The Crystal Structure of Tripotassium Aluminate Octaborate, K₃AlB₈O₁₅

Abstract

A new compound was found recently in the glass forming region of K₂O-Al₂O₃-B₂O₃ three component system. EPMA and chemical analyses showed that this crystal is K₃AlB₈O₁₅. Single crystals up to 0.1mm were grown by the slow cooling method from a stoichiometric melt in a Pt cruicible. Unit cell dimensions and standard errors are a=10.107±0.005Å, b=11.485±0.005Å, c=12.760±0.005Å and β=91.54±0.02°. The observed and the calculated densities are 2.10 and 2.11g/cm³, respectively. The formula units in the cell is Z=4. The crystal was weakly hygroscopic and covered with collodion film. Weissenberg photographs were obtained using Cu K_α radiation for the zero to the 2nd layer on the rotation around the b-axis and for the zero layer on the rotation around the c-axis. Intensity data were collected at 3670 reciprocal lattice points with a four-circle single crystal automatic diffractometer by using Mo K_α radiation. The systematic extinctions for the reciprocal lattice points closely obey the requirements for the space group C⁵_2h-P2₁/c. The true space group seems to be of lower symmetry, C²_s-Pc, however. So the structure was analysed for the both space groups. The structures were determined by the direct methods, using the MULTAN 78 program system. The structures were refined by the method of least-squares using a block diagonal matrix program. Atomic scattering factors corrected for dispersion were used. The structure refined to R values of 0.050 and 0.051 for space groups Pc and P2₁/c, respectively. As very similar structures were obtained, the space group P2₁/c was used for a description of the structure. The aluminoborate polyanion in this crystal is built up from three different groups, viz. the double ring pentaborate group, the single ring triborate group and a single AlO₄ tetrahedron. The groups are mutually connected by sharing of oxygen atoms to an infinite two dimensional double layer, which runs almost perpendicular to the a-axis of the crystal. In this layer, upper and lower anion networks are connected by a bridge between pentaborate groups and AlO₄. The average B-O bond lengths for the BO₄ tetrahedra and the BO₃ triangles are normal, and 1.474 and 1.364Å, respectively. The AlO₄ tetrahedra are distorted, and one Al-O bond is longer than the other three Al-O bonds. The coordinating O atoms do not surround the K atoms in a symmetrical fashion. Anionic networks of oxygen polyhedron have similar configuration as β-Na₂O⋅3B₂O₃ crystal, except for the replacement of B by Al at the tetrahedral sites. In alkaliborate glasses, introduced Al atoms often occupy tetrahedral sites just as in this crystal.