Abstract

A new structure type (L) is reported for disilicates of the middle and heavy rare earth elements (REE) quenched from 10 GPa, 1600–1700 °C. Crystal data are: triclinic, space group P1̅, Z = 6; Dy2Si2O7: a = 6.5971(3), b = 6.6504(2), c = 18.0582(6) Å, α = 83.791(2), β = 88.653(2), γ = 88.498(2)°, V = 787.2 Å3, and Dx = 6.242 g/cm3; Tm2Si2O7: a = 6.5499(2), b = 6.5876(2), c = 17.8916(7) Å, α = 83.828(1), β = 88.368(1), γ = 88.152(2)°, V = 766.9 Å3, and Dx = 6.574 g/cm3; Lu2Si2O7: a = 6.5240(2), b = 6.5553(2), c = 17.7909(6) Å, α = 83.977(2), β = 88.074(2), γ = 87.846(2)°, V = 755.8 Å3 , and Dx = 6.830 g/cm3. The type-L structure comprises linear trisilicate [Si3O10]8− and orthosilicate [SiO4]4− ions cross linked by REE3+ in one sevenfold and five eightfold coordinated positions (to 3.0 Å), and is assembled from alternating (001) strips of the type-B structure of REE2Si2O7 and a sheared structure containing structural elements found in the type-B structure but with Si distributed with 50% occupancy over two face-sharing tetrahedra. The geometry of these half-occupied tetrahedra is consistent with decomposition of a SiO5 trigonal bipyramid during quenching of the pressure.

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