Structure of the fully ferroelectric/fully ferroelastic orthohombic room-temperature phase of cobalt bromine boracite, Co3B7O13Br and nickel chlorine boracite, Ni3B7O13Cl

The X-ray crystal structures of optically controlled single-domain crystals of fully ferroelectric/fully ferroelastic cobalt bromine boracite, Co3B7O13Br (Co-Br) at 298 K [Mr = 540.38, orthorhombic, Pca21, a = 8.5614 (2), b = 8.5657 (2), c = 12.1196 (3) A, V = 888.78 (4) A3, Z = 4, Dx = 4.04 Mg m-3, λ(Mo Kα) = 0.7107 A, μ = 10.61 mm-1, F(000) = 1020, R = 7.0, wR = 5.4%, 2824 reflections] and of nickel chlorine boracite, Ni3B7O13Cl (Ni-Cl) at 298 K [Mr = 495.25, orthorhombic, Pca21, a = 8.5105 (4), b = 8.4984 (4), c = 12.0324 (5) A, V = 870.25 (7) A3, Z = 4, Dx = 3.78 Mg m-3, λ(Mo Kα) = 0.7107 A, μ = 6.8 mm-1, F(000) = 960, R = 3.5, wR = 3.1%, 2082 reflections] are reported. The metal surroundings of Co-Br and Ni-Cl were analyzed in detail and show two metal sites (Co2, Co3; Ni2, Ni3) with chemically similar environments and one metal site (Co1; Ni1) with a different environment. Six B atoms have a tetrahedral or slightly distorted tetrahedral coordination, whereas one B atom (B4) has triangular surroundings in both compounds. (orig.)