Preparation of lanthanide orthophosphates by crystallisation from phosphoric acid solution

Highly crystalline solids of the composition of LnPO4 · nH2O (Ln = La, Ce, Pr, Nd. Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) were prepared by crystallisation from boiling phosphoric acid solution with the initial concentration 2 M/1 of H3PO4 and 0.02 M/1 of Ln. The number of molecules of water (n) was close to 1. The crystallisation process was completed within two hours. Scanning electron micrographs showed that the diameter of the LnPO4 · H2O crystalline particles was increasing within the group of elements from below 1 μm (LaPO4 · H2O) to above 5 μm (TmPO4 · H2O). Such large crystals of lanthanide phosphates cannot be obtained by usually applied precipitation methods. The light lanthanides (La → Sm) formed clusters of shapeless particles with the tendency to form ‘rod-like’ crystals. The heavy lanthanides (Ho → Lu, and YPO4 · H2O) consisted of spherical globules. The TbPO4 · H2O and DyPO4 · H2O, formed a mixture of the spherical and rod-like crystals.