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Strontium-cadmium substitution in hydroxy- and fluor-apatites
Strontium-cadmium
substitution
STRONTIUM-CADMIUM
in apatites
Ann. Chim.
SURSTITUTION J.L. LACOUT,
Sci. Mat, 1998,23,
pp. 57-80
IN HYDROXY- AND FLUOR-APATITEX
A. NOUNAH*,
M. FERHAT*
Laboratoire des Materiaux, Physico-Chimie des Solides, Ecole Nationale Superieure de Chimie de Toulouse (UPRESA 5071), 38, rue des 36 Ponts, 31400 Toulouse, France. * Laboratoire de Chimie Physique Generate, Departement de Chimie, Faculte des Sciences, Avenue Ibn Battouta, Rabat, Maroc.
Summary : Samples of hydroxyapatites and fluorapatites containing both strontium and cadmium were prepared in aqueous medium. They were characterized by chemical analysis, X-ray diffraction and infrared absorption spectroscopy. The limits of strontium-cadmium substitution were determined and
the splitting of the vl phosphate infrared band was interpreted.
Des tchantillons Rbsumb:: d’hydroxyapatites et de fluorapatites contenant it la fois du strontium et du cadmium ‘sent pr@ar& par voie humide. 11sont th? caract&& par analyse chimique, diffraction des rayons X et specttometrie d’absorption infrarouge. La limite de substitution strontium-cadmium a Cte ddterminee et le dedoublement de la bande vl des ions phosphate a et& interprete. 1. INI’RODUCTION Apatites form a large family of isomotphous compounds of hydroxyapatite, CaIo(PC4)6(OH)2. They generally crystallize ln the hexagonal system (space group P63hn). The almost compact assembly of PO4 orthophosphate ions defines two channels in which the cations are located. One of the main characteristics of the apatitic structure is to allow large and varied substitutions for both cations and anions : in the cationic Ca2+ sites, in the anionic P043- sites or in the channel OH- sites [ 11. Calcium-cadmium and calcium-strontium substitutions have already been studied by numerous authors. Strontium is known to substitute for calcium in biological apatites. Its role in pathological calcification and its possible effects in preventing tooth decay has been shown [2]. The structural differences resulting from the replacement of calcium with strontium in the apatite have been examined [3]. Likewise, cadmium hydroxyapatite 141, and cadmium-containing calcium fluorapatites have been studied [S]. Thermal stability of cadmium-containing hydroxy- and fluor-apatites, and distribution of cadmium between the two non-equivalent crystallographic cationic sites in the apatitic structure have been studied by ourselves [6, 71. However, no work relates to solid solutions between strontium apatite and cadmium apatite. In the present paper, we describe the preparation of strontium-cadmium in phospho-apatites, and we analyse their physico-chemical properties and the structural modifications resulting from the substitution of strontium for cadmium. 2. MATERIALS Strontium-cadmium apatites were prepared in aqueous medium by a double decomposition method. Sr2+, Cd2+ and PO43- had the stoichiometric ratio of the composition of the solids that we expected to be formed. ---m
: J.L. LACOUT, L&o ra toire des ----Mat&iaux - Physico-Chitnie des Solides,38, rue des 36 Ponts, 31400 TOULOUSE, FRANCE
58
J. L. Lacout
et al.
Cation solutions (A) (1 1, 0.2 M) were prepared from strontium nitrate (Sr(NO3)2) and cadmium nitrate (Cd(N03)2.4H20). The pH of the solution was adjusted to 11.2 by the addition of ammonia solution (200 ml, d=O.92). Anion solutions (B) (0.6 1.0.2 M) were prepared from diammonium phosphate (NH4)2HPO4). In the case of fluorapatite synthesis, a large excessof NHqF was added to the solution. In all the cases, 200 ml of ammonia solution (d=O.92) was added to anion solution. The pH was 11.8. After a slow precipitation (3 h). the solid was agged at boiling point and refluxed for 1 h in the mother solution, then was filtered and dried at 80°C for 15 h. All the reagents were of analysis grade (Prolabo). All the samples were characterized by X-ray diffraction, IR spectmmetry and chemical analysis.
