Rare earth disilicates R₂Si₂O₇ (R = Gd, Tb, Dy, Ho): type B

Abstract

The type B structure of the single rare earth element disilicates has been revised using room temperature and pressure single-crystal X-ray diffraction measurements on Gd₂Si₂O₇, Tb₂Si₂O₇ and Ho₂Si₂O₇ synthesized at 2–2.5 GPa, 1400–1500 °C in a piston-cylinder apparatus and Dy₂Si₂O₇ synthesized hydrothermally at 0.1 GPa. Crystal data are: triclinic, space group P1̅, Z = 4; Gd₂Si₂O₇ – a = 6.6609(2), b = 6.7081(2), c = 12.1390(4) Å, α = 94.277(1), β = 90.577(1), γ = 91.441(1)°, R = 0.033, and D_x = 5.929 g/cm³; Tb₂Si₂O₇ – a = 6.6331(3), b = 6.6799(2), c = 12.0967(2) Å, α = 94.128(1), β = 90.609(2), γ = 91.541(1)°, R = 0.038, and D_x = 6.041 g/cm³; Dy₂Si₂O₇ – a = 6.6158(2), b = 6.6604(2), c = 12.0551(4) Å, α = 94.373(2), β = 90.836(2), γ = 91.512(2)°, R = 0.031, and D_x = 6.156 g/cm³; and Ho₂Si₂O₇ – a = 6.5960(2), b = 6.6328(3), c = 12.0214(6) Å, α = 94.479(1), β = 90.856(1), γ = 91.749(2)°, R = 0.034, and D_x = 6.313 g/cm³. Substitution of Gd³⁺, Tb³⁺, Dy³⁺, and Ho³⁺ cations into the type B structure results in proportional linear decrease in size of the REEO_n polyhedra and unit-cell parameters, and leaves the silicate stereochemistry essentially unchanged.