mixed phosphate-sulfate fluor apatites as possible materials in dental fillers

Mixed phosphate-sulfate fluor apatites as possible materials in dental fillers

ANDREAS PIOTROWSKI1, VOLKER KAHLENBERG2 and REINHARD X. FISCHER3

1 Heraeus Kulzer GmbH, Research & Development, Philipp-Reis-Str. 8, D-61273 Wehrheim/Ts., Germany2 Institut für Mineralogie und Petrographie, Leopold Franzens Universität Innsbruck, Innrain 52,

A-6020 Innsbruck, AustriaCorresponding author, e-mail: [email protected]

3 Universität Bremen, Fachbereich Geowissenschaften (Kristallographie), Klagenfurter Straße,D-28359 Bremen, Germany

Abstract: A series of sulfate-phosphate apatite samples of formal composition NaxCa10–x(SO4)x(PO4)6–xF2 (x = 1,2,...,6) wereprepared by solid state reactions and hydrothermal techniques. Rietveld analyses based on X-ray diffraction data clearly indicate thatthe samples synthesized by solid state reaction of the reactants Na2SO4, CaSO4, CaF2, and Ca3(PO4)2 consist of a mixture of twoapatite end-members Na6Ca4(SO4)6F2 and Ca10(PO4)6F2 which contradicts previous work assuming a solid solution series betweenthe end-members. The X-ray diffraction powder patterns of the samples synthesized by hydrothermal methods show a splitting ofreflections which can be interpreted as a monoclinic reduction of the hexagonal symmetry, thus representing an apatite-like phase ofthe solid solution series with sulfate and phosphate groups. The hydrothermally synthesized compounds exhibit physical propertiesthat are required for dental applications where apatite-like phases are used as fillers in composite materials.

Key-words: phosphate-sulfate apatite, solid solution, Rietveld study, dental filler, biomaterial.

1. Introduction

Apatite is a generic term for compounds with the generalformula M6M4(ZO4)6X2, which mainly crystallize in thehexagonal space group P63/m. Extensive isomorphic substi-tutions can occur in all of the atomic sites. The most abun-dant composition is Ca10(PO4)6X2 with X = F–, Cl–, OH–, thecalcium-phosphate apatite representing the type materialand mineral apatite (Sudarsanan et al., 1972).

The substitution of phosphate by sulfate groups in apatiterequires a partial replacement of calcium by monovalentcations such as sodium for charge compensation. Examplesare Na6Ca4(SO4)6(OH)2 (cesanite: Tazzoli, 1983; Piotrow-ski et al., 2002a), Na6Ca4(SO4)6F2 (Klement, 1939),Na6Ca4(SO4)6(FxClx–1)2 (Piotrowski et al., 2002b). Otherapatite-like sulfates are Na6Pb4(SO4)6Cl2 (caracolite:Schneider, 1967 and 1969), as well as Na6Cd4(SO4)6Cl2, andNa3Pd2(SO4)3Cl (Perret & Bouillet, 1975). All structurescrystallize in space group P63/m except cesanite whichadopts space group P6 (Piotrowski et al., 2002a). In previ-ous investigations on the solid solution series NaxCa10–x(SO4)x(PO4)6–xF2 based on solid state reactions completemiscibility between the end-members Na6Ca4(SO4)6F2 andCa10(PO4)6F2 was reported (Apella & Baran, 1979 and1981). Furthermore, a symmetry reduction to a monoclinicspace group for members with more than one sulfate groupper formula unit was postulated. No indications for the pres-

ence of distinct apatite phases were found. The ionic con-ductivities of this solid solution series were investigated byLaghzizil et al. (1993). A natural sulfate phosphate apatiteCa5–xNax(P3–xSx)O12(FyOH1–y) with 0.02 e x e 0.41 and0.80 e y e 0.96 was found in the Kushikino mine, Kagoshi-ma Prefecture, Japan, formed in hydrothermal ore deposits(Shiga & Urashima, 1987).

