Radiation Physics and Chemistry 69 (2004) 1–6 Study of apatite behaviour in the presence of the radionuclides U and Rn and local modification of their crystalline and electronic structure F.Z. Boujrhal a,b , E.K. Hlil c, *, R. Cherkaoui El Moursli a a Laboratoire de Physique Nucl ! eaire, Facult ! e des Sciences, Av. Ibn Battouta, B.P. 1014, Rabat, Maroc b D! epartement de Physique Appliqu ! ee, Facult ! e des Sciences et Techniques, B.P. 523, B ! eni Mellal, Maroc c Laboratoire de Cristallographie du CNRS , Av. Martyrs, B.P. 166, Grenoble Cedex 09 38042, France Received 12 November 2002; received in revised form 27 June 2003; accepted 3 July 2003 Abstract The ability of apatites to retain radionuclides and improvement in this ability through annealing, has led us to carry out a study of apatite structure and microstructure. Band structure calculations for a number of reference apatites (fluorapatite; chloroapatite and bromoapatite) have been carried out. The results of these calculations show that the substitution on the A-site causes a slight change in the shape of the density of states without any Fermi level shift or bandgap change. However it has been noticed that a change occurred in the charge and potential distributions in the Ca II environment. The Mulliken population and the projected state density analysis are consistent with a highly ionic model for these apatites. r 2003 Elsevier Ltd. All rights reserved. Keywords: Apatite; Retention and stability; Radon emanation; Crystalline and electronic structure; Mulliken population 1. Introduction Apatites generally crystallize in the hexagonal system (space group P6 3 /m). They have the general chemical formula Me 10 (XO 4 ) 6 Y 2 , where Me generally represents a bivalent cation, XO 4 (B-site) a trivalent anion and Y (A-site) a monovalent anion. Apatites are characterized by the ability to accept substitution at all the three sites from a large array of elements, and have a high resistance to temperature and irradiation processes (Boujrhal et al., 2001a; Carp! ena and Lacout, 1997; Carp! ena, 1998). The apatites from the natural nuclear reactor at Oklo (Gabon) show a high ability to confine actinides and fission products, having done so for over two thousand million years (Bros et al., 1996; S" ere, 1996; Carp! ena et al., 1998). Therefore, it could be used in the future as a matrix for conditioning high separate radioactive wastes (Carp! ena and Lacout, 1997; Carp! ena et al., 1998). This work focuses on the radionuclide retention ability and stability to both heat treatment and radio- active damage (fission fragment tracks) of the Moroccan sedimentary apatite material (phosphate and fossilized teeth). The potential for restoring self-irradiation has been also examined (Boujrhal et al., 1999, 2001a, b). In order to understand better the retention property of this mineral, we study the effect of substitution on the surrounding atoms of Ca on site II, in a defined apatite. We examine here the effect of the anion Y substitution (Hlil et al., 2002). The electronic structure calculations of fluorapatite (FAp) [Ca 10 (PO 4 ) 6 F 2 ], chloroapatite (ClAp) [Ca 10 (PO 4 ) 6 Cl 2 ] and bromoapatite (BrAp) [Ca 10 (PO 4 ) 6 Br 2 ] were carried out using the CRYSTAL code (Saunders et al., 1998), and a comparison between the results is presented. ARTICLE IN PRESS *Corresponding author. Tel.: +33-476-88-74-22; fax: +33- 476-88-10-38. E-mail address: hlil@grenoble.cnrs.fr (E.K. Hlil). 0969-806X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00460-2