In vanadocalcic oxyapatite Ca-10(VO4)(6)O , the pair (Ca2+, ) can be substituted by the pair (Ln(3+), 1/2O(2-)) (Ln(3+) is a rare-earth ion). In theory, this substitution allows for introducing up to two rare-earth ions into the apatitic lattice. However, a limitation of this substitution depending on the nature of rare-earth was observed. Thus, this substitution is maximum in the case of La3+ and Pr3+ ions and leads to lanthanum or praseodym dioxyapatites Ca(8)Ln(2)(VO4)(6)O-2 (Ln = La, Pr). With regards to rare earths such as neodym, samarium, europium, gadolinum, and terbium, the Ln(3+) content limit varies from one rare earth to another. It decreases when the rare-earth ionic radius declines.