Crystal structure of Pb (sub 6) [Ge (sub 6) O (sub 18) ].2H (sub 2) O, a lead cyclo-germanate similar to the mineral dioptase

Pb (sub 6) [Ge (sub 6) O (sub 18) ]2H (sub 2) O crystallizes as transparent colourless trigonal needles, with space group R<$E 3 back 40 up 40 ->, a 15.6476, c 7.3119 Aa; Z = 3; D 6.44 g/cm (super 3) . Powder XRD data after least-squares refinement to R 0.033 shows the structure to consist of sandwiched hetero-polyhedral rings with straight channels down [001]. Isolated and partly expanded rings of six corner-sharing GeO (sub 4) tetrahedra are bonded together vertically by crown-shaped Sechser rings of flat PbO (sub 5+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 approximately 2.5 Aa and confine well-ordered pairs of water molecules which can be degassed by heating. A crystal-chemical and morphological comparison is drawn with dioptase, Cu (sub 6) [Si (sub 6) O (sub 18) ]6H (sub 2) O. This lead cyclo-germanate is the first compound found to have pure [Ge (sub 6) O (sub 18) ] (super 12-) single rings.