Oxidation of Cr (III) to Cr (VI) in the Synthesis of Bismuth-Chromium Oxide Ceramics

Bismuth-containing chromates, Bi_xCrO_1.5x+3 , were synthesized using a ceramic method from bismuth (III) and chromium (III) oxides, with a variable molar ratio of 1 ≤ n (Bi)/n (Cr) ≤ 38. The calcined samples exhibit a color variation from green to dark red, depending on the n (Bi)/n (Cr) ratio. The ceramic materials undergo a color change from green to red when a mixture of oxides with a significant predominance of bismuth oxide n (Bi₂O₃)/n (Cr₂O₃) ≥ 3 is subjected to a high-temperature treatment in air (at 650°C). It is noteworthy that the calcination of chromium (III) oxide under similar conditions occurs without the oxidation of chromium ions. X-ray diffraction analysis confirmed the formation of chromates, including Bi₆Cr₂O₁₅, Bi₁₀Cr₂O₂₁, Bi₃₁Cr₅O_61.5, and Bi₁₄CrO₂₄ . X-ray spectroscopy studies of the samples revealed that the NEXAFS Cr2p spectra of the red bismuth-chromium ceramics exhibited similarities in the main details of the K₂CrO₄ spectrum, indicating the presence of chromium in the oxide ceramics in the form of tetrahedral \({\text{CrO}}_{4}^{2-}\) ${\text{CrO}}_4^{2-}$ ions. In addition, scanning electron microscopy data demonstrated that the samples exhibited a dense, low-porosity microstructure.