The crystal structure of pentapotassium enneakaidekaborate, 5K2O.19B2O3

Potassium borate glasses of the tetraborate composition, K20.4B203, may be crystallized to give a compound of the composition K20.38B203 or 5K20.19B203. This phase is monoclinic, space group C2/c, with unit-cell dimensions at 22 °C: a = 17.888 _+ 0.002, b = 11.479 _+_ 0-001, c = 12-973 _+ 0-002 A, fl = 95.52 +_ 0.01 °. The calculated density is 2-247 g cm -a with two formula units of 5K20.19B203 in the cell. The structure was determined by direct methods from three-dimensional X-ray data taken with Mo Ke radiation. A full-matrix least-squares refinement resulted in an R value of 0-030 (0-026 for the weighted R value). The borate anion polymer in this structure forms a three-dimensional framework consisting of interconnected pentaborate groups, triborate groups, BO4 tetrahedra and BOa triangles. Boron-oxygen bond lengths (standard deviation 0.002 A) are normal and with the usual differences depending on the location within the groups. The structure has three crystallographically different potassium atoms. The closest potassium-oxygen approach is 2681 A.