LAUSR

Abstract Despite huge potential, Al2O3–CaO–Cr2O3 system has been one of the least investigated one due to the generation of carcinogenic and toxic Cr(VI) compounds. Herein, we investigated the system under air atmosphere varying Cr2O3 while keeping Al2O3:CaO ratio constant in order to identify the Cr(VI) dominant region, eventually to avoid it. The Ca4Al6CrVIO16 phase predominantly formed in the air atmosphere with Cr2O3 content up to ∼12 mol%. However, an unprecedented Cr(III) phase appeared under air at higher Cr2O3 content (26.43 mol%). We then synthesized the new polycrystalline ternary Cr(III) compound (CaAl2Cr2O7) at 1500 °C under air atmosphere for the first time. A trigonal symmetry of hexagonal crystal family with space group P3 (143), lattice parameters a = b = 7.7909 A and c = 7.6506 A were determined from the X-ray powder diffraction pattern study. Electron microscope studies revealed uniform hexagonal microcrystals with similar lattice parameters. Most significantly, the binding energies of 586.1 and 576.2 eV for Cr2p1/2 and Cr2p3/2 respectively implied the +3 oxidation state of Cr in this compound.