Electron paramagnetic resonance and optical absorption studies on Cr-doped mullite precursors

Abstract

Mullite precursors doped with 3 wt% Cr 2O 3 were investigated with X-ray diffractometry (XRD), electron paramagnetic resonance (EPR) and optical absorption spectroscopy. The development of phase assemblages occurs over three temperature ranges. In the first temperature field (450–600 °C) the precursors are amorphous, and in the second field (800–1100 °C) small amounts of γ-Al 2O 3 and crystalline Cr 2O 3 can be observed. Finally, in the third field (≥ 1250 °C) mullite forms, and simultaneously γ-Al 2O 3 and crystalline Cr 2O 3 disappear. The EPR measurements reveal different temperature-controlled structural types of short-range order of Cr in the precursors, which go along with phase developments. Between 450 and 800 °C EPR spectra appear with a main signal at g eff ≈ 1·96, most probably resulting from Cr 5+. Between 800 and 1250 °C, the Cr EPR signal is similar to that of Cr-containing glasses, and above 1250 °C the typical Cr 3+ EPR spectrum of Cr-doped mullite appears. Optical absorption spectroscopy yields evidence for the occurrence of Cr 6+, Cr 5+ and Cr 3+ in the mullite precursors, the concentrations of the different oxidation states being dependent on the calcination temperature: Cr 6+ and Cr 5+ contents are high at low temperatures (450 °C), but gradually decrease with temperature. Simultaneously, increasing amounts of Cr 3+ can be detected. At temperatures > 1100 °C, Cr 6+ and Cr 5+ cations completely disappear and optical absorption and EPR spectra of samples gradually approach to those of Cr-doped mullite. Although starting materials consist of Cr 3+, slow hydrolysis kinetics of Cr 3+ causes formation of polyanions which contain Cr 6+. Deprotonation on heating results in reduction of [Cr 2 6+O 7] 2− species to form Cr 2 3+O 3.