Thermal and spectroscopic characterization of reaction products of Al-nitrate-Cr-carbonate interaction at different temperatures

The chemical interaction at 250 to 1000°C of aluminium nitrate with chromium carbonate was investigated. The decomposition products of these compounds, and their mixtures with molar ratios of $\text{3}\text{1}$, $\text{2}\text{1}$, $\text{1}\text{1}$,$\text{1}\text{2}$and $\text{1}\text{3}$, were characterized by means of chemical analysis, diffuse reflectance and X-ray diffraction.

At temperatures higher than 215°C mixtures with a chromium carbonate/aluminium nitrate ratio greater than one yielded an amorphous aluminium chromate dichromate with an empirical formula Al_2−x-Cr_xO₃Al₂(CrO₄)₂Cr₂O₇together with crystalline Cr₂O₃. Mixtures with a ratio equal to or less than one yielded an amorphous aluminium hydroxy chromate (empirically, AlCrO₃Al(OH)CrO₄) with Cr₂O₃ and γ-alumina.

Both types of chromates were converted into solid solutions of (Al_0.17Cr_0.83)₂O₃, (Al_0.25Cr_0.75)₂O₃, (Al_0.33Cr_0.67)₂O₃ and (Al_0.5Cr_0.5)₂O₃ at temperatures higher than 480°C. Characteristic phenomena of (Al_1−xCr_x)₂O₃ solid solutions were observed, such as their d spacings and shifts of ⁴A_2g → ⁴T_2g and ⁴T_2g → ⁴T_1g absorption bands towards shorter wavelengths.