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Structural analysis by Rietveld refinement of calcium and lanthanum phosphosilicate apatites
Structural analysis by Rietveld refinement of calcium and lanthanum phosphosilicate apatites Hela Njema1, Khaled Boughzala2,*, Habib Boughzala3, Khaled Bouzouita2 (1. Laboratory of Industrial Chemistry, National School of Engineering of Sfax, BP 1173, Sfax 3038, Tunisia; 2. Preparatory Institute for Engineering Studies of Monastir, Monastir 5019, Tunisia; 3. Laboratory of Materials and Crystallochemistry, Preparatory Institute for Engineering Studies of Nabeul, Nabeul 8000, Tunisia) Received 24 December 2012; revised 15 July 2013
Abstract: Britholites with the general formula Ca10–xLnx(PO4)6–x(SiO4)xF2, (0dxd6) are considered to be promising matrices for the confinement of the by-products in the nuclear industry. A thermodynamic study showed that the stability of these compounds decreased as the substitution rate increased. The present work was an attempt to gain more information about the structural changes induced by the substitution, in order to understand the observed stability decrease. The samples were successfully synthesized as a single-phase apatite by a solid-state reaction between 1200 and 1400 °C. The structural refinement indicated that the La3+ ions preferentially occupied the 6(h) sites. A progressive shift of F– along the c-axis outside its ideal position occurred as a result of the substitution increase. This might be the cause of the observed stability decrease, especially as the energies of the La–O, La–F and Si–O bonds are higher than those of Ca–O, Ca–F and P–O. The distribution of La3+ between the two non-equivalent sites was confirmed by the charge distribution method. Keywords: britholites; Rietveld refinement; infrared spectroscopy; charge distribution method; rare earths
Since the discovery of the Oklo natural reactors in Gabon[1,2], the britholites have gained much interest among the researchers thanks to their potential use as matrices for the storage of the nuclear wastes[3–5]. Indeed, in these reactors, it was observed that these materials have retained actinides and fission products over several million years in their structure[6–9] without significant structure alterations[1,6] while keeping a strong durability on the geological time scale[2,10]. Furthermore, these compounds exhibit good optical and electrical properties[11,12]. These apatites with the general formula Ca10–xLnx(PO4)6–x(SiO4)xF2 where 0dxd6 are obtained by the substitution of (Ln3+,SiO4–4 ) for (Ca2+, PO3–4 ) in the fluorapatite, Ln is a lanthanide. The solid solutions with Ln=La or Nd, are continuous between the two limit formulae Ca10(PO4)6F2 and Ca4Ln6(SiO4)6F2 [13,14]. Through studying their thermodynamic properties, Ardhaoui et al. have found that the stability of these compounds decreases with the increase of the substitution degree; the enthalpy of formation varied from –13350 to –11389 kJ/mol for Ca9La(PO4)5(SiO4)F2 and Ca4La6(SiO4)6F2, respectively[15]. On the other hand, it was shown that the monosilicated apatite is most resistant to irradiation[6]. The explanation for the increase in the enthalpy of formation should therefore be sought in the structural perturbations induced by the substitution, especially as the energies of the bonds of La–O (799 kJ/mol), La–F (598
kJ/mol) and Si–O (799 kJ/mol) are higher than those of Ca–O (402 kJ/mol), Ca–F (527 kJ/mol) and P–O (599 kJ/mol)[16]. The reason for this stability decrease remains unsolved. Although numerous studies have been carried out on the chemical and physical properties of these materials[17–19], to our knowledge, only a few studies, have dealt with their crystal structure, focusing on the monosilicate apatite[20]. Therefore, it seems interesting to conduct a study by the Rietveld method on these solid solutions in order to elucidate the perturbations induced by the replacement of the (Ca2+, PO3–4 ) pair by the (Ln3+, SiO4–4 ) pair into the apatite structure, causing the decrease in stability of these compounds with the substitution increase.