3.1. B X-ray diffraction shows that when solution B was added to solution A, the precipitated and dried samples presented an apatitic Structure with relatively fine X-ray lines, only when the atomic ratio Cd/(Sr+Cd) in the initial solutions was lower than 0.6. The higher the level of cadmium, the larger the Xmy lines became. So, the crystahinity of the apatites formed decreased with the cadmium content. For an atomic ratio Cd/(Sr+Cd) of 0.6 the product was quite amorphous ; but when this atomic ratio was greater than 0.6. the precipitate still crystallized again but did not present an apatitic structure. In order to improve the crystallinity of the samples, they were heated at 600°C in a water vapor atmosphere. Only samples with an atomic ratio Cd((Sr+Cd) 20.4, led to a pure apatitic structure. All the others (including Cd/(Sr+Cd)=O.S and 0.6) led to a mixture containing no apatitic phases. The X-ray lines of Cd-Sr apatites were indexed with comparison to strontium hydroxyapatite [8]. The lattice parameter values vary linearly with cadmium composition and follows then a typical Veganfs law : a = 9.756 - 0.512X 8, e=*0.003 E = 7.286 - 0.353X A e=+0.003 with X = Cd/(Sr+Cd) v = 600.5 - 90.3x A3 e=i0.6 The simultaneous and continuous decrease of the lattice constants, when the cadmium content increases, shows the progressive substitution of the small cadmium ion (rcd2+ = 0.95 A) for the large strontium ion (rSr2+ = 1.18 A) [9]. The introduction of cadmium in the strontium apatitic structure leads to a larger decreaseof B parameter than E parameter. The average c/a ratio is 0.7474. The cadmium hydroxyapatite lattice parameters can be calculated using the mathematical equation of Vegard’s law. The extrapolation values, a = 9.244 A and c = 6.933 A are quite different to the experimental ones determined in a previous work [ 101. It appears that strontium-cadmium hydroxyapatites prepared in aqueous medium follow Vega& law over a limited range of solid solutions (0 LCd/(Sr+Cd) Q.4). Chemical analysis showed that the experimental (Sr/Cd)/P atomic ratios of the samples were close to the stoichiometric value of 1.67. The chemical formula of these apatites can be determined from chemical analysis, according firstly to the well known fact that the apatitic structure never contains any vacancies in PO4 site and that the PO4 number is six per cell, and secondly to the complete hydroxylation of the apatitic channel. It can be noted that the amounts of P, Sr and Cd are quite identical in the initial solution and in the final solids. The infrared spectra corresponding to apatites containing 0. 1. 2, 3 and 4 cadmium per cell, which were heated at 600°C are characteristic of apatite. On the strontium hydroxyapatite spectrum the presence of vibration bands at 3592 cm--l and 542 cm-l can be attributed to the stretching and librational vibrations of the OH ions respectively. The positions are in agreement with results of several authors [ll]. In the case of cadmium hydroxyapatite the bands are at 3536 cm-l and 715 cm- 1 respectively [lo]. The attribution of OH bands is more problematic for mixed apatite due to the very weak intensity of the OH bands. Concerning the PO4 bands, the infrared spectrum of hydroxyapatite, containing one cadmium per cell, shows two particular bands at about 949 cm-l. It corresponds to the splitting of the vl phosphate band. This doublet is visualized in figure. When the proportion of cadmium ions in the lattice varies, the intensities of the two bands move in o posite directions. The band at 947 cm-l is present alone (figure, spectrum A) in the absence of CdT.!+ ions, but it splits and gives a broad one at 950 cm-l for apatite containing 4 atoms per cell (figure 1, spectrum E). The weak but steady variation of PO43- band positions to the weak wavenumber when strontium is progressively substituted for cadmium can be attributed to the lattice volume variation.
