Electric-quadrupole interactions in mixed spinels.

Abstract

The electric quadrupole interactions at the octahedral and tetrahedral sites in the ${\mathrm{NiFe}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{O}}_{4}$ (1.5\ensuremath{\le}x2) spinel system has been studied with $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy. It is found that the analysis of a paramagnetic zero-field spectrum in terms of distribution of quadrupole splittings, caused by the randomization of ions at both sites, gives parameters which explain the shape of an in-field spectrum, whereas the parameters derived from a commonly used two doublet analysis do not. The electric-field-gradient (EFG) calculations at the octahedral site with use of a monopole-point-dipole model, together with an overlap contribution to the EFG, indicate that the EFG sign is positive. The in-field spectrum for the composition x=1.9 demonstrates unambiguously a small weighting to negative sign of the total EFG distributions at both sites. It is proved experimentally, and explained in terms of relaxation effects, that to derive unequivocal information about the sign of the EFG distribution, the in-field spectra should be measured far above the N\'eel temperature. The similarities between the quadrupole interactions in mixed ferric spinels and amorphous compounds are interpreted as evidence for a significant short-range order in the latter.