Abstract :
Thermal decomposition characteristics of lanthanide phosphates LnPO4·H2O (Ln: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) prepared by crystallisation from boiling phosphoric acid solution has been presented. On the basis of the effects observed on DTA and DTG curves, while heated up to 1000 °C, the phosphates can be divided into two groups: (a) the first group includes hexagonal (La→Tb), and orthorhombic Dy phosphates, (b) the second group consists of tetragonal (Ho→Lu) and Y phosphates. The group (a) is characterised by two endotherms below 300 °C, associated with dehydration, and one exotherm above 700 °C, associated with transformation of the hexagonal/orthorhombic into monoclinic modification. The temperature of the polymorphic transformation is increasing with the increase of the atomic number of the lanthanide from <750 °C (La, Ce) to >900 °C (Tb, Dy). The group (b) dehydrates within a wide temperature range (100–600 °C) and maintains the tetragonal crystal structure even after heating at 950 °C. However, an increased crystallinity, contraction of the a- and c-axes, and difference in IR spectra suggests topotactical structural changes in (Ho→Lu) and Y phosphates.