1 Experimental 1.1 Powder preparation The samples with the general formula Ca10–xLax(PO4)6–x(SiO4)xF2, where 0dxd6, were prepared by a solid state reaction from CaCO3, CaF2, La2O3, SiO2, and Ca2P2O7. The latter compound was obtained by heating a stoichiometric mixture of CaCO3 and (NH4)2HPO4 at 900 °C for 10 h. The synthesis reaction occurs as follows: 3CaCO3+(6–x)/2Ca2P2O7+CaF2+xSiO2 +xLa2O3o Ca10–xLax(PO4)6–x(SiO4)xF2+3CO2 (1)
* Corresponding author: Khaled Boughzala (E-mail: [email protected]; Tel.: +216 97 317 794) DOI: 10.1016/S1002-0721(12)60376-7
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JOURNAL OF RARE EARTHS, Vol. 31, No. 9, Sep. 2013
To avoid deviation from stoichiometry, lanthanum oxide was heat-treated at 1000 °C for 24 h just before use. Indeed, when La2O3 was exposed to air, there was formation of the corresponding La2(OH)6–2x(CO3)x with x|1[21]. The starting reagents in suitable proportions, according to the above equation, were thoroughly ground in an agate mortar, pressed into pellets and heat-treated under a dynamic atmosphere of argon at 900 °C for 12 h. After this heat-treatment, which was aimed to remove the volatile species, the pellets were manually crushed and homogenized, and the resulting powders were again uniaxially pressed. According to their SiO2 content and until a pure phase was obtained, the compacts have undergone one or more heat treatments at temperatures between 1200 and 1400 °C for 12 h. The heating rate was 10 °C/min. In the following sections, the compositions with x=0, 1, 2, 3, 4, 5, and 6 will be labeled as Ca10Fap, Ca9LaFap, Ca8La2Fap, Ca7La3Fap, Ca6La4Fap, Ca5La5Fap and Ca4La6Fap, respectively. 1.2 Powder characterization XRD patterns were recorded on a Bruker D8 ADVANCE diffractometer using Cu K radiation. The data were collected in a 2 range from 9° to 80°, with a step size of 0.02° and a scanning step time of 10 s. The crystalline phase identification was done by comparing the experimental XRD patterns to the standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). Structural refinements were performed by means of the Rietveld method[22] using the Fullprof program[23]. The P63/m space group and the atomic positions of the fluorapatite[24] were used as the starting sets in the refinement procedure for the nonsubstituted sample. Then, the refined parameters were utilized as an initial model for the next sample, and so on. A fifth-order polynomial was used to simulate the background. A pseudo-Voigt
function was employed to fit the experimental peak profiles. The refinement of Ca2+ and La3+ occupancy factors has been made by assuming that the two cations are distributed between the two S(1) and S(2) sites, which were imposed to be fully and complementarily occupied. The occupancy factors of oxygen, fluorine, phosphorus and silicon were assumed to be constant, in agreement with the stoichiometry of the apatite. It is worth noting that the refinement has been achieved in several steps, and the parameters obtained after each step were introduced to make the next step. In the last refinement cycles, all parameters were freed. The charge distribution (CD) was calculated using the Chardi-it program[25]. First, a non-integer coordination number (effective coordination number (ECoN)) was computed. Then, the distribution of the formal oxidation number (q) of each atom among all the bonds as a function of ECoN was done. As the computed charges Q is the result of the distribution of q for the cations, the q/Q ratio is a criterion permitting to investigate the isomorphic substitution of cations with different q: a value close to 1 suggests that the site occupancy is well refined[26,27]. The samples were analyzed in the region from 400 to 4000 cm–1 by a FT-IR spectrometer (FT-IR-8400S, Shimadzu Corporation) using the KBr pellet technique.
