ABSTRACT
We review recent understanding of the crystallographic phases and its relation to ferroelectricity in ammonium lithium sulphate (NH4LiSO4) using the crystal structural data reported before and present our recent idea about its ferroelectricity. In this paper, we also report phase transitions and crystal structure at room temperature of tori-ammonium lithium di-sulphate ((NH4)3Li(SO4)2) from our recent experimental results. NH4LiSO4 which is one of the (NH4)2SO4 family ferroelectrics and (NH4)3Li(SO4)2 which is one of the A3B(XY4)2-type derived from the ABXY4-type were crystallized by evaporating the aqueous solution at room temperature. The phase transition and crystal structure were studied by means of thermogravimetric analysis, differential scanning calorimetry and single-crystal X-ray diffraction. In addition, phase transitions and ferroelectricity of NH4LiSO4 will be discussed from the crystal structural changes.