The crystal structure of the normal orthophosphates of barium and strontium

The compounds Ba3(PO4)2 and Sr3(PO4)2 are rhombohedral and isostructural. The unit cell contains one molecule. The dimensions of the unit rhombohedron and the calculated densities are: Ba3(PO4)2: a = 7.696 ± 0.002 kX, α = 42° 35′ ± 2′, ρ = 5.25 g.cm−3; Sr3(PO4)2: a = 7.280 ± 0.002 kX, α = 43° 21′ ± 2′, 4.53 g.cm−3. The space group is R\overline 3m and the atomic positions are: 1 Ba in (0, 0, 0), 2 Ba in ±(u1, u1, u1), 2 P in ±(u2, u2, u2), 2 O in ±(u3, u3, u3), 6 O in ±(x, x, z) (x, z, x) (z, x, x); with parameter values u1 = 0.208, u2 = 0.412, u3 = 0.337, x = 0.285 and z = 0.742.