X-RAY STUDY OF CRYSTAL-STRUCTURES AND PHASE-TRANSITION IN FERROELECTRIC AMMONIUM FLUOBERYLLATE (NH4)2BEF4

The crystal structures of (NH4)2BeF4 have been refined at −84°C (paraelectric phase) and −140°C (ferroelectric phase). The three-dimensional data were collected on a four-circle automatic diffractometer with monochromatized MoKα radiation. The final discrepancy factors are 0.039 (−84°C) and 0.036 (−140°C). The change of the distribution of hydrogen atoms in N(2)H4 ion in the ferroelectric phase is due to the reorientation of this ion. In the ferroelectric phase, BeF4 and NH4 ions slightly deviate from the paraelectric structure, which results in stronger N–H·F hydrogen bonds. Because of the hydrogen bonds, N(21)H4 and N(22)H4 ions, which are considered to owe the occurence of ferroelectricity, order in opposite direction along the polar b axis respectively. This suggests that (NH4)2BeF4 is a ferrielectric rather than a ferroelectric. © 1979, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.