Pb₁₀Al₁₀Si₈O₄₁: lead aluminosilicate with the narsarsukite-related structure

Krivovichev S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 6. P. 110-121

https://doi.org/10.31857/S0869605520060052

Full text is available on eLIBRARY.RU

Language: English

Abstract

The crystal structure of Pb₁₀Al₁₀Si₈O₄₁ (orthorhombic, Fddd, a = 16.2940(17), b = 21.666(2), c = 22.655(2) Å, V = 7997.8(13) ų, 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 Si_0.5Al_0.5. 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.

 Keywords: aluminosilicate, lead, crystal structure, narsarsukite, superstructure, structural complexity