Crystal structure of copper polysilicate, Cu[SiO3]

For 40 years the copper polysilicate Cu[SiO3], isotypic with the germanate prototype Cu[GeO3], was considered non-existent. Recently however we discov ered this compound as a metastable decomposition product of the silicate mi neral dioptase, CU6[Si6O18] . 6 H2O. Its orthorhombic unit-cell was refined from Guinier data giving a = 4.6357(6) Angstrom, b = 8.7735(11) Angstrom a nd c = 2.8334(4) Angstrom.Using powder diffraction data, the crystal struct ure was determined in the space group Pbmm with Z = 2 formula units, d(x)= 4.024 g cm(-3). The reliability factors after least-squares refinement are R-F = 0.035 and R-W(2) = 0.047 The crystal structure consists of einer sing le chains of silicate tetrahedra running along [001]. These chains are conn ected by chains of edge-sharing CuO4+2 'octahedra' with two more distant ox ygen neighbours, a result of the Jahn-Teller effect. The interatomic distan ces were found to be (2x) 1.582(7) A and (2x) 1.640(7)Angstrom forSi-O; (4x ) 1.941(4) Angstrom and (2x) 2.926(7) Angstrom for Cu-O. Using this data, t he cationic bond valence sums were calculated in accordance with the valenc ies and found to be 3.96 for Si4+, respectively 2.00 for Cu2+. The valence angles of about 119. 5 degrees obtained between siliconatoms and bridging o xygen atoms (ideally 109.47 degrees for the tetrahedral O-Si-O angles and a bout 139 degrees for the Si-O-br-Si one) are evidently the topological limi ts for condensed silicate anions. Being a quasi-onedimensional antiferromag netic material, the prototype Cu[GeO3] belongs to a few inorganic compounds that exhibit a Spin-Peierls transition. It is expected that Cu[SiO3] will also show magnetic properties of considerable interest.