2 Results and discussion 2.1 Rietveld refinement XRD analysis indicated that all the prepared powders consisted of a pure apatite phase, matching well with the JCPDS card #01-076-0558 for Ca10Fap. Then, the XRD data were submitted to the Rietveld refinement. Details of the refinement conditions, lattice parameters and reliability factors are given in Table 1, while the determined positional parameters, occupancy factors and isotropic thermal factors are compiled in Table 2. Fig. 1 shows a
Table 1 Unit cell parameters and details of Rietveld refinement of samples Formula
Ca10Fap
Ca9LaFap
Ca8La2Fap
Ca7La3Fap
Ca6La4Fap
Ca5La5Fap
Formula weight
1002.60
1104.54
1200.48
1272.42
1392.36
1482.30
Ca4La6Fap 1584.24
System
hexagonal
hexagonal
hexagonal
hexagonal
hexagonal
hexagonal
hexagonal
Space group
P63/m
P63/m
P63/m
P63/m
P63/m
P63/m
P63/m
Z
1
1
1
1
1
1
1 0.9638(4)
Unit cell a/nm
0.9389(4)
0.9436(2)
0.9480(2)
0.9520(5)
0.9562(7)
0.9601(0)
c/nm
0.6886(7)
0.6923(4)
0.6963(4)
0.7000(2)
0.7040(4)
0.7070(2)
0.7113(2)
Volume/nm3
0.52882(3)
0.53390(1)
0.54198(3)
0.5494158)
0.55757(0)
0.56441(7)
0.57227(9)
Density/(g/cm3)
3.148
3.435
3.678
3.845
4.146
4.360
4.596
Zero point 2/(°)
0.0293
0.03630
0.0395
0.0288
0.0294
0.0300
0.0621
Rp
11.00
10.70
9.05
5.68
4.67
3.98
5.11
Rwp
13.80
13.80
12.00
7.36
5.98
5.11
6.68
RB
9.55
8.3
4.81
4.00
3.72
2.72
4.49
RF
4.69
4.79
3.74
2.71
3.12
2.77
4.14
Reliability factors
Hela Njema et al., Structural analysis by Rietveld refinement of calcium and lanthanum phosphosilicate apatites
899
Table 2 Positional and thermal parameters with their standard deviations, and occupancy factors after Rietveld refinement of samples Formula
Ca10Fap
Ca9LaFap
Ca8La2Fap
Ca7La3Fap
Ca6La4Fap
Ca5La5Fap
Ca4La6Fap
Atom
M
X
Y
Z
Site occupancy factor
Beq/10–2 nm
P Ca(I) Ca(II) O1 O2 O3 F P/Si Ca(I) La(I) Ca(II) La(II) O1 O2 O3 F P/Si Ca(I) La(I) Ca(II) La(II) O1 O2 O3 F P/Si Ca(I) La(I) Ca(II) La(II) O1 O2 O3 F P/Si Ca(I) La(I) Ca(II) La(II) O1 O2 O3 F P/Si Ca(I) La(I) Ca(II) La(II) O1 O2 O3 F P/Si Ca(I) La(I) Ca(II) La(II) O1 O2 O3 F
6h 4f 6h 6h 6h 12i 4e 6h 4f 4f 6h 6h 6h 6h 12i 4e 6h 4f 4f 6h 6h 6h 6h 12i 4e 6h 4f 4f 6h 6h 6h 6h 12i 4e 6h 4f 4f 6h 6h 6h 6h 12i 4e 6h 4f 4f 6h 6h 6h 6h 12i 4e 6h 4f 4f 6h 6h 6h 6h 12i 4e
0.4037(10) 1/3 0.0174(7) 0.3121(15) 0.5827(19) 0.3482(12) 0.00000 0.4038(10) 1/3 1/3 0.0143(7) 0.0143(7) 0.3238(15) 0.5868(18) 0.3440(10) 0.00000 0.4005(5) 1/3 1/3 0.0123(3) 0.0123(3) 0.3281(9) 0.5871(10) 0.3411(6) 0.00000 0.3969(9) 1/3 1/3 0.0141(4) 0.0141(4) 0.3304(15) 0.5822(18) 0.3375(10) 0.00000 0.3978(4) 1/3 1/3 0.0132(20) 0.0132(20) 0.3357(8) 0.5872(9) 0.3364(5) 0.00000 0.3991(4) 1/3 1/3 0.0137(7) 0.0137(7) 0.3484(8) 0.5886(9) 0.3346(5) 0.00000 0.4009(6) 1/3 1/3 0.0136(2) 0.0136(2) 0.3245(11) 0.5941(13) 0.3351(8) 0.00000
0.3689(9) 2/3 0.2526(6) 0.4872(17) 0.4576(17) 0.2623(11) 0.00000 0.3747(9) 2/3 2/3 0.2495(6) 0.2495(6) 0.4836(17) 0.4583(17) 0.2653(11) 0.00000 0.3712(5) 2/3 2/3 0.2489(2) 0.2489(2) 0.4866(10) 0.4600(10) 0.2554(7) 0.00000 0.3681(9) 2/3 2/3 0.2526(3) 0.2526(3) 0.4871(7) 0.4659(16) 0.2469(11) 0.00000 0.3672(4) 2/3 2/3 0.2529(16) 0.2529(16) 0.4881(8) 0.4131(8) 0.2497(5) 0.00000 0.3672(5) 2/3 2/3 0.2539(4) 0.2539(4) 0.4904(9) 0.4630(9) 0.2594(6) 0.00000 0.3676(9) 2/3 2/3 0.2527(3) 0.2527(3) 0.4942(5) 0.4689(12) 0.2506(7) 0.00000
