Potassium beryllium selenate dihydrate, K2Be(SeO4)2$2H2O: Preparation, structure and infrared spectroscopy

The solubility in the three-component system K2SeO4–BeSeO4–H2O at 25 8C is studied by the method of isothermal decrease of supersaturation. A new compound, K2Be(SeO4)2$2H2O, is formed in a wide concentration range (from solutions containing 39.40 mass% beryllium selenate and 8.50 mass% potassium selenate up to solutions containing 7.02 mass% beryllium selenate and 51.14 mass% potassium selenate). K2Be(SeO4)2$2H2O crystallizes in the monoclinic space group P21/c (aZ11.503(2), bZ11.846(1), cZ7.491(1) A ° , bZ95.77(1)8, VZ1015.5 A ° 3, ZZ4, R1Z0.026 for 4220 FoO4s(Fo) and 153 variables). The crystal structure is composed of [Be(SeO4)2(H2O)2]2K groups, formed by a BeO4 tetrahedron sharing two corners with SeO4 tetrahedra. These units are linked via potassium cations and comparatively strong hydrogen bonds to double layers and further to a three-dimensional framework structure. K2Be(SeO4)2$2H2O is isotypic with the corresponding sulfate compound, K2Be(SO4)2$2H2O. The strengths of the hydrogen bonds in the title compound as deduced from the infrared wavenumbers of the uncoupled OD stretches of matrix-isolated HDO molecules (isotopically dilute sample) are discussed in terms of the Ow/O hydrogen bond distances, the different hydrogen bond acceptor capabilities of the selenate oxygen atoms and the strong Be–OH2 interactions (synergetic effect). The intramolecular O–H bond lengths are derived from the nOD vs. rOH correlation curve [J. Mol. Struct. 404 (1997) 63]. q 2005 Elsevier B.V. All rights reserved.