In the framework of the 1991 French law concerning nuclear waste management, several studies have been carried out in order to elaborate crystalline matrices for specific immobilization of the radionuclides. In the case of high level and long-lived minor actinides (Np, Am and Cm), which are high level and long-lived radioactive elements, monazite, a light rare earth (Re) orthophosphate with general formula Re3+PO4 (with Re = La to Gd), has been proposed as a host matrix, thanks to its high resistance to self irradiation and its low solubility. Monazite crystallizes in the monoclinic space group P21/n. In this structure, trivalent cations (Re3+) could be substituted by an equivalent amount of bivalent (A2+) and tetravalent (B4+) cations, allowing the simultaneous incorporation of Am3+, Cm3+ and Np4+. According to Podor's work1, the limit of a tetravalent element incorporation in monazite is related to its size in the ninefold coordination (RIX).