Crystal structure of copper polysilicate, Cu[SiO3]

Abstract

Abstract For 40 years the copper polysilicate Cu[SiO3], isotypic with the germanate prototype Cu[GeO3], was considered non-existent. Recently however we discovered this compound as a metastable decomposition product of the silicate mineral dioptase, Cu6[Si6O18] · 6 H2O. Its orthorhombic unit-cell was refined from Guinier data giving a = 4.6357(6) Å, b = 8.7735(11) Å and c = 2.8334(4) Å. Using powder diffraction data, the crystal structure was determined in the space group Pbmm with Z = 2 formula units, dx = 4.024 g· cm-3. The reliability factors after least-squares refinement are RF = 0.035 and wR 2 = 0.047. The crystal structure consists of einer single chains of silicate tetrahedra running along [001]. These chains are connected by chains of edge-sharing Cu04+2 ‘octahedra’ with two more distant oxygen neighbours, a result of the Jahn-Teller effect. The interatomic distances were found to be (2×) 1.582(7) Å and (2×) 1.640(7) Å for Si–O; (4×) 1.941(4) Å and (2×) 2.926(7) Å for Cu–O. Using this data, the cationic bond valence sums were calculated in accordance with the valencies and found to be 3.96 for Si4+, respectively 2.00 for Cu2+. The valence angles of about 119.5° obtained between silicon atoms and bridging oxygen atoms (ideally 109.47° for the tetrahedral O–Si–O angles and about 139° for the Si–O br –Si one) are evidently the topological limits for condensed silicate anions. Being a quasi-onedimensional antiferromagnetic 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.