CaAl2Cr2O7: Formation, synthesis, and characterization of a new Cr(III) compound under air atmosphere in the Al2O3–CaO–Cr2O3 system

Despite huge potential, Al 2 O 3 –CaO–Cr 2 O 3 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 Cr 2 O 3 while keeping Al 2 O 3 :CaO ratio constant in order to identify the Cr(VI) dominant region, eventually to avoid it. The Ca 4 Al 6 Cr VI O 16 phase predominantly formed in the air atmosphere with Cr 2 O 3 content up to ∼12 mol%. However, an unprecedented Cr(III) phase appeared under air at higher Cr 2 O 3 content (26.43 mol%). We then synthesized the new polycrystalline ternary Cr(III) compound (CaAl 2 Cr 2 O 7 ) 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 Å and c = 7.6506 Å 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 Cr2p 1/2 and Cr2p 3/2 respectively implied the +3 oxidation state of Cr in this compound.