Introduction of boron in hydroxyapatite: synthesis and structural characterization

Apatites doped with rare-earth ions have been extensively studied due to their potential applications as phosphors or laser hosts. The structure of apatite is based on a network of only tetrahedral PO4 groups. When boron is added, phosphate and OH groups would be partially substituted by borate groups and new calcium borohydroxyapatite with nominal stoichiometry Ca-10{(PO4)(6-x)(BO3)(x)}{(BO3)(y)(BO2)(z)(OH)(2-3y-z)} is proposed. When P/B ratio=7.22, boron atoms are totally introduced in the apatitic lattice, but from P/B= 11 samples are biphased borohydroxyapatite and Ca(OH)(2) and when P/B is lower than 7.22, Ca-3(BO3)(2) is also observed. The infra-red (IR) and Raman spectroscopy and B-11 MAS (magical angle spinning)-NMR experiments prove that boron is introduced as two-fold coordinated boron BO2- in the channels of the apatitic structure and as triangular BO(3)(3-)groups substituting PO4 and OH groups leading to a AB-type borohydroxyapatite. A comparison with a free boron hydroxyapatite shows that P and proton sites are split into several sites in the substituted compounds. (C) 2002 Elsevier Science B.V. All rights reserved.