Birkenstock (1993) also studied synthetic apatites withmixed sulfate-phosphate compositions synthesized by solidstate reactions. He found that the solid solution series postu-lated by Apella & Baran (1979, 1981) and Laghzizil et al.(1993) most likely consists of a mixture of the two end-members rather than of a mixed sulfate-phosphate com-pound.

The aim of this work is to investigate and to characterizethe synthesis products with a formally mixed sulfate-phos-phate composition based on two different synthesis meth-ods: solid state and hydrothermal reaction of the reactants.The knowledge of the existence or non-existence of mixedcompounds is especially important for dental applicationswhere apatites are used as fillers in composite materials be-cause of their chemical and structural resemblance to toothhard tissue. The whole composite is supposed to have an ad-justable transparency, good polishing properties, highstrength, and the capacity to release ions into a biologicalenvironment (Rentsch, 1999). A crucial point of apatite ap-plication in dental fillers is its optical behaviour, especially

Eur. J. Mineral.2004, 16, 279–284

DOI: 10.1127/0935-1221/2004/0016-02790935-1221/04/0016-0279 $ 2.70

ˇ 2004 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

Fig. 1. SEM photograph of the hydrothermally synthesized solid so-lution compound Na4Ca6(SO4)4(PO4)2F2.

the refractive index. In order to match the optical parametersof the organic matrix, thus avoiding light scattering, the av-erage refractive index of the inorganic filler should be below1.58. The hydroxyl apatite Ca10(PO4)6(OH)2 as well as fluorapatite Ca10(PO4)6F2 do not have the transparency requiredfor dental fillers, and so yield opaque materials that poorlytransmitt light for photopolymerization. Mixed sulfate-phosphate apatites would be suitable candidates as their op-tical properties closely resemble the appearance of naturalenamel. Since the sulfate end-member is soluble in water, asdescribed already by Dihn & Klement (1942), a mixture ofthe end-members is undesirable and efforts should be fo-cused on the synthesis of a solid solution product.

While comprehensive crystallographic studies on dentalenamel apatites exist (see, e.g., Wilson et al., 1999 and refer-ences therein), mostly consisting of hydroxylapatite con-taining some carbonate, there is little knowledge concerningthe phosphate-sulfate solid solution apatites studied here.

2. Experimental details

Sulfate-phosphate apatite samples of the composition Nax-Ca10–x(SO4)x(PO4)6–xF2 were prepared by two differentmethods: solid state reactions and hydrothermal techniques.For the synthesis of a compound with formal compositionNaxCa10–x(SO4)x(PO4)6–xF2 (x = 1,2,...,6) mixtures ofNa2SO4, CaSO4, CaF2, and Ca3(PO4)2 in stoichiometric pro-portions were ground and homogenized in an agate mortar.The initial reagents CaSO4, CaF2, and Ca3(PO4)2 were cal-cined separately at 1000°C to obtain pure anhydrous materi-als.

For solid state reaction the mixtures were pressed intopellets and heated in open corundum crucibles between 500and 800°C for 12 to 72 h. The samples were subsequentlycooled to 100°C at a rate of 100°C/h. The sulfate rich sam-ples NaxCa10–x(SO4)x(PO4)6–xF2 with x = 6, 5, and 4 weremolten at 800°C.

For synthesis under hydrothermal conditions the mix-

tures were heated in externally heated autoclaves (Morey-type) with a teflon reaction vessel and a reaction volume of25ml. The experiments were carried out at 250°C for 7 and14 days using 3 g of the starting material and 1.5 ml H2O.Fig. 1 shows an SEM photograph of the hydrothermallysynthesized compound with the characteristic needle-likecrystals of the phosphate-sulfate solid solution phase.

For phase analysis X-ray powder diffraction patternswere recorded using a Philips X’Pert powder diffractometerin Bragg-Brentano geometry operated at 45kV and 40mA.The diffractometer is equipped with a primary beamGe(111) monochromator. The high resolution achieved bythe monochromatic CuK [ 1 radiation ( † = 1.54056 Å) madepossible the resolution of reflections which are overlappedor split by symmetry reduction and cannot be separated bystandard diffractometers. The fine-grained powders werefront loaded in round sample holders of 13 mm diameter.Data were collected at room temperature with a fixed slitconfiguration in consecutive steps of 0.02° 2 ’ covering arange between 10 and 120° 2 ’ . The counting time was 15s/step. All phase analyses with the Rietveld method were per-formed with the Philips PC-Rietveld plus program package(Fischer et al., 1993).

