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This relatively low-temperature process involves complete rearrangement of the crystal structure-from a 2D architecture featuring slabs [Pb 2 F 2 ] 2+ formed by fluorine-centered tetrahedra into a structure characterized by 1D motifs based on [OPb 2 ] 2+ chains of oxocentered tetrahedra. The comparative crystal chemistry of the obtained anion-centered structural architectures is discussed. (Siidra et al. 2013c(Siidra et al. , 2015. Most of these structures can be notionally decomposed into tetragonal subcells with an a parameter similar to that of litharge (commonly 3.75-4.35 Å, depending on the layer composition), while c can vary over a wide range (from 5 to more than 75 Å) as might be expected for layered structures (Charkin 2008). Within these "archetypic" subcells, the 2 ∞ [XM] (X = anion, M = cation) slabs contain two X and two M atoms; they are positively charged and interleave with various anionic species. The OM 4 oxocentered tetrahedra, with M = Pb 2+ , Bi 3+ or rare-earth elements, are the most common contributors to these slabs. Second rank the FM 4 fluoride-centered tetrahedra with M being rare-earth or alkaline-earth elements, but most commonly Pb 2+ . [Pb 2 F 2 ] 2+ slabs have been found in the structures of the minerals matlockite [Pb 2 F 2 ](Cl) 2 (c = 9 Å) (Pasero and Perchiazzi 1996), grandreefite [Pb 2 F 2 ](SO 4 ) (c = 14 Å)

27 Br 3.54 )(SeO 3 ) 4 Br 2 ), all but one exhibiting new complex layered or open-framework architectures, organized via the host-guest principle. Five of these structures contain only bivalent copper: three with well ordered 2D structures and completely filled atomic positions, two structures have 3D frameworks. The four remaining compounds contain both, Cu 2+ and Cu + , and comprise layers formed by edge-and vertex-sharing of oxocentered OCu 2+ 4-n Pb n tetrahedra decorated with SeO 3 2À and Br À anions. The interlayer spaces are filled by partially disordered Cu + -Br and Pb-Br associations. One remarkable feature of the latter group of compounds is the presence of tetrahedral (Cu 2+ ) 4 complexes with arrangements derivable from a kagome network. research papers Acta Cryst. (2018). B74, 712-724 Siidra et al. Copper lead selenite bromides 713 research papers Acta Cryst. (2018). B74, 712-724 Siidra et al. Copper lead selenite bromides 723

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