Neutron structures of ammonium tetrafluoroberyllate

It is thought that hydrogen bonding is responsible for the ferroelectricity in ammonium tetrafluoroberyllate, (NH4)(2)BeF4. In the past X-ray data hav e been collected, but these did not permit accurate determination of the H- atom positions. In order to obtain more accurate information the neutron st ructures have now been determined for the paraelectric and ferroelectric ph ases. Going from the paraelectric to the ferroelectric phase, both the BeF4 2- and the NH4+ ions rotate and shift from the mirror planes of the paraele ctric phase. This results in removal of the mirror-plane symmetry and forma tion of a superlattice with the a axis doubled. Along the polar c axis, the NH4+ ions move towards the BeF42- ions within chains of molecules and the chains move slightly relative to one another. The rotations and translation s give rise to stronger hydrogen-bonding interactions.