¼ 0.0033(6) ¼ ¼ ¼ 0.07571(11) 0.25000 ¼ 0.0011(15) 0.0011(15) ¼ ¼ ¼ ¼ 0.0787(11) 0.2500(12) ¼ –0.0012(7) –0.0012(7) ¼ ¼ ¼ ¼ 0.0713(7) 0.2500(3) ¼ –0.0019(11) –0.0019(11) ¼ ¼ ¼ ¼ 0.0710(10) 0.2633(3) ¼ –0.0016(5) –0.0016(5) ¼ ¼ ¼ ¼ 0.0696(6) 0.2848(3) ¼ –0.0029(5) –0.0029(5) ¼ ¼ ¼ ¼ 0.0750(6) 0.2926(4) ¼ –0.0019(2) –0.0019(2) ¼ ¼ ¼ ¼ 0.0619(5) 0.2998(16)
1 1 1 1 1 1 0.5 1 0.991 0.009 0.840 0.160 1 1 1 0.5 1 0.954 0.047 0.699 0.301 1 1 1 0.5 1 0.863 0.138 0.594 0.406 1 1 1 0.5 1 0.749 0.251 0.504 0.497 1 1 1 0.5 1 0.666 0.334 0.393 0.607 1 1 1 0.5 1 0.425 0.576 0.388 0.612 1 1 1 0.5
1.57(6) 0.42(3) 0.42(3) 1.57(6) 1.57(6) 1.57(6) 0.004(8) 0.89(6) 1.39(8) 1.39(8) 1.39(8) 1.39(8) 0.81(3) 0.81(3) 0.81(3) 0.57(5) 1.67(10) 2.09(11) 2.09(11) 2.09(11) 2.09(11) 2.48(3) 2.48(3) 2.48(3) 3.35(2) 2.47(9) 1.76(3) 1.76(3) 2.01(7) 2.01(7) 0.79(5) 0.79(5) 1.20(5) 1.20(5) 2.26(8) 2.27(9) 2.27(9) 2.27(9) 2.27(9) 2.42(8) 2.42(8) 2.42(8) 0.59(6) 2.13(3) 2.22(6) 2.22(6) 2.22(6) 2.22(6) 2.41(2) 2.41(2) 2.41(2) 0.53(6) 3.736 2.14(2) 2.14(2) 2.14(2) 2.14(2) 4.25(4) 4.25(4) 4.25(4) 1.71(5)
900
Fig. 1 Experimental and calculated X-ray diffraction patterns and their difference for samples (a) x=2; (b) x=4; (c) x=6
good agreement between the observed and calculated XRD patterns for Ca8La2Fap, Ca6La4Fap and Ca4La6Fap samples, taken as examples. For the nonsubstituted sample, the values of a= 0.9389(6) nm and c=0.6886(6) nm are in close agreement with the data reported in the literatures[13,14]. With the substitution of the (La3+, SiO4–4 ) pair for the (Ca2+, PO3–4 ) pair, the refined lattice parameter values increased progressively with x, leading to an expansion of the lattice volume (Table 1). The increase of the lattice parameters confirms that La3+ and SiO4–4 have entered the apatite structure. The explanation of the lattice expansion can be found in the difference in the sizes of the substituted ions. Indeed, the La3+ and SiO4–4 ions are slightly bigger than Ca2+ and PO3–4 , respectively. The radius of Si4+ (0.042 nm) is greater than that of P5+ (0.035 nm), whereas the ionic radii of La3+ and Ca2+ in 7-fold coordination are 0.110 and 0.106 nm, respectively[28]. In the Ca10Fap structure, the fluoride ions are located at the ideal 2a (0 0 ¼) positions, corresponding to the centers of the Ca(2)-triangles. Nonetheless, in manysubstituted Ca10Fap, it is observed that the substitution of Ca+ with bigger ions induces the displacement of F– ions outside the plans containing the equatorial triangles. This was attributed to a steric effect: when the substituent is more voluminous than Ca2+, the space at the center of these triangles is significantly reduced, and the F– ions
JOURNAL OF RARE EARTHS, Vol. 31, No. 9, Sep. 2013
must be accommodated outside[29]. However, in other cases like that of the Sr-La-apatites, although the substituent is less voluminous, it was observed that the Fions also moved outside their ideal positions, when the substituent content increased[30]. Hence, the refinement of the F– ion coordinates was carried out taking into account its delocalization along the z-axis, i.e., 4e position. From Table 2, one can see that for the nonsubstituted apatite, the fluoride ion is located at the ideal position (z=0.25), while for the substituted samples, especially for the high x values, F– is moved progressively along the z-axis. The refined parameter z is 0.2998(16) for Ca4La6Fap. Several studies have analyzed the occupancy of lanthanide ions in the apatite structure. The obtained results indicate that these ions occupy preferentially the S(2) sites[31,32]. Other