The structure of dicalcium potassium heptahydrogen tetrakis(phosphate) dihydrate, Ca2KH7(PO4)4.2H2O, by X-ray and neutron diffraction

Msub(r) = 542.25, triclinic, a = 5.676 (1), b = 12.210 (2), c = 6.292 (1) A, α = 104.10 (3), β = 115.16 (2), γ = 84.25 (2)0, V = 382.8 (6) A3, Z = 1, Dsub(x) = 2.352 Mg m-3, μ = 1.51 mm-1, F(000) = 274 (X-rays), μ = 0.14 mm-1, F(000) = 96.5 fm (neutrons), T = 298 K. For X-rays, space group P1-bar, Mo Kα, = 0.7107 A, R = 0.024 for 2040 independent observed reflections. For neutrons, space group P1, lambda = 1.273 A, R = 0.051 for 1383 independent observed reflections. The structure is isomorphous with Ca2(NH4)H7(PO4)4.2H2O. Although K could occupy a center of symmetry, it apparently does not, and two of the three hydrogen bonds that would cross centers of symmetry in P1-bar are also markedly asymmetric. Because of the greater sensitivity of neutrons to hydrogen atoms, it is concluded that the correct space group is P1. (Auth.)