Control over crystal, local atomic and electronic structures of cerium chromates/chromites via the synthesis conditions

Abstract

The influence exerted by specific synthesis protocol conditions on the crystal, local atomic and electronic structures of various types of Ce chromates/chromites prepared by the coprecipitation is studied using synchrotron X-ray diffraction, X-ray absorption fine structure (XAFS), Raman and Fourier transform infrared (FT-IR) spectroscopies, and simultaneous thermal analysis. Cerium chromate heptahydrate [Ce2+3(Cr+6O4)3(H2O)5]⋅2H2O with the monoclinic structure (sp. gr. P21/c) has been synthesized and characterized structurally for the first time.It has been established that calcination of all types of precursors in air at a temperature ≥ 450 °C leads to the formation of a mixture of CeO2 and Cr2O3 phases. The calcination in vacuum at 1200 °C affords CeCrO3 (with the orthorhombic structure, sp. gr. Pnma(62)), which upon repeated heating to above 650–700 °C in air tends to decompose into CeO2 and Cr2O3 phases. Long-range XRD data correlate well with the results of techniques sensitive to the local structure parameters, such as XAFS, Raman and FT-IR. XANES spectra measured at the Cr K- and Ce L3- edges strongly imply that the synthesis procedures are accompanied by concerted redox transformations Cr6+→ Cr3＋ and Ce3+⟷ Ce4+.