Study of lanthanum orthophosphates polymorphism, in view of actinide conditioning

Abstract In order to perform researches on the substitution lanthanide–actinide in a view of actinide conditioning, a preliminary study of the polymorphism of lanthanum orthophosphates has been carried out by different techniques. LaPO 4 formed by reaction of lanthanum nitrate with phosphoric acid contains 0.5 mol of water in open channel of the hexagonal structure (rhabdophane-type). The combination of thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction and 31 P solid-state nuclear magnetic resonance clearly shows the different steps of the thermal treatment. The zeolitic water evaporates between 180 and 280 °C. After heating up to 700 °C, a monoclinic structure (monazite-type) is formed by compacting the chains of PO 4 tetrahedron alternating with LaO 9 polyhedron.