A new compound was found recently in the glass forming region of K
2O-Al
2O
3-B
2O
3 three component system. EPMA and chemical analyses showed that this crystal is K
3AlB
8O
15. 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/cm
3, 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 C
52h-
P2
1/
c. The true space group seems to be of lower symmetry, C
2s-
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
P2
1/
c, respectively. As very similar structures were obtained, the space group
P2
1/
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 AlO
4 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 AlO
4. The average B-O bond lengths for the BO
4 tetrahedra and the BO
3 triangles are normal, and 1.474 and 1.364Å, respectively. The AlO
4 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-Na
2O⋅3B
2O
3 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.
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