ELECTRON-PARAMAGNETIC-RESONANCE AND OPTICAL-ABSORPTION STUDIES ON CR-DOPED MULLITE PRECURSORS

Mullite precursors doped with 3 wt% Cr2O3 were investigated with X-ray diffractometry (XRD), electron paramagnetic resonance (EPR) and optic al absorption spectroscopy. The development of phase assemblages occur s over three temperature ranges. In the first temperature field (450-6 00 degrees C) the precursors are amorphous, and in the second field (8 00-1100 degrees C) small amounts of gamma-Al2O3 and crystalline Cr2O3 can be observed. Finally, in the third field (greater than or equal to 1250 degrees C) mullite forms, and simultaneously gamma-Al2O3 and cry stalline Cr2O3 disappear. The EPR measurements reveal different temper ature-controlled structural types of short-range order of Cr in the pr ecursors, which go along with phase developments. Between 450 and 800 degrees C EPR spectra appear with a main signal at g(eff) approximate to 1.96, most probably resulting from Cr5+. Between 800 and 1250 degre es C, the Cr EPR signal is similar to that of Cr-containing glasses, a nd above 1250 degrees C the typical Cr3+ EPR spectrum of Cr-doped mull ite appears. Optical absorption spectroscopy yields evidence for the o ccurrence of Cr6+, Cr5+ and Cr3+ in the mullite precursors, the concen trations of the different oxidation states being dependent on the calc ination temperature: Cr6+ and Cr5+ contents are high at low temperatur es. (450 degrees C), but gradually decrease with temperature. Simultan eously, increasing amounts of Cr3+ can be detected. Ar temperatures > 1100 degrees C, Cr6+ and Cr5+ cations completely disappear and optical absorption and EPR spectra of samples gradually approach to those of Cr-doped mullite. Although starting materials consist of Cr3+, slow hy drolysis kinetics of Cr3+ causes formation of polyanions which contain Cr6+. Deprotonation on heating results in reduction of [Cr26+O7](2-) species to form Cr23+O3.