The background was corrected manually, and the valueswere determined by linear interpolation of the successivedata points. The pseudo-Voigt function was chosen for thesimulation of the peak shape with two variable parametersdefining the Lorentzian and the Gaussian character of thepeaks as a function of 2 ’ . The angular variation of the linewidth was accounted for using the Cagliotti function. Initialstructure parameters used in the refinements were takenfrom the structure models of anhydrite CaSO4 (Hawthorne& Ferguson, 1975), fluorite CaF2 (Cheetham et al., 1971),calcium oxide CaO (Primak et al., 1948), sulfate apatiteNa6Ca4(SO4)6F2 (Birkenstock, 1993), and phosphate apatiteCa10(PO4)6F2 (Sudarsanan et al., 1972). In the first step ofthe refinements, the global parameters (2 ’ -zero, instrumen-tal profile) were refined. In the next step, the lattice parame-ters and preferred orientation were refined consecutively.The values of atomic coordinates, displacement factors andsite occupancies were fixed.

A further investigation of the powder patterns was car-ried out by indexing the reflections, which were assigned tothe apatite-like phases. The automatic indexing was per-formed with the program DICVOL91 (Boultif & Louer,1991). For a sample of the hydrothermal reactions with x=1,a whole-pattern profile fit using the structure-independentLeBail method (LeBail et al., 1988) was performed with theprogram FULLPROF.2k (Rodriguez-Carvajal, 2000).

To examine the solubility of the products in water 3 g offinely-granulated powder were mixed in 150 ml distilledwater for 2 h and 5 d, respectively. After the water was de-canted, the precipitates were dried in air at 50°C. The pow-der was analyzed with X-ray powder diffraction methods asdescribed above. The test of solubility pursues two differentaims: (i) determination if a solid solution series was formedduring the syntheses or if mixtures of the end-membersNa6Ca4(SO4)6F2 and Ca10(PO4)6F2 are present, and (ii) in-vestigation of the properties with regard to its application asan inorganic filler for dental composite materials. In case of

280 A. Piotrowski, V. Kahlenberg, R.X. Fischer

Fig. 2. Region of the XRD powder patterns between 31° to 35° 2 ’ forvarious NaxCa10–x(SO4)x(PO4)6–xF2 compositions synthesized bysolid state reactions; (a): x = 5, (b): x = 4, (c): x = 3, (d): x = 2, (e): x= 1. The peak marked with (*) belongs to CaSO4.

a solid solution series the powder will be unaltered, orcompletely decomposed, after the solubility test. On the oth-er hand, in mixtures of the two apatite end-members,Na6Ca4(SO4)6F2 and Ca10(PO4)6F2, the latter phosphate willremain as a precipitate as it is sparingly soluble compared tothe sulfate apatite.

3. Results and discussion

3.1. Solid state syntheses

Syntheses to obtain compounds of composition NaxCa10–x(SO4)x(PO4)6–xF2 were performed between 500°C and800°C for 12 to 72 h. However, only samples prepared at700° for 48h were characterized as this product was consid-ered representative for these types of compounds. Highertemperatures and longer sintering periods did not alter phaseformation. In contrast to prior assumptions, the reactions are

Fig. 3. Observed (crossed) and calculated (line) XRD powder patternfrom a Rietveld refinement using a two phase apatite model of thesample with x = 4 synthesized by solid state reaction (only the inter-val from 31° to 35° 2 ’ is shown).

