The Crystal Structure and Luminescence Properties of Europium(II) Haloborates

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Abstract
The crystal structures of Eu2B5O9X (X=Cl and Br) were determined by the method of X-ray diffraction analysis and refined by the least-square method to give R=0.054 for 1967 observed reflections of Eu2B5O9Cl and R=0.047 for 1979 observed reflections of Eu2B5O9Br. These haloborates are isostructural and belong to the orthorhombic (pseudotetragonal) system, with the Pnn2 space group and with four formula units in a cell of those dimensions: a=11.364(3), b=11.301(3), and c=6.504(2) Å for Eu2B5O9Cl, and a=11.503(3), b=11.382(3), and c=6.484(2) Å for Eu2B5O9Br. The structure consists of a three-dimensional (B5O9)∞ network, the B5O12 groups of three BO4 tetrahedra and two BO3 triangles being linked together with one another. The Eu and X atoms are located alternately in tunnels of the (B5O9)∞ network extending along the c axis. Each Eu atom is surrounded by two X atoms and seven O atoms, and is isolated from the neighboring Eu atoms attributable to borate units and X atoms. This arrangement of anions around Eu atoms relates to the fact that the Eu2+-activated phosphors of alkaline earth analogs give emissions with a high quantum efficiency.

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