study has shown that the nature of the tunnel anion plays a significant role in the distribution of La3+ between the two cationic sites[33]. Furthermore, the nature of this anion seems to have a strong influence on the stability of this kind of apatite[15,34]. Table 2 shows that for the low content, La3+ preferentially occupied the S(2) sites, but this trend decreased and tended to a statistical distribution for the high content. This can be corroborated by the comparison of the refined ratio “La3+ ions in S(1) sites/La3+ ions in S(2) sites” with the theoretic statistical one “2/3”: 0.037 and 0.626 for Ca9LaFap and Ca4La6Fap, respectively. The preferential localization of La3+ in the S(2) sites is in agreement with its slightly greater radius with respect to Ca2+. However, in the “La-Sr-britholites”, although La3+ is smaller than Sr2+, it displays the same preference for the S(2) sites[31]. Several properties other than size such as the electronegativity and/or polarizability have been invoked to explain the localization of a given substituent in one of the two sites[29,35]. In the present case, for the couple Ca-La: (1) the difference in ionic radii is small, (2) the electronegativity values are practically the same, (3) however, the difference in the polarizability values is large, and (4) the ionic charges are different (Table 3). It seems, therefore, that the relevant factor for the preferential localization of La3+ in the S(2) sites is its ionic charge, which is higher than that of Ca2+. The alignment of S(1) sites in columns, and the shorter distances between these sites: 0.3416(4) and 0.3529(2) nm for Ca10Fap and Ca4La6Fap, respectively cause strong repulsions between these ions, while the higher distances between the S(2) sites, 0.3886(7) and 0.4041(4) nm, respectively make them more suitable to minimize the repulsions. Furthermore, the La3+ polarizability, much higher than that of Ca2+, should certainly play a role in the distribution of this ion between the two sites, and consequently on the delocalization of F– along the z-axis. Indeed, the slight difference in the radii of these cations cannot be the sole driving force for the delocalization of F–, especially as indicated above, the same
Hela Njema et al., Structural analysis by Rietveld refinement of calcium and lanthanum phosphosilicate apatites
delocalization was observed for the Sr-La system[30], La3+ being less voluminous than Sr2+ (Table 3). Referring to the HSAB theory[36], Ca2+ is a hard acid, giving mainly ionic interactions with the fluoride ion, whereas La3+ is also a hard acid, but its polarizability is of about 2.2 times higher than that of Ca2+ (Table 3); it must rather display a tendency to provide interactions with a partial covalent character. On the other hand, F– is a hard base, having a little affinity for polarizable cations. Therefore, with increasing La3+ content in S(2) sites, it tended to move away from these cations, leading to the weakening of the Ca/La–F bond, the increase of its length, and consequently to the decrease of the structure stability. It seems hence that the La3+ polarizability outweighs the steric effect. A confirmation of the previous statement can be found in the study of Laghzizil et al. showing that the fluoride ion mobility is notable in the presence of polarizable cations[37]. Also, supporting evidences for the previous hypothesis can be marshaled from the data of the oxybritholites (Ca10–xLax(PO4)6–x(SiO4)xO). Compared to fluorbritholites, the oxy-britholites differ only by the tunnel anion. If their analogous Ca-oxybritholites behaved like ‘‘Sr-oxybritholites’’, we can transpose, for the first series, the refined results obtained for the latter compounds, for which the refined ratios “La3+ ions in S(1) sites/La3+ ions in S(2) sites” are 0.004 and 0.529 for Sr9LaOap and Sr4La6Oap, respectively[38]. Thus, the selectivity for the S(2) sites is maintained. Otherwise, unlike to the “Sr-fluorbritholites”, a progressive shift of the free oxygen, O(4), towards the centers of the triangles formed by the Sr/La-atoms in the (6 h) positions was observed for the”Sr-oxybritholites”. Its position along the z-axis is z=0.326 and 0.265 for Sr10Oap and Sr4La6Oap, respectively[38]. This progressive shift can be explained by the increase of the covalent character of the Sr/La(2)–O(4) bond with the substitution. Accordingly, this bond is enhanced[31,39], providing a greater stability to the apatite structure. Thus, for Ca-Laoxybritholites, the polarizability and the ionic charge of lanthanum, higher than that of the calcium, probably play a role in the rearrangement of oxygen atoms around the S(2) site. This rearrangement leads to a more compact oxygen packing around this site and a strengthening of the La…O(4) interactions, contributing to the increase of the stability of the compounds. Therefore, all others being equal, the nature of the tunnel anion seems to play an important role in the stability of the britholites.
901
As seen in Fig. 2, there is a good correlation between the site occupancy and the stoichiometric compositions. To confirm the La3+ distribution between the two nonequivalent sites, the charge distribution (CD) analysis was carried out using the Chardi-it program[25]. The CD analysis is given in Table 4. The computed charges of both cationic and anionic sites are consistent with their formal charges, the values of the q/Q ratios are close to 1. Furthermore, the difference between the sum of the positive charges and the sum of the negative charges is lower than 0.05 for all the studied compositions, confirming that the increase in the cationic charge of sites due to the substitution of Ca2+ by La3+ is balanced by the substitution of SiO4–4 for PO3–4 . All this indicates that the structures of all compositions are well-refined. As seen from Table 5, with the incorporation of SiO4–4 into the apatite structure, the mean
values increased in agreement with the difference of the Si–O and P–O distances in SiO4–4 and PO3–4 respectively. Nonetheless, the mean Si–O value (0.1660 nm) in the silicate apatite (x=6) is more important than the Si–O distance (0.162 nm) in the SiO4 tetrahedron[28]. Otherwise, the mean
Table 3 Radii, electronegativity and polarisability of Ca2+, Sr2+ and La3+ Ion characteristics
Coordination
Radii/nm Electronegativity –30
Polarisability/(10
3
m)
Ca2+
Sr2+
La3+
Coord 7
0.106
0.121
0.110
Coord 9
0.118
0.131
0.122
1.00
0.95
1.10
5.91
10.81
13.07
Fig. 2 Correlation between the site occupancy and the stoichiometric composition
902
JOURNAL OF RARE EARTHS, Vol. 31, No. 9, Sep. 2013
Table 4 Charge distribution of samples Formula
Ca10Fap
Ca9La Fap
Ca8La2 Fap
Ca7La3 Fap
Ca6La4 Fap
Ca5La5 Fap
Ca4La6 Fap
Elements
q
Q
q/Q
P
5.000
5.018
0.997
Ca (I)
2.000
1.924
1.040
Ca (II)
2.000
2.033
0.984
O1
Q
+50.00
0.060
–50.00
0.092
+50.00
0.022
–50.02
0.125
–2.000 –1.988 1.006
O2
