Ionic Conductivity of Strontium Fluoroapatites Co-doped with Lanthanides

*Khouloud Kthiri, Mohammed Mehnaoui, Samira Jebahi, Khaled Boughzala and Mustapha Hidouri*

## **Abstract**

Britholites derivatives of apatite's that contain lanthanium and neodymium in the serial compounds Sr8La2�xNdx(PO4)4(SiO4)2F2 with 0 ≤ x ≤ 2 were subject of the present investigation. The solid state reaction was the route of preparing these materials. Several techniques were employed for the analysis and characterization of the synthesized powders. The chemical analysis results indicated that molar ratio SrþLaþNd PþSi was of about 1.67 value of a stoichiometric powder. The X-ray diffraction data showed single-phase apatites crystallizing in hexagonal structure with P63/m space group were successively obtained. Moreover, the substitution of lanthanium by neodymium in strontium phosphosilicated fluorapatite was total. This was confirmed by the *a* and *c* lattice parameters contraction when (x) varies coherently to the sizes of the two cations. The infrared spectroscopy and the 31P NMR (MAS) exhibited the characteristic bands of phosphosilicated fluorapatite. The pressureless sintering of the material achieved a maximum of 89% relative density. The sintered specimens indicated that the Nd content as well as the heating temperature affected the ionic conduction of the materials and the maximum was 1.73 � <sup>10</sup>�<sup>6</sup> S cm�<sup>1</sup> obtained at 1052 K for x = 2.

**Keywords:** fluorobritholites, lanthanium-neodymum substitution, sintering, ionic conductivity

## **1. Introduction**

The phosphosilicate apatites containing a coupled substitution of the divalent cation by a trivalent lanthanide or a tetravalent actinide ion and the trivalent groupment PO4 by a tetravalent SiO4 groupment in the general formula Me(XO4)6Y2 (Me: divalent cation; XO4: anionic groupment and Y: monovalent anion) allow to obtain materials called britholite [1–4]. Such materials were found in the natural nuclear reactors Alko of Gabon which demonstrated that they are storing some radionuclides such as uranium U, thorium Th, plutonium Pu and minor actinides like neptinium Np, americium Am and curium Cm [5–8]. Moreover, silicate based apatite samples were found to contain up to 50 wt% of lanthanides (La, Ce, Nd) and actinides (U, Th) in Ouzzal site of Algeria [9]. Hence, britholites were considered as natural nuclear waste disposal and allowing the confinement of radionuclides and some fission byproducts produced by the nuclear industry [10–12]. In fact, many studies indicated that britholites are able to confine radionuclides with continuous irradiation for millions of years with conserved structure and thermal and chemical stability [13, 14]. Indeed, due to the stability and flexibility of their structure, apatites offer many possibilities for substitutions. Moreover, britholite materials favored many cationic and anionic substitutions in their crystallographic structure. These later might be in a total or limited range [15–17]. Therefore, these substitutions are governed by the ionic sizes, the valence, the electronegativity and the polarizability [18]. In this context, several processes have been developed for the preparation of these materials containing various elements such as actinides and lanthanides via solid state reaction or mechanical synthesis [19–27].

On the other hand, many investigations have revealed that britholites might be a good ionic conductor for their use in fuel cells. The conductivity was proved as a thermal process at intermediate temperature range 400–900°C [28–32]. Therefore, the electrical properties allow using the materials as a solid electrolyte in solid oxide fuel cells (SOFCs) [33, 34].

Like-apatite, phosphosilicate apatites have a hexagonal structure and a space group P63/m [15, 35]. Their framework is built on the sixth XO4 groups and the Me is divided between two crystallographic sites: four are located in the site Me(1), coordination 9, and six other are located in the site Me(2), coordination 7. Hence, in order to highlight the capacity of these materials to store non radioactive elements similar to radionuclides as well as their potentialities as ionic conductors, the sintered materials series Sr8La2�xNdx(PO4)4(SiO4)2F2 with 0 ≤ x ≤ 2 were investigated.
