Study of lanthanum orthophosphates polymorphism, in view of actinide conditioning

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. LaPO4 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 31P 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 PO4 tetrahedron alternating with LaO9 polyhedron.