–2.000 –2.144 0.933
O3
–2.000– –1.933 1.035
F
–1.000 –1.007 0.993
P/Si
4.871
4.876
0.999
Ca/La (I)
2.023
2.025
0.999
Ca/La (II)
2.138
2.108
1.014
O1
2.000
–1.827 1.095
O2
–2.000 –2.127 0.940
O3
–2.000 –2.023 0.989
F
–1.000 –1.006 0.994
P/Si
4.763
4.776
0.997
Ca/La (I)
2.027
1.985
1.021
Ca/La (II)
2.301
2.233
1.030
O1
– 2.000 –1.982 1.009
O2
–2.000 –2.083 0.960
O3
–2.000 –1.961 1.020
F
–1.000 –1.007 0.993
P/Si
4.594
4.601
0.998
Ca/La (I)
2.118
2.040
1.038
Ca/La (II)
2.406
2.372
1.014
O1
–2.000 –1.987 1.007
O2
–2.000 –2.146 0.932
O3
–2.000 –1.934 1.034
F
–1.000 –1.003 0.997
P/Si
4.336
4.357
0.995
Ca/La (I)
2.231
2.229
1.001
Ca/La (II)
2.496
2.490
1.002
O1
–2.000 –1.928 1.037
O2
–2.000 –2.036 0.982
O3
–2.000 –2.011 0.995
F
–1.000 –1.003 0.997
P/Si
4.159
4.197
0.991
Ca/La (I)
2.314
2.253
1.027
Ca/La (II)
2.607
2.634
0.990
O1
–2.000 –1.958 1.021
O2
–2.000 –2.147 0.932
O3
–2.000 –1.934 1.034
F
–1.000 –1.008 0.992
Si
4.000
4.021
0.995
Ca/La (I)
2.555
2.585
0.988
Ca/La (II)
2.612
2.589
1.009
O1
–2.000 –1.855 1.078
O2
–2.000 –2.085 0.959
O3
–2.000 –2.021 0.990
F
–1.000 –1.002 0.998
+50.00
0.057
–49.94
0.054
+50.00
0.060
–50.01
0.093
+50.00
0.016
–49.92
0.047
+50.00
0.054
–49.85
0.096
+50.00
0.170
–49.89
0.098
given in Fig. 3. The assignments of the absorption bands were done according to the literature[14,41,42]. The x=0 sample presents the absorption bands associated to the PO3–4 groups in an apatitic environment between 1074 and 1040, 600 and 575 and 467 cm–1. These frequencies correspond to the vibration modes 3, 4 and 2, respect-
Fig. 3 FTIR spectra of samples
tively, whereas the absorption band associated to the symmetric stretching mode ( 1) was detected at 961 cm–1. For the substituted samples, the spectra exhibit some changes, which should be ascribed to the substitution of SiO4–4 for PO3–4 within the apatite lattice. For the x=1 sample, the additional bands detected at 927, 885 and 513 cm–1 and the shoulder at 448 cm–1 can be assigned to the SiO4–4 groups. They correspond to the Si–O stretching vibration modes ( 3 and 1), and Si–O–Si bending modes ( 4 and 2), respectively. The appearance of these bands suggests that P was partially substituted by Si. With increasing Si content, it can be observed that: (1) the intensities of the SiO 4– 4 -derived bands increase, and concomitantly those of the PO3–4 groups decrease; (2) roughly, the frequencies of the vibration modes of the silicate group shift towards the lower values. For the PO3–4 group, the frequencies 1 and 2 modes decrease gradually according to x, while for the vibration modes 3, the bands move towards the higher values. For the latest group, the shift of the frequencies of the vibration modes might be due to the increase of the Si content, on the one hand, and the decrease of the covalent character of the P-O bonds on the other hand. Indeed, with the substitution, there is increase of the covalence of the Ca/La–O bonds with the insertion of La3+ within the apatite lattice, La3+ having a polarisability value much higher than that of Ca2+ (Table 3)[29]. In the same way, the decrease of the frequencies for SiO 4– 4 can be explained by the reinforcement of the Ca/La–O bonds. This shift might also be related to the increase of the disorder around the P/Si sites due to the substitution[36]. When all the phosphate groups were replaced by the silicate groups, the spectrum was typical of the silicate apatite showing only the SiO4–4 -derived bands.
