A low-frequency internal friction peak was observed at 200C in bar specimens of sintered material which had been annealed at temperatures of between 400 and 650C. The peak was explained in terms of diffusional jumps, of O atoms in the basal plane, between the sub-lattice sites, OA(½,0,0) and OB(0,½,0). These 2 sites were not crystallographically equivalent in the orthorhombic symmetry. The O diffusivity for planar migration in the basal planes was deduced on the basis of a 1-dimensional random-walk process. This yielded:

D = ¼d2[CB/(CA + CB)]vo exp[-HB/kT] = Doexp[-HB/kT]

where HB = HA + dE. The parameters, CA, CB, vo and HA, were obtained from internal friction data for specimens with a known O deficiency. The potential energy difference, dE, between OA and OB sites was found from the Boltzmann distribution law,

CA/CB = exp[-dE/kT]

It was found that the pre-exponential factor, Do, of 0.00035cm2/s and the 200C internal friction peak activation energy, HA, of 1.03eV were both temperature-insensitive. On the other hand, dE was temperature-dependent. That is, it was equal to 0.23eV at 400C and decreased as the temperature increased. It vanished at 670C, which was the temperature of the orthorhombic to tetragonal transition temperature; where OA and OB sites became indistinguishable. A non-Arrhenius relationship existed between O diffusivity and temperature.

X.M.Xie, T.G.Chen, Z.L.Wu: Physical Review B, 1989, 40[7], 4549-56