This work deals with the synthesis and crystal structure study of new bismuth oxo-arsenates and their oxo-phosphates homologous: Bi₆ZnO₇(AsO₄)₂ vs Bi₆ZnO₇(PO₄)₂ and Bi_3.667Cd₃O₄(AsO₄)₃ vs Bi₃Cd₄O₄(PO₄)₃. Their crystal structures were solved using single crystal X-Ray Diffraction. These are two other examples of crystal structures built on ribbon-like polycations formed of the linkage of oxo-centered O(Bi,M)₄ tetrahedra sharing edges and surrounded by isolated XO₄ groups (X= As or P), where the O(Bi,M)₄ units are derived from the fluorite topology structure. Dealing with Bi₆ZnO₇(PO₄), its acentric space group was confirmed by preliminary second harmonic generation (SHG). The P/As substitution led to a centrosymmetric space group due to local reorientation of oxo-anions. This is strongly related to steric effects between AsO₄ (d As-O= 1.6-1.7Ã…) and PO₄ (d P-O= 1.4-1.5Ã…). Concerning Bi_3.667Cd₃O₄(AsO₄)₃ and Bi₃Cd₄O₄(PO₄)₃, they show a second example of the reorientation of the XO₄ groups depending of the X chemical nature. Finally, we present an original topology of oxo-centered units obtained with Bi₅KO₅(AsO₄). The photoluminescence properties of Bi₅KO₅(AsO₄) and Bi₆ZnO₇(AsO₄)₂ were also investigated. The first one emits at room temperature in the reddish-orange range (single band peak at 615nm assigned to the Bⁱ³⁺: ³P₁→¹S₀ transition whereas the second exhibits a weak emission in the green range (peak at 530nm). Its intriguing temperature dependence is discussed in the paper.

Mixed single PbFBr_1−xI_x crystals have been prepared. X-ray powder diffraction structure determinations show that all samples crystallize with the matlockite structure. However, the single crystal structure of PbFBr_0.5I_0.5 involves not only fractional occupation of one site corresponding to the stoichiometry, but also split positions of the Pb²⁺ ion. Raman spectra reveal the presence of new additional bands with respect to PbFBr and PbFI. DFT calculations of lattice vibrations for PbFI show good agreement with experimental spectra. The calculated phonon dispersion curve suggests that for the mixed crystals the centre of inversion is conserved locally. These combined results suggest the presence of domains with ordered F–Pb–Br–Br–Pb–F and F–Pb–I–I–Pb–F layers in the mixed crystals. Calculations on PbFBr_0.5I_0.5 show that this suggested structure is more stable than the structure consisting of the F–Pb–Br–I–Pb–F arrangement.