The Crystal Structure of the High Pressure Phase CaB204(III)

Calcium metaborate can crystallize with four different structures by varying the pressure at which the synthesis is carried out. Single crystals of CaB204(III) were grown at 20 kbar and 900 °. They were orthorhombic with a = 11.380 + 0-002, b = 6.382 + 0.002, c = 11.304 + 0.002/~, Z= 12, dc= 3.052 g.cm -3, space group symmetry Pna21, and all atoms in general positions. The structure of CaB204(III) has been solved by three-dimensional Patterson and Fourier syntheses. The positional and anisotropic thermal parameters have been refined by the least-squares method based on 2118 structure factors. The final conventional R index is 0-039. The structure contains units [B6012] -6 consisting of four boronoxygen tetrahedra and two boron-oxygen triangles. These units are linked together to form a threedimensional network. The mean bond distances are B-40 = 1.480/~ and B-30 = 1.373/~. There are three types of calcium atoms. Ca(I) and Ca(2) are each surrounded by an 8-oxygen polyhedron, Ca(3) is surrounded by a 10-oxygen polyhedron. The average Ca-O distances are Ca(1)-O= 2-530/~, Ca(2)-O = 2.443/~ and Ca(3)-O = 2.638/~.