Spectroscopic study of spinel ZnCr2O4 obtained from mechanically activated ZnO–Cr2O3 mixtures

A mixture of starting ZnO and Cr2O3 powders in equimolar quantities was mechanically activated by grinding using a high energy vibro-mill for 0, 40 and 80 min. The spinel ZnCr2O4 was prepared from activated mixtures by a conventional solid-state reaction at the temperature of 900 °C during 240 min. Raman scattering (RS) and Far-infrared (IR) spectroscopy were applied to study the local structure of zinc chromium oxide spinels. We report on an analysis of the vibrational spectra of the spinel ZnCr2O4 structure using both the classical factor-group theory (Oh7 spectroscopic symmetry) and a local environmental model. The structural modifications were studied on the basis of vibrations of ZnO4 tetrahedral and CrO6 octahedral units building the crystal lattice. The RS and IR line intensities and positions remained in good agreement with the Fd3m space group. The broadness of IR and Raman bands and the fact that more vibration modes than expected were observed, may be attributed to certain disorder in the crystal symmetry of cubic spinel ZnCr2O4. Formation of the defect spinel structure can be assigned to mechanical activation of the starting oxide mixture.