Ammonium beryllium selenate dihydrate, (NH4)2Be(SeO4)2·2H2O: Preparation, X-ray powder diffraction, and vibrational spectra

Abstract The solubility in the three-component (NH 4 ) 2 SeO 4 –BeSeO 4 –H 2 O system is studied at 25 °C by the method of isothermal decrease of supersaturation. (NH 4 ) 2 Be(SeO 4 ) 2 ·2H 2 O crystallizes from solutions containing 31.35 mass% beryllium selenate and 30.66 mass% ammonium selenate up to solutions containing 26.84 mass% beryllium selenate and 46.84 mass% ammonium selenate. The X-ray powder diffraction data show that (NH 4 ) 2 Be(SeO 4 ) 2 ·2H 2 O is isostructural with the respective K 2 Be(SeO 4 ) 2 ·2H 2 O, K 2 Be(SO 4 ) 2 ·2H 2 O and Rb 2 Be(SO 4 ) 2 ·2H 2 O. (NH 4 ) 2 Be(SeO 4 ) 2 ·2H 2 O crystallizes in the monoclinic space group P 2 1 / c : a  = 11.747(3) A, b  = 12.212(4) A, c  = 7.649(2) A, β  = 96.94(3)°, V  = 1089.3(3) A 3 , Z  = 4. Vibrational spectra (infrared and Raman) of the title compound are presented and discussed with respect to the internal modes of both the ammonium and the selenate tetrahedra, hydrogen bond strengths and the lattice vibrations of the BeO 4 tetrahedra (skeleton vibrations).