SHORT CR3-CR3+ DISTANCES IN MAGNETOPLUMBITE TYPE SRCR9GA3O19()

The crystal structure of SrCr9Ga3O19 has been refined in space group P 6(3)/mmc with an R-value of 0.043 (R(w) = 0.038). The lattice paramete rs are a = 5.7960(3) Angstrom and c = 22.658(2) Angstrom, Z = 2. Galli um is located on tetrahedral and trigonal bipyramidal sites of the mag netoplumbite structure type. All chromium ions occupy octahedral sites . In pairs efface-sharing octahedra the interatomic Cr3+-Cr3+ distance of 2.634(4) Angstrom is comparable to that in corundum type Cr2O3 wit h Cr3+-Cr3+ = 2.650(1) Angstrom. These short metal-metal separations s uggest cation-cation interactions in both oxides. The displacement of Ga3+ from the center of the adjacent trigonal bipyramid is reduced fro m 0.16 Angstrom in SrGa12O19 to 0.13 Angstrom in SrCr9Ga3O19.