Strontium-cadmium
substitution
in apatites
59
B
960.0
940 .O
cm-1
FIG. 1 - vl IR bands of PO4 group in hydroxyapatites with atomic ratio Cd/(Cd+Sr) : A=O, B=O.l, C=O.2, Lk0.3, E=O.4 3.2. m
To obtain the precipitation of fluoride apatites, cation solutions containing Sr2+ and Cd2+ were added to anion solutions containing PG43- and F-. X-ray diffraction shows that dried samples crystallized in the apatitic structure, only when the atomic ratio Cd/(Sr+Cd) was lower than 0.6 ; for greater values phase mixtures were formed. In order to improve the crystallinity of the apatites synthesized, dried samples were heated to 6OO’C. This treatment led to a noticeable refinement of X-ray lines which can be indexed in the hexagonal system. A linear decrease of lattice parameters I and I; according to Vegard’s law was observed. The following equations are verified :
A A v = 592.8 - 91.7x A3
a = 9.699 - 0.354~ h = 7.279 - 0.628X
e=+0.003
e=;t0.003
with X = Cd/(Sr+Cd)
e = f 0.6
A solid solution exists between strontium fluorapatites and cadmium fluorapatites only in the range 0 5 Cd/(Sr+Cd) I 0.6. The simultaneous decrease of lattice parameters a and E shows the substi tion of the smaller cadmium ions for strontium. The strontium fluorapatite arameters (a = 9.714 “A, c = 7.281 A) am close to those proposed by Kriedler [12] (a = 9.719 A, c = 7. !?76 A). Unliie in hydroxyapatites, the substitution of cadmium ions for strontium in fluorapatites leds to a greater decrease of parameter E than of parameter 8, and consequently to a decrease of the lattice ratio c/a (c/a = 0.7564 - 0.0381X, e = 0.0008). Contraction of axis E is mom important than that of axis 8. The chemical analysis of the samples showed that all the solids have an atomic ratio (Sr/Cd)/P close to the stoichiometric one 1.67. The infrared spectra of strontium-cadmium fiuorapatites show the absence of OH bands at 3590 cm-l and 540 cm-l. Cadmium - strontium substitution in fluorapatites principally affects the ~1 phosphate ion band, which can be split in mixed apatites (Fieure 2). When the proportion of cadmium introduced into the strontium fluorapatite lattice varies, the intensities of the two bands move in opposite directions. The band at 950 cm-l is only present in the absence of cadmium ions (Fieure2. spectrum A) ; this band disappears and is replaced by another larger one at 946 cm-l when the apatite contains 6 atoms of cadmium per cell (Fiaure 2, spectrum F). Between these two limits, the intensity of the band at 950 cm-l decreases with the quantity of cadmium, while the intensity of that situated at 946 cm-l increasesm2. spectra B, C, D and E). The two band intensities am approximately equal when the quantity of cadmium ions is 3 atoms of cadmium per cell. 4. DISCUSSION The pure apatitic phase, Sr(10-x)Cdx(P04)6Y2 only exists in the range 0 5 x 5 4 for Y = OH and 0 I x I 6 for Y = F. It is possible to substitute cadmium ions, smaller than strontium ions, in sttontium hydtoxy- and fluor-apatites structure, but the reverse is not true.
