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

The solubility in the three-component (NH4)2SeO4-BeSeO4-H2O system is studied at 25 °C by the method of isothermal decrease of supersaturation. (NH4)2Be(SeO4)2·2H2O 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 (NH4)2Be(SeO4)2·2H2O is isostructural with the respective K2Be(SeO4)2·2H2O, K2Be(SO4)2·2H2O and Rb2Be(SO4)2·2H2O. (NH4)2Be(SeO4)2·2H2O crystallizes in the monoclinic space group P21/c: a = 11.747(3) Å, b = 12.212(4) Å, c = 7.649(2) Å, β = 96.94(3)°, V = 1089.3(3) Å3, 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 BeO4 tetrahedra (skeleton vibrations).