J Sol-Gel Sci Technol (2009) 51:287–294 DOI 10.1007/s10971-008-1854-5
5TH INTERNATIONAL CONFERENCE ON SOL-GEL MATERIALS- TRZEBIESZOWICE, POLAND
Effect of strontium substitution on the composition and microstructure of sol–gel derived calcium phosphates G. Renaudin Æ E. Jallot Æ J. M. Nedelec
Received: 2 September 2008 / Accepted: 24 October 2008 / Published online: 18 November 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Sr-doped calcium phosphates have been prepared by sol–gel chemistry. All samples exhibit two phases: hydroxyapatite (HAp) and tricalcium phosphate (b-TCP). With respect to undoped sample, the Sr-doped samples exhibit higher proportion of b-TCP phase but the quantity appears to be quite independent of the doping level. To explain the mismatch with the nominal stoichiometry, the presence of amorphous CaO and SrO compounds have been postulated and their proportions evaluated. The insertion of Sr2? ions in the two crystalline phases HAp and b-TCP is almost total for low doping levels but quite incomplete for the highest doping level. The majority of the inserted Sr2? ions are in the b-TCP phase. Considering the acknowledged beneficial effect of Sr2? on the bone regeneration process, the effective partial substitution of Sr in biphasic calcium phosphate makes these materials very interesting for clinical applications. The Sr-substituted HAp and b-TCP cell parameters agree fairly well with the Vegard’s law and Sr2? ions substitute preferentially for Ca2? in the Ca2 site for hydroxyapatite and in the Ca4 site for b-TCP. The microstructural parameters confirm the previous observation and give a new evidence of clear stabilizing effect of Sr2? ions towards the b-TCP structure. G. Renaudin J. M. Nedelec (&) Laboratoire des Mate´riaux Inorganiques CNRS UMR 6002. Clermont Universite´, Universite´ Blaise Pascal & Ecole Nationale Supe´rieure de Chimie de Clermont-Ferrand, 24 avenue des Landais, 63177 Aubie`re Cedex, France e-mail:
[email protected] E. Jallot Laboratoire de Physique Corpusculaire de Clermont-Ferrand CNRS/IN2P3 UMR 6533. Clermont Universite´, Universite´ Blaise Pascal, 24 avenue des Landais, 63177 Aubie`re Cedex, France
Keywords Calcium phosphates Strontium Osteoporosis Rietveld refinement Microstructure Bone Substitutes Implants
1 Introduction Osteoporosis is becoming a major health issue because of the global aging of the population. This disease characterized by an important bone mass loss is particularly severe for women especially over 60. Preventive treatments have been proposed to avoid osteoporotic bone fractures usually localized in long bones, hips and vertebra. The objective is really to prevent the first incident to occur. Unfortunately, in many cases, a fracture is observed and the oral treatment in this case aims at reducing the risk of a second accident. Oral treatments are mainly based upon calcium. Other drugs like Raloxifene are especially designed to regulate the hormonal system which is an important cause of osteoporosis for post-menopausal women. More recently, strontium based drugs have been proposed like strontium ranelate. In effect, strontium is naturally present in bone and various studies investigating the therapeutical and detrimental effects of Sr have been carried out on animals and humans [1–7]. In vitro and in vivo studies have indicated that oral strontium intake not only increases bone formation, the number of bone-forming sites and the bone mineral density, but also reduces bone resorption [1, 5–7]. Sr is used for treatment of osteoporosis [8, 9], and was found to induce osteoblast activity when introduced in biocompatible bone cements [10–14]. Very recently, serious concerns appeared with conventional osteoporosis treatments. Drug Rash with Eosinophilia and Systemic Symptoms (DRESS) appeared in several
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patients treated with Protelos (strontium ranelate) leading to modified recommendation with the use of this drug. Calcium itself is also questioned. Zhu et al. [15] recently showed in a five years study that calcium supplements even if successful initially in reducing bone turnover, just do not do better that placebo after 3 to 5 years. Another study by Bolland et al. [16]. even showed an increase of occurrence of heart attacks and other cardiovascular problems in healthy postmenopausal women taking calcium supplements. The use of drugs to fight against osteopenia (preosteoporosis) is also strongly criticized by a recent study [17] on raloxifene showing that presentation of the results over-exaggerates the effectiveness of the treatment. As a consequence, the development of alternative bone substitutes and coatings for metallic prostheses is thus a very topical issue. In this case, the material replacing bone after a first fracture is designed to decrease the risk of further fracture. The incorporation of strontium in calcium phosphate cements has been largely studied in the last decade [10, 12, 18–27]. Due to its acknowledged biological activity, we have been considering strontium as a doping element in bioactive ceramics to prepare materials with specific properties such as anti-inflammatory or anti osteoporotic [28]. The remarkable properties of these materials are connected to their ability to release strontium at physiological levels during their interaction with the biological medium. Recently, we proposed a detailed description of strontium substitution in hydroxyapatite [29]. The purpose of the present study is to examine the effect of Sr2? concentration on the composition and microstructure of calcium phosphates samples prepared by sol–gel chemistry. Sol–Gel process has been used because of its many advantages like the lower temperature involved in the preparation and the possibility to prepare materials as thin films particularly valuable for metallic prostheses coating. Furthermore, as far as doping is concerned, sol–gel process has proven its superiority in term of homogeneity compared to classical solid state routes. This is directly connected to the definitive homogeneity reached by the use of precursors in solution.
