Structural complexity of lead silicates: Crystal structure of Pb21[Si7O22]2[Si4O13] and its comparison to hyttsjoite

• DOI: 10.2138/am.2014.4723
• Publication Date: 2014-04-01T11:47:32Z
• Authors: O. I. Siidra; D. S. Zenko; S. V. Krivovichev
• Keywords: Tetrahedron; Lone pair

The crystal structure of Pb21[Si7O22]2[Si4O13] has been solved on crystals grown by crystallization from melt. compound is hexagonal, P63/m, a = 9.9244(5), c 34.2357(16) Å, V 795.28(6) Å3, R1 0.042 for 3361 unique observed reflections. contains five symmetrically independent Si sites tetrahedrally coordinated O atoms. Si1O4, Si3O4, and Si4O4 tetrahedra share corners to form branched heptameric [Si7O22]16- units, whereas the Si2O4 Si5O4 tetrameric [Si4O13]10- anions. six Pb with PbOn coordination polyhedra distorted due stereochemical activity lone electron pairs. can be described as stacking layers two types, A B. A-type layer Pb1, Pb2, Pb3, Pb4 sites, B-type anions, together Pb5, Pb6, Pb6A sites. Stacking sequence ...AA′BAA′B..., where A′ denote opposite orientations tripod-shaped silicate heptamers. many similarities that hyttsjöite, which same consisting In both title anions are stacked in such way ellipsoidal cavities dimensions ca. 10 × 6 Å3 created. occupied ClPb6 octahedra hyttsjöite "empty" Pb6 Pb21[Si7O22]2[Si4O13]. Analysis structural chemical complexity PbO-SiO2 system indicates most chemically complex phases (in terms relations between components) appear point view well. phase structurally system. Structural organization crystalline controlled Pb:Si ratio. For <2, their structures contain Pb2+ ions "additional" atoms, i.e., atoms not bonded Si. These OPb4 tetrahedra, next strongest subunits after <2 therefore based upon polynuclear cationic units edge- corner-sharing tetrahedra.