Table 1. Weight percentages of the phases obtained in the system Nax-Ca10–x(SO4)x(PO4)6–xF2 from solid state reactions using the Rietveldmethod. The final weighted profile R-values (Rwp) varied between14.0 and 16.0%. Theoretical weight percentages were calculated fromthe bulk composition. The estimated standard deviations of the weightpercentages obtained from the refinements are about ±0.5%.

x phase refinedweightpercentages

theoreticalweightpercentages

lattice parameters[Å] of the apatitephases afterrefinement

5 sulfate apatite 76.2 81.9 a=9.412(5),c=6.898(4)

phosphate apatite 20.9 18.1 a=9.379(8),c=6.886(8)

fluorite 1.8 –anhydrite 1.0 –



fluorite 2.0 –anhydrite 0.7 –



fluorite 0.8 –calcium oxide 0.2 –



fluorite 0.9 –calcium oxide 0.9 –

1 sulfate apatite 8.2 15.3 not refinedphosphate apatite 90.5 84.7 a=9.376(6),

c=6.886(3)fluorite 0.7 –calcium oxide 0.6 –

Mixed phosphate-sulfate fluor apatites 281

Fig. 4. Region of the XRD powder patterns between 31° to 35° 2 ’ forvarious NaxCa10–x(SO4)x(PO4)6–xF2 compositions prepared from hy-drothermal reactions; (a): x = 5, (b): x = 4, (c): x = 3, (d): x = 2, (e):x=1. The tickmarks correspond to the monoclinic cell derived fromthe auto-indexing for the phase with x = 1.

not stoichiometric. Small amounts of the reactants CaSO4and CaF2 as well as CaO were observed in the X-ray diffrac-tion (XRD) patterns, although apatite-type compounds weredetected as main phases. All reflections belonging to apatiteconsists of two distinct sets of reflections with relative in-tensities changing as a function of composition. However,the positions of the reflections remain unaltered. Fig. 2shows a characteristic region of the XRD pattern.

Two possible explanations for the splitting of the reflec-tions are: a monoclinic reduction of the hexagonal symme-try or the formation of two apatite phases. The first assump-tion can be excluded, as indexing based on 15 reflectionsfailed and a unit cell could not be found. However, the XRDpatterns can be interpreted as a two-phase mixture of apa-tites consisting of the two end-members Na6Ca4(SO4)6F2and Ca10(PO4)6F2. The good agreement between observedand calculated intensities in the Rietveld refinements basedon the two structure models of Na6Ca4(SO4)6F2 (Birken-

Fig. 5. Whole-pattern profile fitting without structural model of thesolid solution member with x = 1 obtained by hydrothermal reaction.The tickmarks indicate the peak positions of the phases, the lowerplot shows the difference between observed and calculated intensi-ties.

stock, 1993) and Ca10(PO4)6F2 (Sudarsanan et al., 1972) un-ambiguously confirms the presence of two separate phases(Fig. 3). All reflections in the pattern can be assigned to oneof the two phases. Additionally, the lattice parameters arenearly unchanged in all samples with various compositions.Furthermore, the ratio of sulfate to phosphate apatite in thesamples calculated from the Rietveld refinements approxi-mately corresponds to the values expected from the amountsof the initial reagents (Table 1), though, with a consistentlylower sulfate content. A possible reason is the non-stoichio-metric reaction of Na2SO4, CaSO4, CaF2, and Ca3(PO4)2 dueto volatility of Na2SO4.

3.2. Hydrothermal syntheses

The products of the hydrothermal reactions are fine crystal-line materials. Crystals are not visible under the petrograph-ic microscope with a 400 fold magnification. The productsconsist of an apatite-like phase (main phase), CaSO4, andCaF2 (minor phases). Their XRD patterns show clear differ-ences to the patterns of the solid state reaction products.


