Abstract

Cu₆[Ge₆O₁₈] · 6H₂O is obtained from aqueous solution as a crystalline, greenish-turquoise powder, having space group R[unk], lattice parameters a = 15.0347(4) Å and c = 7.9493(2) Å, with Z = 18 CuGeO₃ · H₂O formula units per unit cell. The density is d_x = 3.88g cm⁻³. X-ray powder diffraction data revealed a crystal structure isotypic with that of the mineral dioptase, Cu₆[Si₆O₁₈] · 6H₂O. 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 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) Å and c = 7.9761 (8) Å 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 Cu₆[Ge₆O₁₈] · 6H₂O and Cu₆[Si₆O₁₈] · 6H₂O (dioptase) as well as Pb₆[Ge₆O₁₈] · 6H₂O, which represents the first example observed with pure [Ge₆O₁₈]¹²⁻ single sechser rings.