Chromium aluminum oxides
Chromium aluminum oxides are inorganic compounds made up of aluminum oxide (Al 2 O 3 ) and varying amounts of chromium (III) oxide (Cr 2 O 3 ). The most famous chrome aluminum is the ruby , of the corundum - Minerals counts. Artificial rubies were included in the first lasers .
Structural considerations
Since the ion and atomic radii of aluminum and chromium are of similar size, the formation of mixed crystals can be expected for a mixed system of Al 2 O 3 and Cr 2 O 3 . Due to the small differences in the radii and the different bond character of the metal-oxygen bonds, the mixed crystals are structurally distorted depending on their composition and there is even a temperature-dependent miscibility gap that only disappears at 800 ° C.
Well-known types
- Mixed crystals with a minimal Cr 2 O 3 content ( doped aluminum oxide ) are reddish-transparent with a corundum structure and are used as artificial rubies in lasers .
- Mixed crystals of α-aluminum oxide with a chromium (III) oxide content from one to less than nine mol% are homogeneous and show a red color. Ceramic- like chromium aluminum oxide of this composition used as a coating material for cutting tools with the PVD process is pink in color and has properties similar to pure aluminum oxide. These thermally stable mixed crystals are produced by melting together α-Al 2 O 3 with α-Cr 2 O 3 .
- Mixed crystals of α-Al 2 O 3 with exactly nine mol% Cr 2 O 3 form a cracked, reddish-colored material which, in comparison to corundum, turns out to be a clearly distorted crystal in the X-ray structure analysis.