The structure of ammonium hydrogen sulphate in its ferroelectric phase and the ferroelectric transition

Abstract

The structure of ammonium hydrogen sulphate (AHS) has been determined in an (010) projection from X-ray diffraction data collected photographically at - 81°C. At this temperature AHS is in its ferroelectric phase. The asymmetric unit of the pseudo-orthorhombic unit cell (space group Ba) contains four formula units which are related in pairs by a centre of inversion in the non-ferroelectric phase above - 3°C. It is shown that the ferroelectric structure corresponds to an ordering of sulphate groups which are disordered in the room-temperature structure, an ordering which is antisymmetric with respect to the room-temperature centre of inversion. This is the principal change: the relative movements of the other atoms are comparatively small.