Structural study of (NH4)(4)H-2(SeO4)(3) in the low-temperature phase

The crystal structure of tetraammonium dihydrogen triselenate, (NH4)(4) H-2 (SeO4)(3), in the low-temperature phase at 141 K is studied by X-ray diffr action. The space group symmetry (triclinic P1) and the structure parameter s are determined. By the shift of the Se atom from the center of the SeO4 t etrahedron, it is suggested that the (NH4)(4)H-2(SeO4)(3) crystal in the lo w-temperature phase is ferroelectric with the electric dipole moment along the c-axis. The rotation of the SeO4 tetrahedron suggested by electron para magnetic resonance measurements is discussed.