3 Conclusions A thermodynamic study showed that for the series Ca10–xLax(PO4)6–x(SiO4)xF2 with 0x6, obtained by the substitution of the (La3+, SiO4–4 ) pair for the (Ca2+, PO3–4 )
Hela Njema et al., Structural analysis by Rietveld refinement of calcium and lanthanum phosphosilicate apatites
903
Table 5 Bond lengths (10–1 nm) and angles (°) with their standard deviations of samples Formulae
Ca10Fap
Ca9LaFap
Ca8La2Fap
Ca7La3Fap
Ca6La4Fap
Ca5La5Fap
(P/Si)–(O1)
1.603(8)
1.541(5)
1.554(12)
1.552(2)
1.615(9)
1.487(11)
Ca4La6Fap 1.712(6)
(P/Si)–(O2)
1.462(18)
1.529(13)
1.532(9)
1.533(16)
1.568(8)
1.577(8)
1.612(9)
(P/Si)–(O3) x2
1.477(9)
1.498(7)
1.565(6)
1.608(9)
1.600(4)
1.579(5)
1.658(2)
<(P/Si)–O>
1.510
1.517
1.554
1.575
1.596
1.555
1.660
(O1)–(P/Si)–(O2)
116.1(19)
115.8(7)
114.1(11)
108.9(18)
111.3(6)
106.1(9)
110.2(6)
(O1)–(P/Si)–(O3) x2
108.5(14)
110.2(9)
111.3(9)
113.6(14)
111.4(9)
113.7(7)
109.2(7)
(O2)–(P/Si)–(O3) x2
102.6(19)
107.3(6)
107.3(8)
108.8(10)
108.7(6)
110.0(5)
110.1(7)
(O3)–(P/Si)–(O3)
108.3(8)
105.8(7)
105.2(5)
102.9(7)
105.0(4)
103.2(4)
107.6(2)
109.43
109.43
109.42
109.43
109.42
109.45
109.40
(Ca1/La1)–(O1) x3
2.340(14)
2.430(14)
2.426(8)
2.454(13)
2.436(6)
2.515(7)
2.416(5)
(Ca1/La1)–(O2) x3
2.449(14)
2.439(11)
2.431(10)
2.510(14)
2.473(9)
2.468(6)
2.494(8)
(Ca1/La1)–(O3) x3
2.785(12)
2.812(10)
2.832(8)
2.863(8)
2.884(7)
2.916(5)
2.911(9)
<(Ca1/La1)–O>
2.525
2.560
2.563
2.609
2.598
2.633
2.607
(Ca1/La1)–(Ca1/La1)x3
3.411(11)
3.461(16)
3.463(7)
3.486(11)
3.498(5)
3.494(5)
3.529(2)
(Ca1/La1)–(Ca2/La2)x3
3.924(6)
3.960(8)
3.991(3)
4.000(5)
4.006(3)
4.019(18)
4.041(4)
(Ca2/La2)–(Ca2/La2)x3
3.991(7)
3.972(8)
3.986(3)
4.065(6)
4.085(5)
4.114(3)
4.109(5)
(Ca2/La2)–(O1)
2.543(12)
2.640(12)
2.699(7)
2.715(11)
2.696(7)
2.860(6)
2.724(1)
(Ca2/La2)–(O2)
2.51(2)
2.689(2)
2.492(12)
2.45(2)
2.463(3)
2.469(11)
2.438(6)
(Ca2/La2)–(O3) x2
2.392(8)
2.413(8)
2.326(7)
2.384(7)
2.374(3)
2.412(4)
2.340(2)
(Ca2/La2)–(O3) x2
2.595(10)
2.561(10)
2.539(5)
2.516(8)
2.542(2)
2.539(4)
2.602(7)
<(Ca2/La2)–O>
2.505
2.546
2.487
2.494
2.499
2.539
2.508
(Ca2/La2)–(F) x2
2.304(6)
2.293(6)
2.302(2)
2.403(6)
2.371(7)
2.394(20)
2.398(9)
pair in the fluorapatite, the stability of the compounds decreased with the increase of the substitution degree. Therefore, this series was prepared by a solid-state reaction and investigated by the X-ray diffraction and infrared spectroscopy. The main result of the structural refinement was the shift of fluorine outside its ideal position (0, 0, 1/4) with the increase of the substitution. This shift could be the cause of the observed stability decrease. The charge distribution method confirmed the distribution of La3+ between the two cationic sites, with a marked preference for the Ca(2) sites for the low La contents. The IR spectroscopy indicated that the substitution of PO3–4 by SiO4–4 group was successfully performed.
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