Small atomic displacements in the molybdenophosphates AMo2P3O12 (A = K, Rb, Tl)

KMo2P3O12, Mr = 515.90, orthorhombic, Pbnm, a = 17.6398(14), b = 9.1761(4), c = 12.3000(8) A, V = 1990.9(4) A3, Z=8, Dx = 3.44 Mg m-3, λ(Mo Kα) = 0.71069 A, μ = 3.42 mm-1, F(000) = 1952. T = 294 K, R = 0.028 for 2123 reflections. RbMo2P3O12, Mr = 562.26, orthorhombic, Pbcm, a = 8.8314(8), b = 9.2368(7), c = 12.3051(9) A, V = 1003.8(4) A3, Z=4, Dx = 3.72 Mg m-3, λ(Mo Kα) = 0.71069 A, μ = 8.08 mm-1, F(000) = 1048, T = 294 K, R = 0.044 for 2073 reflections. The Mo2P3O12 frameworks of the K, Rb and Tl compounds are almost the same. The main difference is in the position of the alkaline-earth ions in the tunnels, which induces, in the potassium compound, a superstructure along a. The alkaline-earth ions are slightly displaced as their size decreases in order that the A-O distances may agree with the sum of the ionic radii. (orig.)