Crystal Chemistry in the Barium Fluoride Chloride System

The crystal chemistry of the barium fluoride chloride system is studied both experimentally and theoretically. Different synthetic approaches yield nanocrystalline materials as well as large single crystals. The crystalline phases identified so far are BaFCl, Ba12F19Cl5, and Ba7F12C12 (in two modifications) and are compared with analogous compounds. It is demonstrated that the compound Ba2F3Cl reported by Fessenden and Lewin 50 years ago corresponds to Ba7F12Cl2. The phase diagram of the BaCl2-BaF2 system is reinvestigated for fluoride mole fractions between 0.5 and 1. The peritectic formation of Ba12F19Cl5 is observed. Periodic density functional theory (DFT) calculations are performed for all structures in this system, including a hypothetical structure for Ba2F3Cl, based on the experimental structure of Ba2H3Cl. The energy of formation of the different barium fluoride chloride compounds from BaCl2 and BaF2 (normalized for one barium atom per formula unit), as calculated by DFT at 0 K, is within only about +/- 15 kJ/mol. Comparison with recent experimental results on calcium and strontium hydride chloride (bromide) compounds suggests the possibility of a mutual exclusion between the M2X3Y and M7X12Y2 (M = Ca, Sr, Ba, Pb, X = II, F, Y = Cl, Br) structures. The single crystal structure of PbFBr is also reported.