Structural study of a high-temperature phase in (ND4)3D(SO4)2

The crystal structure of a new high-temperature phase in deuterated triammonium hydrogen disulphate, (ND4)3D(SO4)2, was determined by the X-ray diffraction. The structure in this phase is found to be trigonal with space group R3m. The reorientation of SO4 tetrahedra occurs at the phase transition from phase I to the new high-temperature phase. The result shows that the shortest length of the O–D–O hydrogen bond which connects the two nearest adjacent SO4 tetrahedra is changed from 3.263(13) to 2.622(9) Å during the structural phase transition. The mechanism of the deuterium migration in the new high-temperature phase is discussed and the new phase is confirmed to be a proton superionic conductor with respect to the crystal structure.