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a white consistent gel and further heated at 80 °C during 10 h to obtain a white powder. Finally, the powder was heated at 1100 °C during 15 h. To prepare Sr-substituted hydroxyapatite, the required amount of Sr(NO3)2 (Aldrich) was added to the solution. Five samples have been studied with various strontium concentrations 0, 0.5, 1, 2, and 5 at.%. The samples are labelled HA for undoped hydroxyapatite (Ca5(PO4)3 OH) and HASrx, x being the Strontium at.%. The doped samples thus correspond to a nominal composition Ca5(1-x/100)Sr5x/ 100(PO4)3 OH. 2.2 Chemical analysis of HASrx samples The chemical composition of the hydroxyapatite powders were determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on a Jobin Yvon 70 Ultima C spectrometer. Solid samples were first submitted to alkaline fusion with LiBO2 (500 mg ? 100 mg sample) in a graphite crucible. The resulting fusion cake was then dissolved with molar nitric acid. Diorite sample was used for calibration. The nominal and experimental compositions of the different HAp samples are listed in Table 1. As usually observed for sol–gel derived ceramics [32], the nominal compositions have been readily achieved. 2.3 X-rays powder diffraction measurements and Rietveld refinements Powder X-ray diffraction (PXRD) patterns were recorded on a X’Pert Pro Philips diffractometer, h–h geometry, equipped with a solid detector X-Celerator and using Cu ´˚ Ka radiation (k = 1.54184 A ). Powder pattern were recorded at room temperature in the interval 3° \ 2h \ 120°, with a step size of D2h = 0.0167° and a counting time of 70 s for each data value. A total counting time of about 70 min was used for each sample. Si powder
Table 1 Nominal and experimental concentrations (weight %) of Ca, P, Sr in HASrX samples determined by ICP-AES
2 Experimental 2.1 Sol–gel elaboration of Sr-substituted calcium phosphates The sol–gel route previously proposed by the authors [30, 31] has been used. Briefly, to produce 2 g of pure HAp powder, 4.7 g of Ca(NO3)2 4H2O (Aldrich), 0.84 g of P2O5 (Avocado Research chemicals) were dissolved in ethanol under stirring and refluxed at 85 °C during 24 h. Then, this solution was kept at 55 °C during 24 h, to obtain
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HA Ca Nominal
HASr0.5 HASr1 HASr2 HASr5
39.88 39.60
39.31
38.73
37.02
Ca Experimental
38.46 37.22
39.30
39.51
38.79
P Nominal
18.50 18.45
18.41
18.32
18.07
P Experimental
17.21 17.29
18.55
18.58
18.42
Sr Nominal
–
0.43
0.86
1.73
4.26
Sr Experimental
–
0.41
0.81
1.61
4.23
% Substitution theoretical % Substitution experimental
– –
0.5 0.48
1 0.94
2 1.87
5 5
2% 1% 0.5%
1.67
–
(Ca ? Sr)/PO4
Sr/(Sr ? Ca)
Minor CaO and SrO amounts were estimated assuming the refined values for the main crystalline phases and the synthesis conditions: (Ca ? Sr)/PO4 = 1.67 and Sr/(Sr ? Ca) = x at.%. The refined compositions of the different phases are also given
