SIMILARITY OF THE SURFACE STRUCTURE OF THE NANOSTRUCTURED MECHANOSYNTHESIZED AND OF MECHANICALLY ACTIVATED ZINC FERRITE

V. Šepelák¹*, P. Druska², U. Steinike² and K.D. Becker³

¹Institute of Geotechnics of the Slovak Academy of Sciences, 04353 Košice, Slovakia ²Institute of Applied Chemistry Berlin-Adlershof, 12484 Berlin, Germany
³Institute of Physical and Theoretical Chemistry, Technical University of Braunschweig, 38106 Braunschweig, Germany

Zinc ferrite prepared by a conventional ceramic method forms the structure of a normal spinel with zinc in the tetrahedral (4) sites and iron in the octahedral (6) sites of a cubic close packing of oxygen atoms Zn⁽⁴⁾Fe⁽⁶⁾₂O₄. New mechanochemical method of zinc ferrite synthesis (mechanosynthesis) from the mixture of zinc oxide and iron oxide has received increased attention in recent years, which offers the possibility of forming of the nanoscale spinel structure [1-3].

The changes in the surface structure of mechanosynthesized zinc ferrite (synthesized by the high-energy milling of the ZnO/Fe₂O₃ mixture) and of mechanically activated zinc ferrite (synthesized by the conventional thermal method, followed by mechanical treatment) as a function of the mechanical treatment time as well as the thermal stability of the mechanically induced surface structural disorder in a temperature range of 800 to 1100 K have been studied using the X-ray photoelectron spectroscopy.

It has been found that, from the qualitative point of view, the surface structure of the nanoscale mechanosynthesized zinc ferrite is similar to that of the mechanoactivated zinc ferrite. The main feature of the surface structural disorder of both mechanoactivated and mechanosynthesized zinc ferrites is the mechanically induced inversion, i.e. a substantial displacement of Fe³⁺ cations to tetrahedral sites and of Zn²⁺ cations into octahedral sites of the cubic close-packed anionic sublattice. In both cases, the inversion degree increases with increasing milling time. At temperatures over 800 K the structural metastability of the surface of mechanically treated zinc ferrite is manifested by a gradual disappearance of the mechanically induced inversion. The re-equilibration terminates by a complete relaxation of the disordered surface structure.

This work was supported by the German Federal Ministry of Education, Science, Research and Technology and the Berlin Senate Department for Science, Research and Culture (project No 03C3005). The authors are indebted to the DFG (project Ste 692/2-1) and to the Alexander von Humboldt Foundation for financial support of one of them (V.Š.) during his stays at the Institute of Applied Chemistry in Berlin.

1. V. Šepelák, A.Yu. Rogachev, U. Steinike, D.-Chr. Uecker, S. Wißmann & K.D. Becker, Acta Cryst., Suppl. Issue, A 52 (1996) C-367.

2. V. Šepelák, A.Yu. Rogachev, U. Steinike, D.-Chr. Uecker, F. Krumeich, S. Wißmann & K.D. Becker, Materials Science Forum, 235-238 (1997) 139-144.

3. V. Šepelák, U. Steinike, D.-Chr. Uecker, S. Wißmann & K.D. Becker, J. Solid State Chem., in press.

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* At present time - a Humboldt Fellow at the Institute of Physical and Theoretical Chemistry, Technical University of Braunschweig