with the latter acid salts. (C) 2001 Elsevier Sc ience B.V. All rights reserved.