Ca-10((CrO4)-O-V)(6)((CrO4)-O-VI), a disordered mixed-valence chromium compound exhibiting inversion twinning

The structure analysis of so-called 9CaO center dot 4CrO(3)center dot Cr2O3 proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The (CrO4)-O-VI tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the (CrO4)-O-V tetrahedra are in general positions and are ordered. The charge is balanced by Ca2+ cations, one of which is located on a threefold axis. The Ca2+ ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO-CrOx system and its formation reduces the refractoriness of calcium-rich compositions in an oxidizing atmosphere