Potassium beryllium sulfate dihydrate, K2Be(SO4)2·2H2O: Crystal structure and infrared spectroscopy

Most of the infrared and Raman bands corresponding to the internal vibrations of the SO42− ions appear as doublets, thus reflecting the existence of two crystallographically different [SO4]-tetrahedra in agreement with the structural data. The strengths of the hydrogen bonds as deduced from the infrared wavenumbers of the uncoupled OD modes of matrix-isolated HDO molecules (bands at 2278, 2352 and 2388 cm−1, liquid nitrogen temperature) are discussed in terms of the the Ow⋯O bond distances, the different hydrogen bond acceptor capabilities of the oxygen atoms and the Be-OH2 interactions. The intramolecular OH bond lengths are derived from the νOD vs. rOH correlation curve [J. Mol Struct. 404 (1997) 63]. The water librations couple intensively with the translatory modes of the Be2+ ions (BeO4 skeleton vibrations) and the normal modes of SO42− ions, thus producing small isotopic shifts. The SO42− guest ions matrix-isolated in the isostructural host lattice K2Be(SeO4)2·2H2O exhibit doublet bands in the region of the stretching modes (the bands corresponding to the bending modes could not be recognized well), thus indicating a statistical distribution of the guest ions over the two available crystallographic positions. The distortion of the SO42− guest ions in the host selenate lattice is weaker than that of the same ions in the neat sulfate owing to the larger unit-cell volume of the host compound, i.e. to the smaller repulsion potential of the host lattice.