Powders with spherical nano-sized particles were obtained in a two-stage method according to [24] (Spherical Grains, SG), first involving the precipitation of lanthanide hydroxides from their nitrate aqueous solutions using ammonium hydroxide then a subsequent transformation of the hydroxide precursors into mixed lanthanide phosphates using (NH4)2HPO4 and hydrothermal conditions at pH = 10, T = 200°C and P = 35 bar for 2 hours.