Re-refinement of sodium ammonium sulfate dihydrate at 170 K.

The title compound, sodium ammonium sulfate dihydrate (SASD), NaNH 4 SO 4 ·2H 2 O, a synthetic analogue of the mineral lecontite, is a well known ferroelectric. The crystal structure of the paraelectric phase has been re-refined at 170 K on the basis of single-crystal X-ray data, improving the previous study [Arzt & Glazer (1994 ▸). Acta Cryst. B 50 , 425-431] in terms of accuracy regarding hydrogen-atom positions and thus details of the hydrogen bonding. O-H⋯O and N-H⋯O hydrogen bonds between the principal building units [Na(OH 2 ) 4 O 2 octa-hedra, SO 4 tetra-hedra and ammonium cations] constitute a three-dimensional network structure.