Acta Crystallographica Section B

Structural Science

Volume 56, Part 6 (December 2000)

research papers

Acta Cryst. (2000). B56, 940-946 [ doi:10.1107/S0108768100008545 ]

Structure and complex twinning of dysprosium disilicate (Dy₂Si₂O₇), Type B

M. E. Fleet and X. Liu

Abstract: Dysprosium disilicate (Dy₂Si₂O₇) is triclinic with a = 6.6158 (2), b = 6.6604 (2), c = 12.0551 (4) Å, [alpha] = 94.373 (2), [beta] = 90.836 (2), [gamma] = 91.512 (2)°, V = 529.4 (1) Å³, space group P $\overline 1$ , Z = 4 and D_x = 6.156 g cm^-3. The structure (single-crystal X-ray, R = 0.033, wR = 0.041) is built from a linear triple tetrahedral group [Si₃O₁₀] and isolated [SiO₄] tetrahedron cross-linked by Dy³⁺ in one sixfold and three eightfold coordinated positions, and corresponds to the presently revised type B structure of Ho₂Si₂O₇. The formation of the unusual linear triple tetrahedral group in the type B structure allows for a more continuous transition in the mean size of REE³⁺O_n (REE = rare earth element) polyhedra in REE disilicates through the 4f transition metal series. The crystal of Dy₂Si₂O₇ investigated was complexly twinned such that the diffraction pattern was also consistent with a larger dimensionally monoclinic unit cell (a = 22.5354, b = 14.2102, c = 6.6158 Å, [beta] = 91.788°), which resulted in an apparent superstructure of the type B structure in space group C1. Lattice coincidence with the type B unit cell appears to have been maintained during crystal synthesis and quenching by the complex sector-zoned growth twin.

Keywords: Twinning; Dysprosium disilicate.