5%
1.67 1.67 1.67 1.67
0.4
3.1 SrO 0.6 SrO 0 SrO 0 SrO SrO Strontia
–
34.8 Ca2.925Sr0.075(PO4)2
CaO 2.3
44.3 Ca2.939Sr0.061(PO4)2
CaO 2.0
33.3 Ca2.941Sr0.059(PO4)2
CaO 2.3
34.8 Ca2.961Sr0.039(PO4)2
CaO 0.5 Lime
8.0 Ca3(PO4)2
CaO
b-TCP
61.7 Ca4.91Sr0.09 (PO4)3(OH)
2.05
52.8 Ca4.952Sr0.048 (PO4)3(OH)
1.44%
64.7 Ca4.964Sr0.036 (PO4)3(OH))
1.13%
63.3 Ca4.970Sr0.030 (PO4)3(OH)
0.8%
91.5 –
Ca5(PO4)3(OH
Sr/(Sr ? Ca)
HAp Estimated assuming the synthesis ratios
(Ca ? Sr)/PO4
%
36.1 Ca2.925Sr0.075(PO4)2
1.60
45.6 Ca2.939Sr0.061(PO4)2
1.59
34.0 Ca2.941Sr0.059(PO4)2
1.61
35.5 Ca2.961Sr0.039(PO4)2
1.60
8.0 Ca3(PO4)2
1.65
b-TCP
Ca4.91Sr0.09 (PO4)3(OH)
Formula %
54.4 Ca4.952Sr0.048 (PO4)3(OH)
Formula %
66.0
Formula
Ca4.964Sr0.036 (PO4)3(OH)
%
64.5 Ca4.970Sr0.030 (PO4)3(OH)
Formula
Ca5(PO4)3(OH) HAp
% Formula Phases Refined
92.0
HASr5 HASr2 HASr1 HASr0.5 HA Sample
Fig. 1 X-Ray Powder patterns for the various samples. Diffraction peaks corresponding to HAp are indicated with * and the ones corresponding to b-TCP with
Table 2 Results of the quantitative phase analyses (wt%) using the Rietveld method
pattern was collected (from pure silicon standard) by using the same experimental conditions in order to extract the instrumental resolution function. The XRD patterns of the different samples are presented in Fig. 1. Powder pattern were analyzed by Rietveld refinement with FullProf.2k [33]. The two following main crystalline phases are considered: hydroxyapatite (HAp) and tricalcium phosphate (b-TCP). The initial structural parameters [34, 35], line profile models and refinement sequences were those described in our previous study on Sr substitution in BCP (biphasic calcium phosphate) [29]. The Sr substitution has been checked for all the Ca crystallographic sites (two sites in the HAp structure, and five sites in the b-TCP structure) by constraining a full cationic occupancy of the sites (i.e. occupancy(Ca) ? occupancy(Sr) = 1). Only the calcium occupancies of the Ca2 site of HAp and the Ca4 site of b-TCP were to found to deviate significantly from unity and were refined. The calcium occupancies of the other Ca sites were fixed at unity. The powder patterns resolution allow to extract accurately the quantitative phase analyses, the lattice parameters and the localisation and quantification of the strontium substituted Ca crystallographic sites. The present study based on laboratory X-ray diffraction patterns allows the Sr substitution refinement only in the main Ca substituted crystallographic sites (in reference to our previous works performed with synchrotron radiation which has indicated two additional Ca sites with a weak level of Sr substitution; the Ca3 and Ca5 sites in the b-TCP structure [29]). Refinement results and reliability factors are summarized in Tables 2 and 3. Representative Rietveld plots of HA and HASr2 samples are presented in Fig. 2.