60
J. L. Lacout et al.
/-----------~-, 910.0
FIG.2-VI
cm-’
910.0
IR bands of PO4 group in fluorapatites with atomic ratio
Cd/(Cd + Sr) : A = 0, B = 0.1, C = 0.2. D = 0.3, E = 0.4, F = 0.6 Concerning the cadmium hydroxyapatite lattice parameters, the values of jb and E. calculated by extrapolation are different from those experimentally determined. It seems that progressive cadmium introduction in lattice induces a greater decrease of B and a smaller decrease of E than expected due to the size of the cation. This can explain the deformation of the lattice, the limit of substitution and the difference with the values of hydroxyapatite parameters. Infrared spectra of strontium-cadmium apatites show a splitting of the vl phosphate ion band. The band, located at about 947 cm-l, is present alone in the absence of Cd2+. When the cadmium ion amount in the lattice increases,two bands appear whose intensities vary in opposite directions. We know that in the apatitic structure, phosphate ions, at six per cell, are isolated from each other and present a Cs site symmetry. The calculation of the vibration numbers according to the site and space groupe P63/m, indicates nine active vibrations in the infrared. In the range we are interested in, from 970 cm-l to 930 cm-I, the apatite allows only one absorption band : symmetric valency band vl. The splitting of the vl band must be correlated with the fact that the six phosphate ions which are normalIy equivalent in the P63/m symmetry are not equivalent when a certain amount of cadmium ions substitutes for strontium ions. Switch et al. [13] have already observed the same phenomenon in the substitution of manganese for calcium in fluorapatites. They have shown, after a detailed study, that the splitting of the vl absorption band of phosphate ions is attributed to the preferential location of manganese in Me(I). 5. CONCLUSION The complete solid solutions strontium/cadmium hydroxy- or fluor-apatite does not exist ; according to the general formula Sr(l@x)Cdx(PO4)6Y2, hydroxyapatite (Y=OH) exists in the range 0 I x I 4, and fluorapatite (Y=F) in the range 0 I x I 6. These limits can be correlated with the different dimensions for strontium and cadmium ion and/or the preferential location of the ions in the two crystallographic cationic sites. A detailed structural study by the Rietveld method can perhaps confirm this hypothesis. 6.i31
PI [lo] [ll] [ 121 [13]
D. MC CONNEL, Apatites, B, New-York, 1973. R. FEITH. T.J.J.H. SLOOFF, I. KAZEM,T.J.H. VANRES, I,Bone M. ANDRES-VERGES, F.J. HIGES, P. FELIX GONZALES-DIAZ, I., IQ 13c y, lL.J.
1978,5&j& 79. 1980,
Powder Diffraction File, Int. Center for Diff. Data Penn. USA, JCPDS file : 14-691. R.D. SHANNON, Acta Crvstallorrr, 1976, A32. Part 5,751. A. NOUNAH. J. SZILAGYI, J.L. LACOUT. Ann.Chim.Fr., 1990, u, 409. G. ENGEL, W.E. KLEE, J.Salid 1972,Z 28. E.R. KRBIDLER, Ph. D. T&& Pennsylvania’University, 1967. P.R. SUITCH, J.L. LACOUT, A. HEWAT, R.A. YOUNG, s, 1985.m.
173.
substitution
STRONTIUM-CADMIUM
in apatites
Ann. Chim.
SURSTITUTION J.L. LACOUT,
Sci. Mat, 1998,23,
pp. 57-80
IN HYDROXY- AND FLUOR-APATITEX
A. NOUNAH*,
M. FERHAT*
Laboratoire des Materiaux, Physico-Chimie des Solides, Ecole Nationale Superieure de Chimie de Toulouse (UPRESA 5071), 38, rue des 36 Ponts, 31400 Toulouse, France. * Laboratoire de Chimie Physique Generate, Departement de Chimie, Faculte des Sciences, Avenue Ibn Battouta, Rabat, Maroc.
Summary : Samples of hydroxyapatites and fluorapatites containing both strontium and cadmium were prepared in aqueous medium. They were characterized by chemical analysis, X-ray diffraction and infrared absorption spectroscopy. The limits of strontium-cadmium substitution were determined and
the splitting of the vl phosphate infrared band was interpreted.
