Crystal structure of Pb-6[Ge6O18]center dot 2H(2)O, a lead cyclo-germanate similar to the mineral dioptase

Pb-6[Ge6O18]. 2H(2)O crystallizes as transparent and colourless trigonal needles, having space group <R(3)over bar>, lattice parameters a = 15.6476(7) and c = 7.3119(4)Angstrom, with Z = 3 formula units per unit cell. The density of 6.44 gcm(-3) is comparatively low. Powder X-ray diffraction data lead to a crystal structure yielding a final residual of R(F) = 0.033 (wR2 = 0.070, R(p) = 0.106, R(wp) = 0.133) after least squares refinement. The structure consists of sandwiched hetero-polyhedral rings with straight channels down [001]. Isolated and partly expanded rings of six corner-sharing GeO4 tetrahedra are bonded together vertically by crown-shaped Sechser rings of flat PbO5+1 pyramids with shared corners, which themselves share edges laterally. Lone electron pairs of lead, which complete the pyramids in a psi-octahedral arrangement, point towards the channel centres. The channels have a free diameter of about 2.5 Angstrom and confine well-ordered pairs of water molecules which can be degassed by heating. A crystal-chemical and morphological comparison is drawn with the mineral dioptase, Cu-6[Si6O18]. 6H(2)O. Lead cyclo-germanate is the first compound observed with pure [Ge6O18](12-) single rings.