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

Dysprosium disilicate (Dy2Si2O7) is triclinic with a = 6.6158 (2), b = 6.6604 (2), c = 12.0551 (4) Angstrom, alpha = 94.373 (2), beta = 90.836 (2), gamma = 91.512 (2)degrees, V = 529.4 (1) Angstrom (3), space group P (1) over bar, 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 Angstrom, beta = 91.788 degrees), which resulted in an apparent superstructure of the type B structure in space group C (1) over bar . 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.