Insights into the structure of disordered layered lead oxychlorides: Twinning and short-range order in janchevite, Pb9VO10.25□0.75Cl2.5

The crystal structure of the rare mineral janchevite was solved using intensity data collected from a twinned crystal from the Kombat mine, Namibia. This study revealed that, in spite of the strong tetragonal subcell, the structure is triclinic (space group P-1) 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) Å3, 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 domains non-merohedral twinning (twinning matrix |-100/0-10/⅕⅗1|), led to an R = 0.0522 for 6037 independent observed reflections [2σ(I) level] and 165 parameters and an R = 0.0964 for all 11701 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, yielding Pb9VO10.25◻0.75Cl2.5 (Z = 2). The presence of additional apical oxygen atoms between Pb layers influences the interlayer thickness, as evident 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 understand and predict the functional properties of this family of materials.