A new rare earth disilicate (REE2Si2O7; REE = Dy, Tm, Lu; type-L): Evidence for non-quenchable 10 GPa polymorph with silicon in fivefold trigonal bipyramidal coordination?

摘要

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 P, Z = 6; Dy₂Si₂O₇: a = 6.5971(3), b = 6.6504(2), c = 18.0582(6) Å, = 83.791(2), ß = 88.653(2), = 88.498(2)°, V = 787.2 Å3, and D_x = 6.242 g/cm3; Tm₂Si₂O₇: a = 6.5499(2), b = 6.5876(2), c = 17.8916(7) Å, = 83.828(1), ß = 88.368(1), = 88.152(2)°, V = 766.9 Å3, and D_x = 6.574 g/cm3; Lu₂Si₂O₇: a = 6.5240(2), b = 6.5553(2), c = 17.7909(6) Å, = 83.977(2), ß = 88.074(2), = 87.846(2)°, V = 755.8 Å3 , and D_x = 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.