Citrate mediated synthesis of uniform monazite LnPO4 (Ln = La, Ce) and Ln:LaPO4 (Ln = Eu, Ce, Ce + Tb) spheres and their photoluminescence

A simple method for the synthesis of spherical LaPO(4) (monazite) particles with narrow size distribution and tailored size in the 150-500 nm range is reported. The procedure is based on a homogeneous precipitation process at low temperature (120 degrees C) from solutions containing La(3+), citrate and phosphate ions under a very restrictive set of experimental conditions, which involves the use of La nitrate, citric acid and phosphoric acid as precursors and ethylene glycol as solvent. The growth mechanism of the spheres was investigated aiming at explaining the differences in particle size and shape observed when varying the experimental conditions. The applicability of this method for the synthesis of spherical particles of other lanthanide (Ce, Tb, Eu) phosphates is also analyzed. Finally, it is shown that the developed procedure can be used to dope the lanthanum phosphate particles with lanthanide cations, which resulted in spherical phosphors as illustrated for the Eu-doped, Ce-doped and Ce, Tb codoped systems, whose luminescent properties are also evaluated.