Chromium(III) oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In the nature it occurs as the rare mineral eskolaite.
Structure and properties
adopts the corundum
structure, consisting of cubic close packed oxides with 2/3 of the octahedral holes occupied by chromium. It is antiferromagnetic
up to 307 K, the Neel temperature. It not readily attacked by acids or bases, although molten alkali gives chromites
The Parisians Pannetier and Binet first prepared Cr2
in 1838 via a secret process. It is derived from the mineral chromite
. The conversion of chromite to chromia proceeds via Na2Cr2O7
, which is reduced with sulfur at high temperatures:
- Na2Cr2O7 + S → Na2SO4 + Cr2O3
Chromium oxide can be converted into elemental chromium metal through a thermite-like reaction: unlike iron oxide thermites, chromium oxide thermites creates few or no sparks, smoke or sound, but glow brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.
Because of its considerable stability, chromia is commonly used pigment and was originally called viridian
. It is used in paints, inks, and glasses. It is the colourant in "chrome green" and "institutional green." Chromium(III) oxide is the precursor to the magnetic pigment chromium dioxide, according to the following reaction:
- Cr2O3 + 3 CrO3 → 5 CrO2 + O2