X-ray Structure Refinement and Vibrational Spectroscopy of Ca8Gd2 (PO4)6 O2

Laboratory of the Physico-Chemistry of Solid States. LR11 ES51. of Sfax. Road of Soukra km 4. Sfax3071. Tunisia. Email: rayda_ayadi @yahoo.fr M. Boujelbene Laboratory of the Physico-Chemistry of Solid States. LR11 ES51. of Sfax. Road of Soukra km 4. Sfax3071. Tunisia. * Corresponding author: m_boujelbene2010 @yahoo.fr T. Mhiri Laboratory of the Physico-Chemistry of Solid States. LR11 ES51. of Sfax. Road of Soukra km 4. Sfax3071. Tunisia. Email: tahermhiri@yahoo.fr Abstract The present paper is interested in the study of compounds from the apatite family with the general formula Ca10 (PO4)6A2. It particularly brings to light the exploitation of the distinctive stereochemistries of two Ca positions in apatite. In fact, GdBearing oxyapatite Ca8 Gd2 (PO4)6O2 has been synthesized by solid state reaction and characterized by X-ray powder diffraction. The site occupancies of substituents is 0.3333 in Gd and 0.3333 for Ca in the Ca(1) position and 0. 5 for Gd in the Ca (2) position. Besides, the observed frequencies in the Raman and infrared spectra were explained and discussed on the basis of unit-cell group analyses.

A compact arrangement of PO4 tetrahedrons constitutes the skeleton of this structure which exhibits two kinds of tunnels parallel to the c-axis.
The first is occupied by four M (1) cations at 4f sites, along a three -fold axis. These cations are coordinated by nine oxygen atoms. The second tunnel which is the larger is occupied, on its periphery, by the six other M (2) cations at 6h sites, along a six-fold axis. These M (2) cations which are surrounded by six oxygen atoms and one Y atom two alternated equilateral triangles at level ¼ and ¾ centred on a sixfold axis where the Y atoms are located.  (2) (see the procedure), confirmed the volatilized of bismuth. Therefore, 16% de bismuth was volatilized. The refined results indicated that the Bi 3+ ions were mainly located in Ca (2) site on 6h position and formed two triangles that rotated 60° from the c-axis. The oxygen atom O4 was located in the center of these Bi-triangles [17].
The compounds Bi Ca4 (PO4)3 O and La Ca4 (PO4)3O have been reported recently [18]. They are isostructural with Bi Ca4 (VO4)3 O [19].Which is closely related to the apatite structure except for the number of cationic sites available. Bi Ca4 (VO4)3 O crystallizes in hexagonal symmetry with space group P63/m have only two types of cationic sites [20,21] where as Bi Ca4 (VO4)3 O is reporter to have three types of cationic sites viz; Ca(1), Ca(2) and Ca (3). The formula can be written as Ca (1) In the present work, we propose to investigate the structure of the phosphate apatite and the vibrational spectroscopy of the Ca8Gd2 (PO4)6O2 compound.

Experiment
The Ca8Gd2 (PO4)6O2 compound was obtained by the solid-state reaction of Gd2O3(Merck. 98. 9%), P2O5 (Merck. 98. 9%) and CaCO3 (Cerac. 99.95%), as shown in the following formula: The resultant powder was subsequently heated at 740°C during 12 h, in slow cooling conditions. X-ray powder diffraction (XRD) pattern was determined by means of a Panalytical XPERT PRO MPD diffractometer equipped with a detector X'cellerator operating with a secondary monochromator and using a CuKα radiation source (Kα1 = 0.15439 nm and Kα2 = 0.15440 nm). The diffraction pattern was recorded under ambient atmosphere over an angular range of 5-80° (2θ) with a step length of 0.033° (2θ).
The Fourier transform infrared (FT-IR) measurements were performed at room temperature. On a JASCO FT-IR 420 spectrometer over the 4000 -400 cm -1 region, in a KBr pellet. Furthermore, Raman spectra were measured with a LABRAMHR 800 triple monochromatic at room temperature under a 50 × LF objective microscope, a He-Ne ion laser operating at about 300 mW was used (on the triple) as an excitation source (514.532 nm), with a spectral steps of 3 cm -1 .

Refinement of the structure
The structure of the compounds in the solid are closely related to those of the common phosphate apatite. They have been frequently described in the literature [22]. They have been commonly determined by X-Ray powder diffraction using the Rietveld method refinement stating from the isostructural phase Ca10 (PO4)6F2.
The analysis of the final adjustments carried out for the observed and calculated diagrams indicated that there were nonindexed lines. The latter could be identified as minor impurities. The latter could be identified as Ca3 (PO4)2.
The final results of this refinement are presented in Table. 1, Table. 2 (for the structure parameters), Table. 3 (for the atomic positions) and Table. 4 (the bond length distances and angles). Besides Fig. 1 shows the observed, calculated and different X-ray profiles of the powder diffraction of these apatite phosphates. N o v e m b e r 2 6 , 2 0 1 3   The cations M (1) (Ca1/Gd1) were coordinated to nine oxygen anions belonging to six distinct tetrahedral. Each polyhedron was linked to three PO4 tetrahedra via corners and to three other tetrahedra via edges (Fig. 3.). The M (2) (Ca2/Gd2) cations are inserted into six -fold sites that constituted the walls of the tunnels. Each polyhedron was linked to four PO4 tetrahedra via corners and to one PO4 via edge and two of the free oxygen O4 (Fig. 4.).
In the case of the M (1)-O distances, the nine distances have an average value of 2.658(5) Å. which is slightly larger than the one observed in calcium-Fluorapatite (2.414(13) Å). In the case of the M (2)-O distances, the average value is 2.558(4) Å, which is similar than in calcium Fluorapatite 2.535(12) Å [23].

Spectroscopy analysis
The IR and Raman spectra are shown in Fig. 5 and Fig. 6 respectively. The spectral data and proposed vibration assignment is listed in Table. 5. As shown in the Raman spectrum. One strong band at 963 cm -1 , was observed, which can be attributed to ν1 (PO4). The position of these bands were similar to those (933 and 963 cm -1 ) previously reported by Toumi [24]. The weaker peaks observed at 1040, 1058 and 1082 cm -1 and those recorded at 539, 607 and 642 cm -1 which can be accredited to the asymmetric stretching ν3 and the asymmetric bending modes ν4 of PO4 groups, respectively. They were observed at 575/600 cm -1 and at 545/575 cm -1 in Pb10 (PO4)6F2 and Ca10 (PO4)6F2 [25], respectively. Regarding the weak lines observed at 432 and 445 cm -1 . They could be assigned to the symmetric bending ν2 mode.

Conclusion
The results from X-ray refinement has shown that the formula assigned to the new Gd substituted Ca-apatite was Ca8Gd2 (PO4)6O2. The analysis of data from vibrational spectroscopy has also provided support for the high symmetry P63/m space group.