The structure of (NH 4 ) 4 H 2 (SeO 4 ) 3 and (ND 4 ) 4 D 2 (SeO 4 ) 3 superionic crystals below 180 K determined by X-ray singlecrystal analysis is presented. The compounds have the following crystal data: (NH 4 ) 4 H 2 (SeO 4 ) 3 (T ؍ 140 K), triclinic, space group P-1, a ؍ 7.647(2) A s , b ؍ 10.342(2) A s , c ؍ 10.558(2) A s , ؍ 114.41(3)3, ؍ 93.22(3)3, ؍ 107.47(3)3, V ؍ 709.4(3) A s 3 , Z ؍ 2, R ؍ 0.0358; (ND 4 ) 4 D 2 (SeO 4 ) 3 (T ؍ 135 K), triclinic, space group P-1, a ؍ 7.660(2) A s , b ؍ 10.345(2) A s , c ؍ 10.552(2) A s , ؍ 114.40(3)3, ؍ 93.27(3)3, ؍ 107.50(3)3, V ؍ 710.3
(3) A s 3 , Z ؍ 2, R 1 ؍ 0.0325. The two 99acid:: protons are involved in short hydrogen bonds of length &2.5 A s linking three SeO 4 tetrahedra into a trimer. The dihydrogen triselenate anions are linked with the ammonium cations by weak H bonds (2.8+3.1 A s ). No drastic changes in bond lengths within SeO 4 tetrahedra were observed between room temperature and &140 K. The tendency to the N+H(D)2O bond network contraction can be related to slowing the ammonium ions:s rotation. The mutual reorientation of SeO 4 tetrahedra in trimers evidenced by torsion angle changes is negligible in the studied temperature range. The obtained results are compared with those previously reported and the reasons are given that at low temperatures both crystals preserve room temperature, nonpolar P-1 space group.