Mixed PbFBr_1-xI_x crystals: structural and spectroscopic investigations

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.