On the structure of Tl${}_2$Pb(SO${}_4$)${}_2$ and crystal chemistry of thallium-containing sulfates

Single crystal of a thallium-containing sulfate Tl${}_2$Pb(SO${}_4$)${}_2$ was prepared through hydrothermal crystallization starting from Tl${}_2$CO${}_3$, Pb(NO${}_3$)${}_2$ and H${}_2$SO${}_4$. The crystal structure was solved from single-crystal X-ray diffraction data: Tl${}_2$Pb(SO${}_4$)${}_2$ crystallizes in space group R$\overline{3}$m with lattice parameters of a=5.5955(2) Å, c=22.1001(9) Å, and Z=3, which is also corroborated by Rietveld refinement on X-ray powder data. The crystal structure can be regarded as [Pb(SO${}_4$)${}_2$]${}^{2-}$ sheets intercalated by double layers of Tl${}^{+}$ along the c-axis. From a mineralogical perspective, Tl${}_2$Pb(SO${}_4$)${}_2$ belongs to the palmierite family and can further be classified within palmierite-supergroup. Validation of the structure models was carried out utilizing various concepts including Bond Valence (BV), Charge Distribution (CD) and Madelung Part of Lattice Energy (MAPLE). Notably, a contrast in stereochemical activity of 6s${}^2$ lone electron pair (LEP) between Tl${}^{+}$ and Pb${}^{2+}$ is interpreted in terms of bonding behaviours as well as Wang-Liebau Eccentricity (WLE) parameters. Moreover, an extensive investigation into the crystal chemistry of thallium-containing sulfates was conducted for the first time, focusing on the coordination environment and bonding behaviour of Tl${}^{+}$ and corresponding WLE parameters. A comparative analysis of these structurally related compounds including Tl${}_2$Pb(SO${}_4$)${}_2$, Tl${}_3$H(SO${}_4$)${}_2$, TlBi(SO${}_4$)${}_2$, TlIn(SO${}_4$)${}_2$, TlRh(SO${}_4$)${}_2$, and TlAl(SO${}_4$)${}_2$ was delineated. (© 2024 Wiley-VCH GmbH)