289 63.9
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Table 3 Rietveld refinements results (standard deviations are indicated in brackets) and conventional Rietveld reliability factors Sample
HAp ´˚ a (A )
´˚ c (A )
a
Sr2 occ. (%)
RBragg
b-TCP ´˚ a (A )
v2 ´˚ c (A )
b
Sr4 occ. (%)
Rp, Rwp
RBragg
HA
9.42442(9)
6.88468(6)
0(–)
0.030
10.4384(3)
37.403(2)
0(–)
0.087
2.37
0.039, 0.053
HASr0.5
9.42594(9)
6.88586(9)
1.0(2)
0.026
10.4439(2)
37.3944(9)
27(3)
0.032
2.04
0.036, 0.048
HASr1
9.4263(1)
6.88638(9)
1.2(2)
0.027
10.4468(2)
37.3951(9)
41(3)
0.033
1.91
0.034, 0.046
HASr2
9.4274(2)
6.8872(1)
1.6(2)
0.031
10.4473(2)
37.3927(9)
43(4)
0.035
2.26
0.035, 0.048
HASr5
9.4336(3)
6.8907(3)
3.0(4)
0.042
10.4536(6)
37.391(2)
53(4)
0.051
3.50
0.039, 0.055
a, b
indicate the substitution levels of the strontium substituted Ca crystallographic sites (respectively for the Ca2 site in HAp and for the Ca4 site of b-TCP structures)
3 Results and discussion 3.1 Quantitative phase analysis Quantitative phase analysis has been performed on the basis of the Rietveld refinements. Two main phases have been refined and quantified (HAp and b-TCP). The low level impurities (lime, portlandite, strontia) detected in our previous work [29] using synchrotron radiation, were not taken into account for these laboratory XRD patterns treatments. Anyway, if present, their contribution must be very small. Results of the quantitative phase analysis are given in Table 2 and summarized in Fig. 3. It is striking to note that upon doping the evolution of the composition is very rapid. Even for the lower doping level (x = 0.5) a clear tendency to favour the formation of b-TCP is observed. The proportion of the b-TCP phase remains almost constant (around 40%) whatever the doping level (compared to the value around 10 wt% of b-TCP for the un-doped HA sample). This allows keeping the phase proportions of the material constant while changing continuously the strontium doping level. This is an important issue for further study of the influence of Sr2? concentration on the biological activity of these ceramics. In effect, the two potential effects (Sr concentration and HAp/b-TCP ratio) can be monitored independently. Synthesis conditions correspond to hydroxyapatite single phase samples; i.e. a (Ca ? Sr)/PO4 ratio of 1.67. However, all syntheses yielded biphasic samples. Such a hydroxyapatite inhibiting formation, favouring b-TCP formation has already been described in the case of smaller Mg2? ions insertion [36]. The HA sample contains 92 wt% of hydroxyapatite but this percentage falls down to 64% for HASr5 sample. The appearance of b-TCP in the Sr-doped apatite samples has been previously reported [19, 21, 29, 37]. The presence of b-TCP, with a lower Ca/PO4 ratio of 1.50 is probably compensated by the presence of CaO and/ or SrO. At high temperature, the Sr-doped b-TCP phase is more stable than the Sr-doped hydroxyapatite phase and its
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Fig. 2 Rietveld plots of HA (top) and HASr2 (bottom) samples. Observed (a, dots), calculated (a, line) and difference (b) powder diffraction patterns are presented. Bragg positions are indicated by vertical bars for hydroxyapatite (c1) and b-TCP (c2)
formation is compensated by the combined extraction of alkaline earth oxide in order to keep the ratio (Ca ? Sr)/ PO4 = 1.67. This observation correlates with the fact that single phase poorly crystalline Sr-HAp samples are
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Quantitative analysis (wt. %)
100 HAp (wt %) β-TCP (wt %)
90 80 70 60 50 40 30 20 10 0 0
1
2
3
4
5
x (at. %) Fig. 3 Quantitative phase analysis showing the proportions (wt%) of the HAp phase (full circles) and b-TCP phase (empty circles) as a function of the nominal Strontium doping level x. Lines are drawn as guide for the eye
polyhedron for calcium atoms. The substitution of Ca by bigger Sr atom stabilises the b-TCP structure [29]. The refined compositions in the present study are certainly under-estimated as no Sr substitution was taken into account in the Ca3 and Ca5 sites (due to numerous peak overlapping and an imperfect line profile modelling). Sr site occupancies in the Ca2 site of HAp structure and Ca4 site of b-TCP structure, as well as the total refined amount of strontium are plotted against the nominal composition in Fig. 4. The level of Sr substitution quickly reaches a value close to 50% for the Ca4 site in b-TCP. These results indicate that a large amount of the Sr introduced during the synthesis is not located in the crystalline phases for the HASr5 sample. This amount of not visible Sr atoms is explained by the under-estimated Sr substitution of the b-TCP phase, and by the presence of minor SrO phase not refined here. Following our previous work, it can be assumed that a large part of it is present in the SrO phase. This soluble
