EN
Differential scanning calorimetry and X-ray diffraction measurements were performed on a deuterated tetraammonium dihydrogen triselenate (ND_4)_4D_2(SeO_4)_3 crystal at low temperatures. A structural phase transition is found at 182.0(3) K. The space group symmetry (triclinic P1) and the structure parameters are determined at 145 K in the low-temperature phase. The isotope effect for an asymmetric O-H-O hydrogen bond by the substitution of deuterium is discussed.