The crystal structure of Cr₈O₂₁ determined from powder diffraction data: Thermal transformation and magnetic properties of a chromium-chromate-tetrachromate

Thermal decomposition of CrO3 was utilized to prepare a powder sample of the chromium oxide usually designated Cr3O8. Combined information from powder diffraction data using synchrotron, conventional X-ray, and neutron radiation allowed determination of the structure. The structure is triclinic (a = 5.433(1), b = 6.557(1), c = 12.117(2) Å, α = 106.36(1), β = 95.73(1) and γ = 77.96(1)°) and was refined in the space group P1. The true composition of the compound is Cr8O21. There are two distinct types of chromium atoms in the structure, which may be designated the oxidation numbers (III) and (VI), respectively. The structure is built from pairs of edge-sharing Cr(III)O6 octahedra linked together by Cr(VI)O4 tetrahedra to form sheets. The sheets are then linked together by tetrachromate groups (Cr(VI)4O13) to form a three-dimensional structure. Thus, the chromium oxide may be described as Cr(III)2(Cr(VI)O4)2(Cr(VI)4O13). The magnetic properties of Cr8O21 were investigated in the temperature range 5 to 300 K. Above 100 K the compound is paramagnetic. Magnetic susceptibility data indicate a transition to antiferromagnetism around 100 K, but only vague indications for additional magnetic reflections were found with neutron powder diffraction.