Insights into the structure of disordered layered lead oxychlorides: Twinning and short-range order in janchevite, Pb₉VO_10.25□_0.75Cl_2.5 Available to Purchase

The crystal structure of the rare mineral janchevite was solved using intensity data collected from a twinned crystal from the Kombat mine in Namibia. This study revealed that, despite the strong tetragonal subcell, the structure is triclinic (space group $P1¯$⁠) with a = 8.8382(7), b = 8.8567(7), c = 11.7103(17) Å, α = 103.385(10), β = 94.192(10), γ = 90.294(6)°, V = 889.13(16) ų, and Z = 2. Chemical data indicate the presence of Mo, As, and Si, besides Pb, Cl, O, and V. The refinement of an anisotropic model, which takes into account a two-domain non-merohedral twinning (twinning matrix |$1¯00/01¯0/$⅕ ⅗ 1|), led to an R = 0.0522 for 6037 independent observed reflections [2σ(I) level] and 165 parameters and an R = 0.0964 for all 11 701 independent reflections. The structure consists of PbO litharge sheets alternating with a layer of Cl⁻ anions in a 2:1 ratio. The incorporation of high-charge cations (V⁵⁺, Mo⁶⁺, As⁵⁺, Si⁴⁺) into the litharge sheet involves the formation of square cavities where MoO₅ square pyramids and (Si,As,V)O₄ tetrahedra are hosted. On the basis of information gained from the chemical, structural, and spectroscopic characterization, the crystal chemical formula was revised to Pb₉VO_10.25□_0.75Cl_2.5 (Z = 2). The presence of additional apical O atoms between Pb layers influences the interlayer thickness, as evidenced by the linear correlation between the interlayer oxygen content and the type of structural defects. A correct comprehension of short- and long-range order is crucial to understanding and predicting the functional properties of this family of materials.