New dihydrogenhypodiphosphates of alkaline-earth metals : preparation, crystal structure, vibrational spectrum and thermal behavior of AE[H2P2O6] · n H2O (AE = Ca, Sr and n = 1.5, 2)

Two new alkaline-earth metal dihydrogenhypodiphosphates, AE[H2P2O6] ⋅ n H2O (AE=Ca, Sr and n=1.5, 2) have been prepared and structurally characterized by single crystal X-ray diffraction. Calciumdihydrogenhypodiphosphate sesquihydrate, Ca[H2P2O6] ⋅ 1.5 H2O (1) (lattice parameters: a=6.619(1) Å, b=10.201(1) Å, c=20.922(2) Å, β=107.95(1)°, VEZ=1343.9(2) Å3, Z=4), crystallizes in the monoclinic space group P21/c and strontiumdihydrogenhypodiphosphate dihydrate, Sr[H2P2O6] ⋅ 2 H2O (2) (lattice parameters: a=7.548(1) Å, b=11.906(1) Å, c=7.880(1) Å, VEZ=708.2(2) Å3, Z=4), in the orthorhombic space group Pbcn. The crystal structure of compound 1 consists of chains of edge- and face-sharing of [CaO6] and [CaO8] polyhedra linked by dihydrogenhypodiphosphate(IV) anions to form a three-dimensional network. The structure of 2 consists of alternating layers of chains of edge-sharing [SrO8] polyhedra and respective anionic units. The different water-content compositions result from the corresponding alkaline-earth metal coordination polyedra and by the preferred number of water molecules in their coordination sphere, respectively. The IR/Raman spectra of the title compounds have been recorded and interpreted, especially with respect to the [H2P2O6]2− group. The phase purity of 2 was verified by powder diffraction measurements.