Pb<sub>10</sub>Al<sub>10</sub>Si<sub>8</sub>O<sub>41</sub>: Lead aluminosilicate with the narsarsukite-related structure
The crystal structure of Pb10Al10Si8O41 (orthorhombic, Fddd, a = 16.2940(17), b = 21.666(2), c = 22.655(2) Å, V = 7997.8(13) Å3, Z = 8) had been solved by direct methods and refined to R1 = 0.042 on the basis of 1393 unique observed reflections. The crystal structure of Pb10Al10Si8O41 contains three symmetrically independent Pb atoms, each having distorted coordination environment due to the stereoactivity of the 6s2 lone electron pair. There are four tetrahedrally coordinated sites in the structure, T1–T4, with the average ❬T–O❭ bond lengths varying from 1.66 to 1.69 Å. The site occupancies for these sites were assigned to Si0.5Al0.5. There is one additional Al site that has the fivefold trigonal bipyramidal (TBP) coordination with the O9 site half-occupied. The crystal structure is based upon tubular tetrahedral chains, [T4O10], parallel to the a axis and linked into a three-dimensional framework via Pb atoms and dimers of corner-sharing AlO5 trigonal bipyramids located in between the chains. The comparison between the experimental X-ray powder diffraction pattern for Pb4Al4Si3O16 (ICDD PDF #32-0505) and theoretical one calculated on the basis of the crystal-structure data shows remarkable similarities, which provides strong evidence in favor of the identity of Pb10Al10Si8O41 with the “Pb4Al4Si3O16” compound reported previously. The crystal-chemical formula of Pb10Al10Si8O41 can be written as Pb10Al2O[(Si2Al2)O10]4 that emphasizes the composition of the tetrahedral chains with disordered distribution of Si and Al over tetrahedral sites. The structure type of the title compound is new for inorganic compounds and may be viewed as a hybrid of the structure types of Pb6O[(Si6Al2)O20] and narsarsukite. The unit cell of the title compound corresponds to the 2 × 2 × 2 (eightfold) supercell relative to both narsarsukite and Pb6O[(Si6Al2)O20]. The information-based structural complexity parameters are higher than those of the parent structure types with the total structural information per cell (590.100 bits) about 6.2–6.5 times larger.