INFRARED SPECTROMETRIC INVESTIGATION OF SUBMICRON METASTABLE CATION-DEFICIENT SPINELS IN RELATION TO ORDER-DISORDER PHENOMENA AND PHASE-TRANSITION

A large variety of cation-deficient spinels with a variable vacancy content and resulting from oxidation at low temperature of mixed-valency submicron ferrites has been investigated by infrared spectrometry. The comparison of spectra of defect ferrites substituted by unoxidizable (K+, Zn2+, Co2+, Cr3+, Al3+, Ti4+) Or oxidizable cations (Mn2+, Mo3+) with the spectrum of ordered, well-crystallized gamma-Fe2O3 indicates the presence of an order of 1:5 between cation and vacancy on octahedral sites for a vacancy/cation ratio varying between 0.17 and 0.24. Increasing incorporation of foreign cations in the maghemite structure destroys or maintains the vacancy ordering, depending not only on the vacancy content but also on their location and on the nature of substituted cation. So, when chromium in Cr-defect phases is able to occupy the totality of octahedral sites and the vacancies of constant content go to tetrahedral sites, the large number of absorption bands suggests a cation vacancy order of 1/2 on these sites. Different degrees of octahedral orders (1:5, 1:3 and 1:2) between Fe3+ ions and vacancies can be observed in Mo-defect phases due to the possibility for Mo6+ ions to occupy the tetrahedral sites. This ordering is destroyed for partially reduced samples which have molybdenum in octahedral coordination. Vibrational spectrometry also provides valuable and complementary information on the precipitation of cr-phases of corundum structure resulting from a structural change in these gamma-defect phases as the effect of temperature. From considerations about the intensity variations of certain absorption bands for divalent defect spinels or the nearly linear shift of a specific band for solid solutions of trivalent oxides, a transition temperature (gamma --> alpha) or an unknown composition can be determined. Concerning the phase transformation in Al- and Mo-defect phases, the IR study gives the evidence of continuous solid solutions and intermediate transformation stages with aluminum and brings structural information on valency and coordination of molybdenum in the spinel lattice.