Acta Montanistica Slovaca Ročník 2 (1997), 3, 266-272 Mechanically induced structural disordering in spinel ferrites Vladimír Šepelák 1 and Klára Tkáčová 1 Mechanicky indukované štruktúrne neusporiadanie vo feritoch - spineloch V práci je študovaný charakter mechanicky indukovaného štruktúrneho neusporiadania a metastability feritov so spinelovou štruktúrou. Vysvetlený je pôvod mechanicky indukovaných zmien magnetických vlastností študovaných látok. Kľúčové slová: mechanická aktivácia, spinel, ferit, katiónová redistribúcia, mechanicky indukované neusporiadanie. Introduction Over the past 20 years there has been a fairly extensive amount of work directed toward investigation of mechanically induced metastable states formation in substances with close packed structure units. Among the most important contributions made in the 1980s by Russian scientists, the influence of the original structure on the nature of mechanically induced structural disordering in ferrites should be mentioned: if large cations are present in the system (e.g. in CdFe 2 O 4 , LaFeO 3 , DyFeO 3 , BiFeO 3 ), the translation invariance completely disappears and amorphisation takes place. A characteristic feature of spinels containing relatively small cations (Zn, Ni, Co, Mg) is the possibility of preserving the long-range order at stressing (Boldyrev, 1994; Pavljukhin, 1987; Pavljukhin et al., 1984 a 1988; Rykov et al., 1989; Yermakov et al., 1982). A study of mechanically activated spinel ferrites has revealed that during energy-intensive milling they are transformed into an activated state whose magnetic properties differ essentially from those of the initial polycrystalline ferrites (Pavljukhin, 1987; Pavljukhin et al., 1984; Rykov et al, 1989). A model of mechanically induced structural disordering in both normal and inverse spinel ferrites has been proposed on the basis of Mössbauer spectroscopy measurements and calculations of radial distribution functions of atom densities (Pavljukhin et al., 1982-1988; Rykov et al., 1989; Boldyrev, 1986). According to this model, in the course of mechanical activation of these substances, all the cations that initially occupied tetrahedral (A) sites move into octahedral [B] positions due to the shear deformation of the close-packed anion layers in the <111> direction. Thus, after the transition into a final activated state only the octa-sublattice remains filled, and the decisive factor determining the modified magnetic properties of mechanically activated spinel ferrites is the interaction of the octa/cation-anion-octa/cation. Further attempts to explain the origin of mechanically induced changes in magnetic properties of a normal spinel ferrite have been based on analogy with a mechanically induced inversion (Rykov et al., 1989; Yermakov et al., 1982; Yermakov, 1991). However, the results of previous work do not sufficiently confirm the model of mechanically activated spinel ferrites based on a random distribution of all cations in octahedral sites. In addition, the causes of a total transition of cations into octahedral positions are also unclear, since the shear deformation of the close-packed anion layers in the <111> direction allows also a reverse transition of cations (Rykov et al., 1989; Petrakovskii, 1981). It is evident from the discussion of the contradictory views that the character of mechanically induced disordering in spinel ferrites is not yet satisfactorily explained. The present paper summarizes the results of the contract cooperation between the Institute of Solid State Chemistry of the Russian Academy of Sciences (Novosibirsk) and the Institute of Geotechnics of the Slovak Academy of Sciences (Košice) aimed at the study of the formation of metastable states in mechanically activated spinel ferrites. 1 RNDr. Vladimír Šepelák, CSc. a Prof. RNDr. Klára Tkáčová, DrSc. Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 04353 Košice, Slovakia, Phone/Fax: +421-95-6323402, E-mail address: vlsep@Linux1.saske.sk (Recenzovali: Dr. Ing. Milan Škorbian, CSc. a RNDr. Peter Baláž, DrSc. Revidovaná verzia doručená 16.10.1997) 266