Shape and size determination for zinc-aluminium-chloride layered double hydroxide crystallites by analysis of X-ray diffraction line broadening

The Rietveld method, extended by a Fourier analysis of line profiles on the basis of the Warren-Averbach method, has been used for analysing the powder X-ray diffraction (PXRD) pattern of a [Zn-Al-Cl] layered double hydroxide, in order to separate size and strain effects. Assuming a given size distribution (Cauchy) and an adjustable strain variation in space, this method allows the simultaneous determination of structural parameters and the size and strain parameters of the sample. The shape, size, size distribution and orientation of the crystallites are determined. It is found that the shape is markedly anisotropic and compatible with plate-like crystallites with a hexagonal symmetry; the structure being described in the <R(3)over bar>m space group. The strain parameter, i.e. the distribution of atomic positions around their equilibrium positions, varies by a factor of 2 while the mean particle size changes from 120 Angstrom to 2200 Angstrom in the [001] and [110] directions, respectively. Such shape and size are in good agreement with the transmission electron microscopy (TEM) observations.