A series of mixed crystals with general formula Ba_7-xNa_yF₁₂Cl_2-zBr_z in the ordered modification (space group P-6) has been studied by single crystal x-ray diffraction. Depending on synthesis conditions, the disorder in the channels (i.e. occupation of 0 0 z sites) can be changed. The disorder is found to be correlated with the refined Na content, and its effect on Ba-Cl(Br) bond length is discussed.

A new barium silico-aluminate phase with the stoichiometry Ba13.35(1) Al30.7 Si5.3 O70 has been found and characterized. The compound crystallizes in the space group P63 /m (No. 176) with a = 15.1683(17) Ã…, c = 8.8708(6) Ã…, V = 1767.5(4) Ã…³ , Z = 1, Rw = 0.026, 32 reï¬�ned parameters. A 3-dimensional matrix of Al/SiO4 tetrahedra with Ba(II) ions located in channels along the c axis builds up the structure. One of these channels is partially ï¬�lled with Ba(II) ions (CN 6+3) in Wyckoff position 2a, leaving ∼ 1/3 of the positions empty. The second and third type of Ba(II) ions occupy channels orientated along the c axis with CN 4+2+2 and 4+3+1, respectively. The structure shows a rare clustered arrangement of six tetrahedra ï¬�lled exclusively by Al(III) and therefore is an exception to Loewenstein’s rule. The other tetrahedral positions show an Al to Si ratio of ∼ 4 : 1. The Al/Si–O bond lengths in the tetrahedral Al/Si positions drawn vs. site occupation show linear behavior similar to the prediction by Vegard’s rule for solid solutions. After doping with Eu(II) the compound shows bright orange-yellow luminescence with an unusual large shift of the Eu(II) emission band.

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.