Local distortions in the structure of nanocrystalline gahnite

Gahnite is a zinc-aluminum spinel with stoichiometric formula ZnAl2O4 and compact cubic structure (fcc), belonging to the space group Fd-3m. The zinc ion shows a tetrahedral coordination with four oxygens, while aluminum is placed in the octahedral site: the degree of inversion (i.e. the fraction of trivalent atoms in the tetrahedral sites) is very small and largely temperature independent. A lot of research work has been done to show the different properties of nanospinels (see [1] for example). These technological properties seem to be enhanced as a function of decreasing grain size (see [2] for example). The primary purpose of this work is the structural study of nanocrystalline gahnite and its possible local distortions. Two samples of nanocrystalline gahnite have been synthesised with a hydrothermal method, and then calcined at different temperatures (340°C, 700°C, 800°C, 900°C). The data collected from the high-energy beamline ID15B at the European Synchrotron Radiation Facility in Grenoble (F) were used applying classical crystallography using the Rietveld method (with many limitations), and the analysis of Pair Distribution Function (PDF) in order to explore its structure, both locally and on average. Pair Distribution Function analysis shows a good agreement with the crystallographic structure in the average region; the local part (below 10Å) most influenced by the diffuse scattering, cannot be properly fit with this structure, even though the fit improved with calcination temperature. The local part was studied using two different distortion approaches, to understand the local structure better, but with the simplest possible models (i.e. the one with least refinable parameters.). The APPROACH I identifies three different zones of distortion, with increasing r: this resulted in good values of the Rw parameter, but such a model was difficult to understand from the physical point of view. The APPROACH II was applied by modelling a distortion in the octahedral site, easier to deform than the very stiff tetrahedral site. This again gave very good Rw values (lower than in the other approach), with the advantage of being easy to comprehend. The distortion obtained is similar for the two powders samples, regardless of the synthesis conditions. In addition, the TEM observations, independent from the diffraction data, confirm the good accuracy in the calculation of the diameters of nanoparticles through Pair Distribution Function methodology. The small distortion refined is compatible with the extreme ordering of the particles observed by TEM (lattice planes are clearly visible also in the sample with the grains of 2 nm). This study provides a model that, though approximate and certainly not unique, gives account of the local distortions with good accuracy, with growing similarities with the crystallographic structure as a function of calcination temperature. [1] I. Miron, C. Enache, M. Vasile, I. Grosescu, Phys. Scr. (2012), 149, 014064 [2] T. Sirikajorn, O. Mekasuwandumrong, P. Praserthdam, J.G. Goodwin Jr. and J. Panpranot, Cat.Lett. (2010) 501, 103