Barite-type Ba(BeF4)x(SO4)1−x (x = 1 and ≈0.5)

Barite-type α-BaBeF4 (a = 8.8594(3), b =5.3265(2), c = 7.0493(2) Å, Pnma (No. 62)) and Ba(BeF4)0.535(7)(SO4)0.465(7) (a = 8.8657(2), b = 5.3902(2), c = 7.1007(2) Å, Pnma (No. 62)) were prepared by precipitation from aqueous solutions and their structures refined from laboratory X-ray powder diffraction data. In the case of α-BaBeF4 it was possible to identify the light atom Be on a difference Fourier map and to refine its positional parameters in the presence of a heavy atom (Ba). Both phases contain almost ideal isolated BX42− tetrahedra (B = Be, Be/S and X = F, F/O) together with Ba, that is 12-fold coordinated by X. The plausibility of the resulting structures was proved with the help of the bond valence model. For both compounds no phase transitions were found up to 550∘C.