  The Raman spectra of single crystals of NiFe2O4 were studied in various
scattering configurations in close comparison with the corresponding spectra of
Ni0.7Zn0.3Fe2O4 and Fe3O4. The number of experimentally observed Raman modes
exceeds significantly that expected for a normal spinel structure and the
polarization properties of most of the Raman lines provide evidence for a
microscopic symmetry lower than that given by the Fd-3m space group. We argue
that the experimental results can be explained by considering the short range
1:1 ordering of Ni2+ and Fe3+ at the B-sites of inverse spinel structure, most
probably of tetragonal P4_122/P4_322 symmetry.
