The crystal structure of paraelectric ammonium fluoroberyllate at room temperature

The crystal structure of paraelectric ammonium fluoroberyllate, (NH4)2BeF4, isomorphous with β-K2SO4, has been refined at room temperature. All atoms including six hydrogen atoms have been determined. The shape of the BeF4 ion is a fairly good tetrahedron with an average vibrationally corrected bond length of 1.548(2) Å. The final discrepancy factor is 0.037 for 446 independent reflections. Large thermal motions are observed on one of the two non-equivalent ammonium ions and on the BeF4 ion. The largest principal value of the r.m.s. displacement and the r.m.s. librational amplitude is 0.166 Å nearly along the b axis and 7.89° around the c axis, respectively. There are two possible hydrogen bonds; N(1)-H(1) … F(1) and N(2)-H(5) … F(2).