Well shaped crystals of Cr2(SO4)3 and Al2(SO4)3 could be prepared by chemical transport reactions using HCl (Cr2(SO4)3) or SOCl2 (Al2(SO4)3) as transport agent. Cr2(SO4)3 and Al2(SO4)3 are isostructural to the rhombohedral modification of Fe2(SO4)3 and crystallize in the trigonal space group R[unk] with a = 8.1245(7) (8.0246(4)) Å and c = 21.944(3) (21.357(1)) Å, Z = 6 (hexagonal setting). The crystal structures were refined to a residual R = 0.035 using 828 unique reflections with |F|≥3 σ|F| for Cr2(SO4)3 and to R = 0.026 (772 unique reflections with |F|≥3 σ|F|) for Al2(SO4)3.
Main building units of the two compounds are MO6-octahedra and SO4−-tetrahedra which are linked via common corners to a 3-dimensional network.
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