Synthesis and properties of novel inorganic red chromophore based on Cr4+ in tetrahedral coordination in Bi24Al2-xCrxO39

The novel orange to red color Bi 24 Al 2- x Cr x O 39 (0≤ x  ≤ 0.5) compounds were successfully synthesized by the solid state method. With the increase of Cr content, the colors of compounds are tuned from orange red to dark red. The X-ray diffraction patterns of all samples are indexed with cubic Bi 24 Al 2 O 39 structure with the space group of I 23. The oxidation state of Cr in the oxide is confirmed to be +4 valence by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Based on the analysis of Cr oxidation state and UV–Visible spectrum, the red hue is due to the d-d transition of Cr 4+ in tetrahedral sites. The as-synthesized red color compounds also show excellent stability in acid and base environments.