Preparation, crystal structure and infrared spectroscopy of the new compound Rb 4Be(SeO 4) 2(HSeO 4) 2·4H 2O

Abstract

A new compound, Rb 4Be(SeO 4) 2(HSeO 4) 2·4H 2O, crystallizes in a comparatively wide concentration range from mixed beryllium rubidium selenate solutions (from solutions containing 29.06 mass% beryllium selenate and 25.75 mass% rubidium selenate up to solutions containing 12.53 mass% beryllium selenate and 55.32 mass% rubidium selenate). Rb 4Be(SeO 4) 2(HSeO 4) 2·4H 2O crystallizes in the acentric orthorhombic space group Pmn2 1 ( a = 32.607(4), b = 10.676(2), c = 6.069(1) Å, V = 2112.8 Å 3, Z = 4, R1 = 0.047 for 4059 F o > 4σ(F o) and 311 variables). The crystal structure is composed of Be(H 2O) 4 tetrahedra arranged in layers at x = 0 and x = ½, alternating with broad layers built up from SeO 4 and HSeO 4 selenate tetrahedra and Rb cations. The beryllium–water layers are linked to the rest of the structure via hydrogen bonds only. The H 2O molecules as well as the OH molecules of the acid HSeO 4 groups form strong to very strong hydrogen bonds with donor–acceptor distances between 2.58 and 2.74 Å. Vibrational spectra (infrared and Raman) of Rb 4Be(SeO 4) 2(HSeO 4) 2·4H 2O are presented and discussed in the region of the fundamentals of both the selenate and the beryllium tetrahedra (skeleton motions) as well as in the region of the OH vibrations at ambient and liquid nitrogen temperature (LNT). The appearance of four Raman bands corresponding to ν 1 of the selenate ions reflects the existence of four crystallographically different selenate tetrahedra in the structure. The spectroscopic experiments reveal that the ν 1 modes of the selenate ions appear at higher frequencies than some components of ν 3. Bands of an AB doublet structure (2950, 2390 cm −1) arising from the OH stretching modes of the HSeO 4 - ions are recognized in the infrared spectra. The appearance of two infrared bands (1308, 1250 cm −1) corresponding to δ(OH) (in-plane bending modes of the OH groups) confirms the structural data regarding the existence of two crystallographically different OH groups. The water librations are also briefly commented. The appearance of a band at a comparatively large wavenumber (1013 cm −1) corresponding to rocking librations of the water molecules indicate that strong hydrogen bonds are formed in the title compound.