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 m (LaPO₄ · H₂O) to above 5 m (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.