Double beryllium iodate dihydrates, M2Be(IO3)4·2H2O (M = K, NH4+, Rb): Preparation, X-ray powder diffraction and vibrational spectra

Abstract The solubilities in the three-component systems MIO 3 –Be(IO 3 ) 2 –H 2 O (M  =  K, NH 4 + , Rb, Cs) were studied at 25 °C by the method of isothermal decrease of supersaturation. It has been established that double salts, K 2 Be(IO 3 ) 4 ·2H 2 O, (NH 4 ) 2 Be(IO 3 ) 4 ·2H 2 O, and Rb 2 Be(IO 3 ) 4 ·2H 2 O, crystallize from the ternary solutions within wide concentration ranges. Both the X-ray powder diffraction and the spectroscopic studies (infrared and Raman) reveal that the title compounds are isostructural. They crystallize in the monoclinic space group P 2/ m with lattice parameters: K 2 Be(IO 3 ) 4 ·2H 2 O – a  = 14.218(5) A, b  = 6.747(2) A, c  = 5.765(2) A, β  = 98.74(4)°, V  = 546.6(2) A 3 ; (NH 4 ) 2 Be(IO 3 ) 4 ·2H 2 O – a  = 14.414(4) A, b  = 6.838(2) A, c  = 5.947(2) A, β  = 99.52(4)°, V  = 578.0(2) A 3 ; Rb 2 Be(IO 3 ) 4 ·2H 2 O – a  = 14.423(4) A, b  = 6.867(2) A, c  = 5.743(3) A, β  = 98.15(3)°, V  = 562.9(3) A 3 . Infrared spectroscopic experiments show that comparatively strong hydrogen bonds are formed in the potassium and rubidium salts as deduced from the wavenumbers of ν OD of matrix-isolated HDO molecules (isotopically dilute samples) owing to the strong Be–OH 2 interactions ( synergetic effect). However, the IO 3 − ions in the ammonium compound are involved in hydrogen bonds with NH 4 + ions additionally to those with water molecules and as a result of these intermolecular interactions the proton acceptor strength of the iodate ions decreases ( anti -cooperative effect), thus leading to the formation of weaker hydrogen bonds in this compound (bonds of moderate strength) as compared to those formed in the potassium and rubidium ones. The normal vibrations of other entities (IO 3 − ions and BeO 4 tetrahedra (skeleton vibrations)) are also discussed.