CRYSTALLIZATION IN THE THREE-COMPONENT SYSTEMS Rb 2 SeO 4 - MeSeO 4 - H 2 O (Me = Mg, Ni, Cu) at 25°C

The solubility in the three-component systems Rb 2 SeO 4 -MeSeO 4 -H 2 O (Me = Mg, Ni, Cu) is studied at 25°C by the method of isothermal decrease of supersaturation. It has been established that double compounds, Rb 2 Me(SeO 4 ) 2 ·6H 2 O (Me = Mg, Ni, Cu), crystallize from the ternary solutions within wide concentration ranges. The X-ray diffraction data reveal that the title compounds crystallize in the monoclinic space group P2 1 /c (C 2h 5 ) with lattice parameters: Rb 2 Mg(SeO 4 ) 2 ·6H 2 O – a = 6.353(3) A, b = 12.674(5) A, c = 9.412(3) A, b = 105.22(2)o , V= 731.2(3) A 3 ; Rb 2 Ni(SeO 4 ) 2 ·6H 2 O – a = 6.349(1) 9.324(2) A, b = 12.617(2) A, c = 9.324(2) A, b = 105.35(1)o, V = 720.3(1) A 3 ; Rb 2 Cu(SeO 4 ) 2 ·6H 2 O – a = 6.361(1) A, b = 12.569(2) A, c = 9.414(2) A, b = 104.68(1), V = 728.1(2) A 3 . The infrared spectra are discussed with respect to the normal vibrations of the selenate ions and water molecules. The unit-cell group theoretical treatment of the double salts is presented. Infrared spectroscopy experiments show that the effective spectroscopic symmetry of the selenate ions is close to C 3v . Comparatively strong hydrogen bonds are formed in the rubidium Tutton selenates as deduced from both the wavenumbers of the stretching modes of the water molecules and the water librations due to the strong proton acceptor strength of the selenate ions.