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 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.