INTRA SITE CATION ORDERING AND CLUSTERING IN NATURAL Mn-Zn FERRITES

Cooling rates of 10 -3 K/yr. permit naturally occurring Mn-Zn ferrites (franklinltes) to be in thermodynamic equilibrium at ambient temperatures with respect to both electrostatic and elastic energies. The elastic strain energy resulting from the occupancy of the A sites by ions of differing sizes is minimized by clustering of.Mn and Zn in different regions of a D/ crystal. Using Fe NGR, these regions have been observed directly in franklinite with low Mn 3+ contents and their structures elucidated. The Mn rich region has a local crystal chemistry very similar to that of MnFe204 and the Zn rich region has a local structure similar to that of ZnFe204. The two regions are crystallographically coherent. Laboratory heat treatments destroy the Mn-Zn ordering and alter the magnetic properties. The Fe spin configurations are also observed to be collinear. The discovery of the intrasite ordering in these materials proves that Goodenough's criteria for square B-H loops in ferrites can be realized. For samples containing Mn 3+, the effects of the Jabn-Teller distortions do not permit a direct observation of Mn-Zn intrasite ordering.