Crystal chemistry of silica-rich Barium silicates

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Abstract
The crystal structures of the two title compounds have been refined with three-dimensional MoKα X-ray intensities. Results for both are given in the order a, b, c, β, Z, space group, number of independent reflections, R(unweighted), R(weighted): Ba2[Si4O10](l): 7.688(1) Å, 4.629(1) Å, 13.523(1) Å, 90°, 2, Pmcn, 611, 0.041, 0.045. Ba2[Si4O10](h): 23.195(2) Å, 4.658(1) Å, 13.613(1) Å, 97.57(1)°, 6, C2/c, 1579, 0.033, 0.034. Both polymorphs contain chains with two [SiO4] tetrahedra in the repeat unit of approximately 4.6 Å (zweier single chains). These chains are linked via every second tetrahedron to zweier single layers parallel (001). The higher degree of corrugation of the layers in the low-temperature phase compared to those of the high-temperature phase is explained by the smaller effective radius of the barium ions at lower temperature. Barium ion coordination number is 9 in the low-temperature and both 9 and 8 + 2 in the high-temperature form respectively. The barium oxygen polyhedra share edges to form layers parallel (001).

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