Synthesis and crystal structure of Cu-6[Ge6O18]center dot 6 H2O, a dioptase-type cyclo-germanate

Cu-6[Ge6O18] . 6 H2O is obtained from aqueous solution as a crystalline, greenish-turquoise powder, having space group <R(3)over bar> lattice parameters tr = 15.0347(1) Angstrom and c = 7.9493(2) Angstrom, with Z = 18 CuGeO3 . H2O 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[Si6O18] . 6 H2O 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 crown-sharing GeO4 tetrahedra alternate vertically with crown-shaped rings of six hydrogen-bonded water molecules. The two ring systems are bonded together by Cu2+ ions in elongated octahedral coordination of four oxygen atoms and two more distant water molecules, Water can be degassed by heating, leaving Cu2+ in at least square planar coordination. Once water is removed completely, the less dense compound with lattice parameters a = 14.8763(6) Angstrom and c = 7.9761(8) Angstrom 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[Ge6O18] . 6 H2O and Cu-6[Si6O18] . 6 H2O (dioptase) as well as Pb-6[Ge6O18] . 6 H2O, which represents the first example observed with pure [Ge6O18](12-) single sechser rings.