Rare earth disilicates R 2Si2O7 (R = Gd, Tb, Dy, Ho): type B

Abstract

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 Dx = 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 Dx = 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 Dx = 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 Dx = 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 REEOn polyhedra and unit-cell parameters, and leaves the silicate stereochemistry essentially unchanged.