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

Dysprosium disilicate (Dy(2)Si(2)O(7)) is triclinic with a = 6.6158 (2), b = 6.6604 (2), c = 12.0551 (4) A, alpha = 94.373 (2), beta = 90.836 (2), gamma = 91.512 (2) degrees, V = 529.4 (1) A(3), space group P1;, 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 [Si(3)O(10)] and isolated [SiO(4)] tetrahedron cross-linked by Dy(3+) in one sixfold and three eightfold coordinated positions, and corresponds to the presently revised type B structure of Ho(2)Si(2)O(7). 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 REE(3+)O(n) (REE = rare earth element) polyhedra in REE disilicates through the 4f transition metal series. The crystal of Dy(2)Si(2)O(7) 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 A, beta = 91.788 degrees ), 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.