Synthesis and crystal structure of Cu6[Ge6O18]*6 H2O, a dioptase-type cyclo-germanate

Cu 6 [Ge 6 O 18 ]. 6 H 2 O is obtained from aqueous solution as a crystalline, greenish-turquoise powder, having space group R3, lattice parameters a =15.0347(4) A and c = 7.9493(2) A, with Z = 18 CuGeO 3 H 2 O formula units per unit cell. The density is d x = 3.88 g cm -3 . X-ray powder diffraction data revealed a crystal structure isotypic with that of the mineral dioptase, Cu 6 [Si 6 O 18 ]. 6 H 2 O. The structure was refined by the Rietveld method to a final residual of R F = 0.036 (R p = 0.067, R wp = 0.088). It consists of sandwiched rings with empty channels down [001]. Isolated and less expanded rings of six corner-sharing GeO 4 tetrahedra alternate vertically with crown-shaped rings of six hydrogen-bonded water molecules. The two ring systems are bonded together by Cu 2+ ions in elongated octahedral coordination of four oxygen atoms and two more distant water molecules. Water can be degassed by heating, leaving Cu 2+ in at least square planar coordination. Once water is removed completely, the less dense compound with lattice parameters a = 14.8763(6) A and c = 7.9761(8) A shows reversible thermochromism from denim-blue at room temperature to rich green at high temperatures, a color which can be frozen by rapid cooling. A crystal-chemical comparison is drawn between Cu 6 [Ge 6 O18]. 6 H 2 O and Cu 6 [Si 6 O 18 ]. 6 H 2 O (dioptase) as well as Pb 6 [Ge 6 0 18 ]. 6 H 2 O, which represents the first example observed with pure [Ge 6 O 18 ] 12- single sechser rings.