Ba2(BO3)1–x(CO3)xCl1+x: A Mixed Borate and Carbonate Chloride Crystallized from High-Temperature Solution

A mixed borate and carbonate chloride Ba2(BO3)1–x(CO3)xCl1+x was obtained by spontaneous crystallization from a high-temperature melt in open air. It crystallizes in the trigonal crystal system with space group of P3m̅1 and lattice constants of a = 5.4708(8) Å and c = 10.640(2) Å. The structure can be viewed as an intergrowth of trigonal Ba2Mg­(BO3)2 (001) slab and (111) slab of the cubic fluorite BaCl2. During Fourier analysis of the single-crystal X-ray diffraction data, additional electron density was found locating at 1b (0, 0, 1/2) site and attributed to chlorine surplus, which was confirmed by chemical titration. Charge balance of the compound was found, unexpectedly in an acidic borate containing high-temperature melt, by partial CO32– group substituting the BO33– group. The existence of CO32– anion in the crystal was detected by thermogravimetry–mass spectrum analysis and Raman spectrum. The transmittance spectrum shows that the crystal is transparent from ultraviolet to infrared with short-wavelength absorption edge at about 220 nm.