Redistribution of cations into tetrahedral and octahedral interstices of the close-packed anion sublattice, crystallite size and microstrains in mechanically activated (ball milled) zinc ferrite have been investigated by X-ray diffraction analysis at room temperature and in the range 600-1000 K. It has been found that mechanical activation of zinc ferrite results in the change of inversion degree as well as in the decrease of both unit cell dimension and oxygen parameter. The mechanically induced contraction of the ferrite crystal lattice is accompanied by the significant decrease of crystallite size and by the increase of microstrains.
Measurements of the temperature dependence of the integral intensities of the diffraction lines (220) and (400) have revealed that in the temperature range 600-1000 K an opposite redistribution of cations in the close-packed anion sublattice takes place than that observed during mechanical activation, i.e. the mechanically induced inversion disappears. The comparison of the temperature dependence of the unit cell dimensions of the non-activated and activated samples have shown that with increasing temperature the mechanically induced lattice compression also disappears. Contrary to the non-activated sample, approximation of the a=a(T) dependence for the mechanically activated sample with a single straight line over the whole temperature range is questionable: in the activated sample, in a narrow temperature range 760-820 K, a sudden increase of the unit cell dimension to values observed for the non-activated sample takes place.
Disappearance of mechanically induced lattice compression is manifested also by a sudden decrease of microstrains in the temperature range about 700-800 K. Above 800 K the mictrostrains decrease slowly and in the considered temperature range no relaxation of microstrains to the zero value takes place. A source of residual microstrains may be seen in the presence of products of partial thermal decomposition in the structure of zinc ferrite. Within 700-800 K the crystallite size doubles quickly to about 30 nm. Above 800 K the size of crystallites remains practically the same and is 50% less than in the non-activated sample.
Thus, the structural metastability of mechanically activated zinc ferrite after heat treatment is connected with its fine-crystallite nature and with the presence of residual microstrains, which are present even after the cations returned completely from the inversion into their equilibrium sites.