The crystal structure and luminescence properties of europium(II) haloborates.
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概要
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The crystal structures of Eu<SUB>2</SUB>B<SUB>5</SUB>O<SUB>9</SUB>X (X=Cl and Br) were determined by the method of X-ray diffraction analysis and refined by the least-square method to give <I>R</I>=0.054 for 1967 observed reflections of Eu<SUB>2</SUB>B<SUB>5</SUB>O<SUB>9</SUB>Cl and <I>R</I>=0.047 for 1979 observed reflections of Eu<SUB>2</SUB>B<SUB>5</SUB>O<SUB>9</SUB>Br. These haloborates are isostructural and belong to the orthorhombic (pseudotetragonal) system, with the <I>Pnn</I>2 space group and with four formula units in a cell of those dimensions: <I>a</I>=11.364(3), <I>b</I>=11.301(3), and <I>c</I>=6.504(2) Å for Eu<SUB>2</SUB>B<SUB>5</SUB>O<SUB>9</SUB>Cl, and <I>a</I>=11.503(3), <I>b</I>=11.382(3), and <I>c</I>=6.484(2) Å for Eu<SUB>2</SUB>B<SUB>5</SUB>O<SUB>9</SUB>Br. The structure consists of a three-dimensional (B<SUB>5</SUB>O<SUB>9</SUB>)<SUB>∞</SUB> network, the B<SUB>5</SUB>O<SUB>12</SUB> groups of three BO<SUB>4</SUB> tetrahedra and two BO<SUB>3</SUB> triangles being linked together with one another. The Eu and X atoms are located alternately in tunnels of the (B<SUB>5</SUB>O<SUB>9</SUB>)<SUB>∞</SUB> 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 Eu<SUP>2+</SUP>-activated phosphors of alkaline earth analogs give emissions with a high quantum efficiency.
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