12018-01-8

dark brown or black powder(s); tetr; used in magnetic tapes [KIR78]; ferromagnetic; rutile structure [MER06]

In magnetic recording tapes; as catalyst.

Chromium dioxide [12018-01-8], chromium(IV)oxide, CrO2, is a ferromagnetic material with a specific saturation magnetization Ms/? of 132 A m2 kg?1 at 0 K, corresponding to the spin of two unpaired electrons per Cr4+ ion. TheMs/n value of CrO2 at room temperature is ca. 100 A m2 kg?1; CrO2 magnetic pigments reach values of 77–92 A m2 kg?1. The material crystallizes with a tetragonal rutile lattice in the form of small needles, which have the desired magnetic shape anisotropy. The morphology of the particles can be varied with several dopants, particularly antimony and tellurium. The coercive field strength can be controlled between ca. 30 and 75 kA m?1 (in addition to shape) by doping with transition metal ions, which modify the magneto-crystalline anisotropy of the material; the Fe3+ ion being industrially important.

The conversion of an intimate mixture of Cr(III) and Cr(VI) compounds into CrO2 under hydrothermal conditions has been developed into an industrial process in autoclaves at ca. 350 °C and 300 bar.
Pure CrO2 slowly disproportionates in the presence of water. The CrO2 crystal surface of commercial pigments is therefore topotactically converted to â-CrOOH, which serves as a protection layer. In the absence of moisture, CrO2 is stable up to ca. 400 °C, above this temperature it decomposes to form Cr2O3 and oxygen.