Decomposition of Higher Oxides of Chromium Under Various Pressures of Oxygen

The formation of Cr 3 O 8 , Cr 2 O 5 , CrO 2 , and Cr 2 O 3 by the decomposition of anhydrous CrO 3 under a high oxygen pressure and their physical and chemical properties are presented. The crystal structure of CrO 2 is a rutile type and its lattice constants do not vary with the ratio O/Cr. The specific gravity of CrO 2 decreases with an increase in the ratio O/Cr. This is explained by assuming that the crystal structure of CrO 2 contains an excess of Cr ions at interstitial sites. This crystal model is supported by the low electrical resistivity of CrO 2 . An electron microscopic examination showed an abnormal grain growth of CrO 2 during the decomposition of Cr 2 O 5 to CrO 2 . The activation energies of the decomposition of Cr 2 O 5 and CrO 2 to Cr 2 O 3 in air are 61.7 and 48.2 kcal. per mole, respectively. The pressure‐temperature diagram of various chromium oxides has been determined. From the pressure‐temperature curve of CrO 2 ‐Cr 2 O 3 the heat of decomposition is found to be 26.3 kcal. per mole.