Comparison of the crystallisation and solid state reaction methods for the preparation of rare-earth orthophosphates

Two easy laboratory methods for preparation of rare-earth orthophosphates (crystallisation from phosphoric acid solution and solid-state reaction with (NH₄)₂HPO₄) were compared on the basis of the products’ properties with a focus on their application as new inorganic pigments. The preparation method has a significant influence on optical properties. The samples prepared by crystallisation have lighter and less rich colour and also change colour more under sunlight irradiation. The surface properties analysed by SEM and presence of a greater amount of phosphoric acid in the crystallisation procedure influence the pH and resistivity of aqueous extract of products and thus their corrosion-inhibition properties. The optical properties of rare-earth orthophosphates and their preliminary corrosion tests show their potential application as highly efficient corrosion-inhibition pigments.