Correlation between crystallite sizes and microstrains in TiO2 nanopowders

Nanopowders of TiO2 doped with by V, Nb, Ta were obtained by laser-induced pyrolysis. The profiles of the diffraction peaks were Fourier analysed with a single-peak method and the microstructural parameters [M] (average crystallite size) and [epsilon(2)](1/2) (mean-square root microstrain) were estimated. A linear correlation between [epsilon(2)](1/2)[M] and 1/[M] was observed in both anatase and rutile phases. For a given crystallite size, the microstrains as well as the dispersion of data were larger in anatase then in rutile. Indeed, one of the factors inducing the anatase-to-rutile transformation may be the presence of larger microstrains in anatase. In rutile, the high correlation found between [epsilon(2)](1/2)[M] and 1/[M] suggests that in this phase the microstrains present in the structure are due only to the geometrical constraints of the surface. Indeed, for infinite crystallite size no microstrains are expected. In anatase, microstrains depend also on the specific cation present in the crystallite. (C) 1999 Elsevier Science B.V. All rights reserved.