obtained by a moderate heat treatment [18, 25]. From the Rietveld refinement and considering the nominal Ca ? Sr/ PO4 ratio of 1.67, it is possible to evaluate the amount of amorphous CaO and/or SrO. Considering the very low quantity involved, this could correspond either to crystalline or amorphous alkaline earth oxides not observed in the XRD pattern. The values are given in Table 2. It is worthy to note that very low amount of CaO can compensate for large stoichiometry mismatch. The presence of CaO/SrO is speculated in this work but was demonstrated in the HA and HASr5 samples previously studied by synchrotron radiation [29]. Many times in the literature, a Calcium Deficient Apatite (CDA) was speculated instead but without any experimental evidence. 3.2 Strontium substitution Rietveld refinements allow locating Sr atoms in both HAp and b-TCP phases, and evaluating the Sr occupancies in these sites (Table 3). The corresponding refined compositions, i.e. the Sr substitution level in the two crystalline phases, are indicated in Table 2. In the HAp phase, strontium ions are found to be located only in the site Ca2 in agreement with previous results. For the b-TCP phase, strontium ions are found only in the site Ca4. Previous results from high resolution multi-patterns synchrotron refinements have indicated the effective presence of strontium in three independent Ca crystallographic sites of the b-TCP structure: the sites Ca3, Ca4, and Ca5. Present refinements performed on laboratory X-ray diffraction patterns allow only to observed, and quantified, the main strontium substituted Ca site; i.e. the Ca4 site which present an unusual and non adapted coordination
Fig. 4 Refined strontium occupancies (top) in Ca2 site of HAp structure (full circles) and in Ca4 site of b-TCP structure (empty circles), and refined value of total Sr substitution level x in the two HAp and b-TCP phases (bottom) as a function of the nominal x value
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amorphous phase would be beneficial for Sr-delivering applications like the development of anti-osteoporosis coatings onto metallic implants. Evaluation of the percentage of the two alkaline earth oxide, considering the nominal Sr/Sr ? Ca ratio, are given in Table 2. For the following Sects. 3.3 and 3.4, the refined y (for Ca5-ySry (PO4)3 OH) and y0 (for Ca3-y0 Sry0 (PO4)2) values have been taken into account.
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neighbouring OH- anion which can partially compensate a weak substitution of Ca atoms by bigger Sr atoms (see Figs. 3 and 4 from [29]). 3.4 Sr-substituted b-TCP study
This study investigated to Ca-rich side of the Sr-apatite solid solution, i.e. Ca5-ySry(PO4)3 OH with y ranging from 0 to 0.09 (refined values). The solid solution extends from y = 0 (i.e. undoped hydroxyapatite) to y = 5 (i.e. belovite Sr5(PO4)3(OH) [38]). Table 3 and Fig. 5 reports the lattice parameters and unit cell volume variations in the solid solution for the various samples. As mentioned in our previous study, the linear Vegard’s law observed for the total solid solution (0 B y B 5) is not perfectly respected close to the Ca-rich side (for y \ 0.10). This behaviour is attributed to the shifted 4e position of the
Rietveld refinements showed that Sr is preferentially inserted in the site Ca4 (Table 3). More generally Sr was inserted in the low density column described and named by Yashima et al. [35]. (composed of the sites Ca4 and Ca5). The Ca4 site is unusually face coordinated to a phosphate tetrahedron (with three too large Ca4-O9 interatomic distances) in pure b-TCP. The insertion of Sr in the Ca4 site has clearly a stabilising effect on the b-TCP structure by decreasing these three large distances, and explains the fact that hydroxyapatite formation was inhibited when strontium was introduced during the synthesis process. It is worthy to note that this effect is observed even for very low Sr substitution ratio. Figure 6 presents the evolution of the cell parameters (see also Table 3) and unit cell volume of the Sr-substituted b-TCP phase. A linear Vegard’s law is globally
Fig. 5 Evolution of the cell parameters a (top, left axis, empty circles) and c (top, right axis, full circles), and of the unit cell volume (bottom, squares) as a function of the substitution y refined value for the Sr-substituted HAp phase