Similar to the mixture of two apatite phases from the solidstate reactions, the powder pattern of the hydrothermalproduct also shows additional reflections and shoulders(Fig. 4) but the separation within reflection doublets isclosely correlated with the degree of substitution of SO4

2–

and Na+ for PO43– and Ca2+. The splitting of reflections is

barely distinguishable with decreasing sulfate content, andthe relative intensities of the reflections are nearly un-changed. All reflections in the powder pattern of the samplewith the bulk composition NaCa9(SO4)(PO4)5F2 could be in-dexed according to a monoclinic cell confirming the sym-metry reduction from hexagonal to monoclinic. Initial unitcell parameters were derived from 15 accurately identifiedreflections and used as starting parameters for the whole-pattern profile fit with the LeBail method. The close agree-ment between calculated and observed intensities (Fig. 5) isa clear indication for the presence of a single phase sulfate-phosphate apatite in contrast to the mixture of sulfate apatiteand phosphate apatite obtained in the solid state reactions.The refined monoclinic unit cell parameters of the apatite-like phase are a = 9.417(3) Å, b = 6.884(5) Å, c = 9.401(5)and q = 120.18(7)°. Therefore, the XRD analysis of the hy-drothermally synthesized products clearly indicates the for-mation of compounds within the solid solution series. Apa-tites with related compositions as e.g. Ca10(SiO4)3(SO4)3(OH)2 (hydroxyellestadite: Sudarsanan, 1980) show similarmonoclinic distortions of structure as the apatite-like phasesin this study.

The small amount of residual CaF2 in the product indi-cates that essentially all fluorine has been incorporated inthe apatite phase, although hydroxyl groups could be ex-pected under hydrothermal conditions.

3.3. Solubility experiments

After the solubility tests, a white precipitate remained ineach experiment. The subsequent phase analyses were per-formed using XRD methods. Fig. 6 shows the same regionof the diffraction patterns of the samples from hydrothermaland solid state reactions with the bulk compositionNa3Ca7(SO4)3(PO4)3F2 before and after solubility tests. It isclear in the diffraction pattern of the solid state reactionproducts that the sulfate-apatite phase is removed by disso-lution. The subsequent Rietveld analysis indicates the pres-ence of a hexagonal apatite phase with lattice parameters a= 9.378(6) Å and c = 6.886(7) Å which corresponds to thephosphate end-member Ca10(PO4)6F2 (Sudarsanan et al.,1972). These results confirm the formation of a mixtureof two apatite phases, as the insoluble phosphate apatiteremains in the precipitate and the sulfate apatite is dis-solved. However, the members of the solid solutionseries NaCa9(SO4)(PO4)5F2, Na2Ca8(SO4)2(PO4)4F2, andNa3Ca7(SO4)3(PO4)3F2, which were synthesized under hy-drothermal conditions, do not show any change in the XRDpatterns after the solubility test. The remaining members ofthe solid solution series with higher sulfate contents,Na4Ca6(SO4)4(PO4)3F2 and Na5Ca5(SO4)5(PO4)F2 decom-pose in CaSO4 x 2H2O, Ca3(PO4)2, CaF2 and Na2SO4 afterexposure to water.

Fig. 6. Influence of the solubility experiments on the XRD powderpatterns for samples with x = 3 synthesized (a) hydrothermally and(b) by solid state reactions (only the region between 31° to 35° 2 ’ isshown).

4. Conclusions

X-ray diffraction experiments, aided by Rietveld analysesand solubility tests, unequivocally show that apatite-likesolid solution compounds of composition NaxCa10–x(SO4)x(PO4)6–xF2 are not formed by solid state reactions ofthe respective reagents given in the experimental section.Instead, a mixture of pure sulfate (Na6Ca4(SO4)6F2) andphosphate (Ca10(PO4)6F2) phases is obtained. Real solid so-lution compounds could be hydrothermally synthesized inthe compositional range 1 e x e 6. Therefore, former studiespresumably performed on solid solution compounds shouldbe revised. For dental applications, where apatites withphysical properties intermediate between the end-membersare needed, hydrothermally synthesized compounds can beused. Mixtures obtained in the solid state reactions are notsuitable as dental fillers due to the high solubility of the sul-fate phase.

Acknowledgments: This work was supported by a grant(Fi442/8) of the Deutsche Forschungsgemeinschaft. The re-views by Timothy J. White and Jordi Rius are greatly appre-ciated.

Mixed phosphate-sulfate fluor apatites 283

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Received 11 July 2003Modified version received 30 September 2003Accepted 3 November 2003


mixed phosphate-sulfate fluor apatites as possible materials in dental fillers

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