Structural Study of (NH$\text{}_{4}$)$\text{}_{4}$H$\text{}_{2}$(SeO$\text{}_{4}$)$\text{}_{3}$ in the Low-Temperature Phase

The crystal structure of tetraammonium dihydrogen triselenate, (NH$\text{}_{4}$)$\text{}_{4}$H$\text{}_{2}$(SeO$\text{}_{4}$)$\text{}_{3}$, in the low-temperature phase at 141 K is studied by X-ray diffraction. The space group symmetry (triclinic P1) and the structure parameters are determined. By the shift of the Se atom from the center of the SeO$\text{}_{4}$ tetrahedron, it is suggested that the (NH$\text{}_{4}$)$\text{}_{4}$H$\text{}_{2}$ (SeO$\text{}_{4}$)$\text{}_{3}$ crystal in the low-temperature phase is ferroelectric with the electric dipole moment along the c-axis. The rotation of the SeO$\text{}_{4}$ tetrahedron suggested by electron paramagnetic resonance measurements is discussed.