Fig. 6 Evolution of the cell parameters a (top, left axis, empty circles) and c (top, right axis, full circles), and of the unit cell volume (bottom, squares) as a function of the substitution y0 refined value for the Sr-substituted b-TCP phase. Structural detail of the Ca4 site environment is inserted
3.3 Sr-HAp solid solution study
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observed once again for the volume of the unit cell describing the solid solution with the general formula Ca3y0 Sry0 (PO4)2. Divergences from the linear fits shown in Fig. 6 should be explained by the under-estimated Sr substitution level (due to the sites Ca3 and Ca5, see Sect. 3.2). According to Fig. 6 we can assume that underestimation of the Sr substitution level in b-TCP is really effective for the HASr5 sample. It can be noticed that the accommodation of Sr2? ions provokes an increase of the a lattice parameter and a decrease of the c lattice parameter. The decrease of the c lattice parameter when introducing big Sr atoms in the Ca4 site is a direct observation of its stabilizing effect. The substitution of Ca by Sr in the site Ca4 leads to a decrease of the three large and badly accommodated Ca4-O9 distances which are mainly oriented along the c axis (see inserted drawing in Fig. 6). The ˚´ in HA sample to Ca4-O9 distances decrease from 3.12 A ´ ˚ 2.79 A in HASr5 sample, whereas the Ca4-O1 distances (oriented in the basal hexagonal plane) increase from 2.53 ´˚ . to 2.63 A 3.5 Micro structural parameters Sample intrinsic micro structural parameters (i.e. average apparent crystallite size and average maximum internal strain) were extracted during the refinement by using the instrumental resolution function determined from the powder pattern recorded on pure silicon standard powder. The evolution of the micro structural parameters as a function of the nominal substitution ratio is presented in Fig. 7 for both the Sr-substituted HAp and b-TCP phases. Upon annealing at 1100 °C, the crystal growth of the HAp phase is highly favoured as reflected by the crystallite size ˚ for HAp, against of the pure HA sample (about 1300 A ˚ only 750 A for the b-TCP phase). Upon doping, the growth of the HAp crystals is hindered resulting in a smaller ˚ for x = 0.09). This average crystallite size (down to 675 A is in perfect agreement with quantitative phase analysis described earlier confirming the stabilising role of Sr2? ion towards the b-TCP phase. In a concomitant way, the average size of b-TCP crystallites increases upon doping ˚ , and becomes larger to the HAp up to about 1,000 A crystallites in the HASr5 sample.
4 Conclusions The sol–gel process has been successfully used to prepare Sr-doped calcium phosphates. All samples are Biphasic Calcium Phosphates (BCP). With respect to undoped sample, the Sr-doped samples exhibit higher proportion of b-TCP phase but the level appears to be quite independent of the doping level for 0.5 \ x \ 5 at.%. To explain the
Fig. 7 Evolution of the microstructural parameters, average apparent crystallite size (top) and maximum internal strains (bottom), as a function of the nominal substitution ratio for the Sr-substituted HAp (d) and b-TCP () phases. Lines are drawn as guide for the eye
mismatch with the nominal stoichiometry, the presence of amorphous CaO and SrO compounds have been postulated. The amount of these phases has been evaluated. The insertion of Sr2? ions in the two crystalline phases HAp and b-TCP is almost total for low doping levels but quite incomplete for HASr5 sample where about 3 wt% of (SrO) is calculated. The majority of the inserted Sr2? ions are in the b-TCP phase. The effective partial substitution of Sr in BCP makes these materials very interesting for orthopaedic applications. It is unexpectedly shown that it is possible to vary continuously the amount of strontium in these ceramics without changing the phase composition significantly. The Sr-substituted HAp and b-TCP cell parameters agree fairly well with the Vegard’s law and Sr2? ions substitute preferentially for Ca2? in the Ca2 site for hydroxyapatite and in the Ca4 site for b-TCP. The microstructural parameters confirm the previous observation and give a new evidence of clear stabilizing effect of Sr2? ions towards the b-TCP structure. Further work will include the study of solution behaviour of these materials in particular their ability to release
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strontium in physiological conditions and of the biological activity of these materials. Acknowledgements Financial support from ANR under project Nanobonefiller (PNANO 2006) is gratefully acknowledged. The authors are grateful to J. Cellier for his technical assistance in XRD patterns collection.
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