Structural phase transition and crystal structure of Na3H(SO4)2 in the low temperature phase

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Differential scanning calorimetry and X-ray diffraction measurements are performed on trisodium hydrogen disulphate, Na3H(SO4)2, crystals at low temperatures. A structural phase transition is found at 178(3) K. The space group symmetry (triclinic P1) and the crystal structure are determined at 139K in the new low-temperature phase. It is found that the H atoms on the O─H─O hydrogen bond are not located on the center of the bond. By the shift of the S atom from the center of the SO4 tetrahedron, it points out that Na3H(SO4)2 crystal in the new phase is possible to be ferroelectric with the dipole moment almost along the [1 2 0] direction.