Orthorhombic phase of nickel bromine boracite Ni3B7O13Br: Room temperature ferroelectric–ferroelastic crystal structure

Nickel bromine boracite Ni3B7O13Br is a fully coupled ferroelectric–ferroelastic crystal with a Curie temperature of 398 K that crystallizes in the orthorhombic system with space group Pca21 and four formula units per unit cell. The lattice constants at 298 K are a = 8.5218±0.0002, b = 8.5127±0.0002, and c = 12.0408±0.0002 A. The integrated intensities of 11 682 reflections were measured with an Enraf‐Nonius CAD‐4 diffractometer using monochromatic Mo Kα radiation and a small single‐domain crystal. The crystal structure was refined by the method of least squares, using the coordinates of isomorphous Mg3B7O13Cl as the initial model, with 1531 symmetry‐independent Fmeas. The final agreement factor is 0.0492, based on anisotropic thermal vibrations for all atoms. Six independent boron atoms form tetrahedra with nearest‐neighbor oxygen atoms at an average B–O distance of 1.478 A. The remaining B(3) is 0.140 A from a plane formed by three oxygen atoms, wth average B–O = 1.378 A, and is 2.198 A from a fourth oxygen. Each Ni has four O neighbors at 2.024 A, one Br at 2.667 A, and a more distant Br at 3.370 A, on average. Two ferroelectric–ferroelastic tranformation mechanisms are applicable, each fully coupled. In the first, the sense of the spontaneous polarization is reversed as the spontaneous strain is reoriented: it has a magnitude calculated to be about 0.9×10−2 C m−2, based on a point charge model. The largest atomic displacement in this transformation is 0.72 A for a Ni2+ ion: All other displacements are less than 0.5 A. In the second mechanism, the spontaneous polarization is rotated through 90° as the spontaneous strain is also reoriented: it has a calculated magnitude of 2.6×10−2 C m−2. The largest corresponding atomic displacement is 0.60 A for a Ni2+ ion. As‐grown crystals could be only partially detwinned in an electric or a stress field. Ni3B7O13Br is closely isomorphous with Mg3B7O13Cl, the largest difference between coordinates of corresponding atoms being only 0.175 A, with an average difference of 0.055 A.