Cubic structure of chromium-bromine boracite at 298 and 113 K

Cr3B7O13Br, Mr=519.55, cubic, Fanti 43c, Z=8, λ(Mo Kα)=0.71073 A, F(000)=1968. At 298 K: a=12.1524(1) A, V=1794.68(3) A3, Dx=3.845 Mg m-3, μ=8.533 mm-1, final R=0.020 for 206 unique reflections with I>3σ(I); shortest interatomic distances (A): Cr-O=2.061(2), Cr-Br=3.03810(3), B-O=1.436(3), O-O=2.388(2). At 113 K: a=12.1340(3) A, V=1786.54(13) A3, Dx=3.863 Mg m-3, μ=8.533 mm-1, final R=0.025 for 281 unique reflections with I>3σ(I); shortest interatomic distances (A): Cr-O=2.057(2), Cr-Br=3.0335(1), B-O=1.436(3), O-O=2.390(2). The planarity of the metal-centred CrO4 groups does not differ significantly from that in other chromium-based cubic boracites and generally increases as the temperature is lowered. (orig.)