Des tchantillons Rbsumb:: d’hydroxyapatites et de fluorapatites contenant it la fois du strontium et du cadmium ‘sent pr@ar& par voie humide. 11sont th? caract&& par analyse chimique, diffraction des rayons X et specttometrie d’absorption infrarouge. La limite de substitution strontium-cadmium a Cte ddterminee et le dedoublement de la bande vl des ions phosphate a et& interprete. 1. INI’RODUCTION Apatites form a large family of isomotphous compounds of hydroxyapatite, CaIo(PC4)6(OH)2. They generally crystallize ln the hexagonal system (space group P63hn). The almost compact assembly of PO4 orthophosphate ions defines two channels in which the cations are located. One of the main characteristics of the apatitic structure is to allow large and varied substitutions for both cations and anions : in the cationic Ca2+ sites, in the anionic P043- sites or in the channel OH- sites [ 11. Calcium-cadmium and calcium-strontium substitutions have already been studied by numerous authors. Strontium is known to substitute for calcium in biological apatites. Its role in pathological calcification and its possible effects in preventing tooth decay has been shown [2]. The structural differences resulting from the replacement of calcium with strontium in the apatite have been examined [3]. Likewise, cadmium hydroxyapatite 141, and cadmium-containing calcium fluorapatites have been studied [S]. Thermal stability of cadmium-containing hydroxy- and fluor-apatites, and distribution of cadmium between the two non-equivalent crystallographic cationic sites in the apatitic structure have been studied by ourselves [6, 71. However, no work relates to solid solutions between strontium apatite and cadmium apatite. In the present paper, we describe the preparation of strontium-cadmium in phospho-apatites, and we analyse their physico-chemical properties and the structural modifications resulting from the substitution of strontium for cadmium. 2. MATERIALS Strontium-cadmium apatites were prepared in aqueous medium by a double decomposition method. Sr2+, Cd2+ and PO43- had the stoichiometric ratio of the composition of the solids that we expected to be formed. ---m
: J.L. LACOUT, L&o ra toire des ----Mat&iaux - Physico-Chitnie des Solides,38, rue des 36 Ponts, 31400 TOULOUSE, FRANCE
58
J. L. Lacout
et al.
Cation solutions (A) (1 1, 0.2 M) were prepared from strontium nitrate (Sr(NO3)2) and cadmium nitrate (Cd(N03)2.4H20). The pH of the solution was adjusted to 11.2 by the addition of ammonia solution (200 ml, d=O.92). Anion solutions (B) (0.6 1.0.2 M) were prepared from diammonium phosphate (NH4)2HPO4). In the case of fluorapatite synthesis, a large excessof NHqF was added to the solution. In all the cases, 200 ml of ammonia solution (d=O.92) was added to anion solution. The pH was 11.8. After a slow precipitation (3 h). the solid was agged at boiling point and refluxed for 1 h in the mother solution, then was filtered and dried at 80°C for 15 h. All the reagents were of analysis grade (Prolabo). All the samples were characterized by X-ray diffraction, IR spectmmetry and chemical analysis.
3.1. B X-ray diffraction shows that when solution B was added to solution A, the precipitated and dried samples presented an apatitic Structure with relatively fine X-ray lines, only when the atomic ratio Cd/(Sr+Cd) in the initial solutions was lower than 0.6. The higher the level of cadmium, the larger the Xmy lines became. So, the crystahinity of the apatites formed decreased with the cadmium content. For an atomic ratio Cd/(Sr+Cd) of 0.6 the product was quite amorphous ; but when this atomic ratio was greater than 0.6. the precipitate still crystallized again but did not present an apatitic structure. In order to improve the crystallinity of the samples, they were heated at 600°C in a water vapor atmosphere. Only samples with an atomic ratio Cd((Sr+Cd) 20.4, led to a pure apatitic structure. All the others (including Cd/(Sr+Cd)=O.S and 0.6) led to a mixture containing no apatitic phases. The X-ray lines of Cd-Sr apatites were indexed with comparison to strontium hydroxyapatite [8]. The lattice parameter values vary linearly with cadmium composition and follows then a typical Veganfs law : a = 9.756 - 0.512X 8, e=*0.003 E = 7.286 - 0.353X A e=+0.003 with X = Cd/(Sr+Cd) v = 600.5 - 90.3x A3 e=i0.6 The simultaneous and continuous decrease of the lattice constants, when the cadmium content increases, shows the progressive substitution of the small cadmium ion (rcd2+ = 0.95 A) for the large strontium ion (rSr2+ = 1.18 A) [9]. The introduction of cadmium in the strontium apatitic structure leads to a larger decreaseof B parameter than E parameter. The average c/a ratio is 0.7474. The cadmium hydroxyapatite lattice parameters can be calculated using the mathematical equation of Vegard’s law. The extrapolation values, a = 9.244 A and c = 6.933 A are quite different to the experimental ones determined in a previous work [ 101. It appears that strontium-cadmium hydroxyapatites prepared in aqueous medium follow Vega& law over a limited range of solid solutions (0 LCd/(Sr+Cd) Q.4). Chemical analysis showed that the experimental (Sr/Cd)/P atomic ratios of the samples were close to the stoichiometric value of 1.67. The chemical formula of these apatites can be determined from chemical analysis, according firstly to the well known fact that the apatitic structure never contains any vacancies in PO4 site and that the PO4 number is six per cell, and secondly to the complete hydroxylation of the apatitic channel. It can be noted that the amounts of P, Sr and Cd are quite identical in the initial solution and in the final solids. The infrared spectra corresponding to apatites containing 0. 1. 2, 3 and 4 cadmium per cell, which were heated at 600°C are characteristic of apatite. On the strontium hydroxyapatite spectrum the presence of vibration bands at 3592 cm--l and 542 cm-l can be attributed to the stretching and librational vibrations of the OH ions respectively. The positions are in agreement with results of several authors [ll]. In the case of cadmium hydroxyapatite the bands are at 3536 cm-l and 715 cm- 1 respectively [lo]. The attribution of OH bands is more problematic for mixed apatite due to the very weak intensity of the OH bands. Concerning the PO4 bands, the infrared spectrum of hydroxyapatite, containing one cadmium per cell, shows two particular bands at about 949 cm-l. It corresponds to the splitting of the vl phosphate band. This doublet is visualized in figure. When the proportion of cadmium ions in the lattice varies, the intensities of the two bands move in o posite directions. The band at 947 cm-l is present alone (figure, spectrum A) in the absence of CdT.!+ ions, but it splits and gives a broad one at 950 cm-l for apatite containing 4 atoms per cell (figure 1, spectrum E). The weak but steady variation of PO43- band positions to the weak wavenumber when strontium is progressively substituted for cadmium can be attributed to the lattice volume variation.
Strontium-cadmium
substitution
in apatites
59
B
960.0
940 .O
cm-1
FIG. 1 - vl IR bands of PO4 group in hydroxyapatites with atomic ratio Cd/(Cd+Sr) : A=O, B=O.l, C=O.2, Lk0.3, E=O.4 3.2. m
To obtain the precipitation of fluoride apatites, cation solutions containing Sr2+ and Cd2+ were added to anion solutions containing PG43- and F-. X-ray diffraction shows that dried samples crystallized in the apatitic structure, only when the atomic ratio Cd/(Sr+Cd) was lower than 0.6 ; for greater values phase mixtures were formed. In order to improve the crystallinity of the apatites synthesized, dried samples were heated to 6OO’C. This treatment led to a noticeable refinement of X-ray lines which can be indexed in the hexagonal system. A linear decrease of lattice parameters I and I; according to Vegard’s law was observed. The following equations are verified :
A A v = 592.8 - 91.7x A3
a = 9.699 - 0.354~ h = 7.279 - 0.628X
e=+0.003
e=;t0.003
with X = Cd/(Sr+Cd)
e = f 0.6
A solid solution exists between strontium fluorapatites and cadmium fluorapatites only in the range 0 5 Cd/(Sr+Cd) I 0.6. The simultaneous decrease of lattice parameters a and E shows the substi tion of the smaller cadmium ions for strontium. The strontium fluorapatite arameters (a = 9.714 “A, c = 7.281 A) am close to those proposed by Kriedler [12] (a = 9.719 A, c = 7. !?76 A). Unliie in hydroxyapatites, the substitution of cadmium ions for strontium in fluorapatites leds to a greater decrease of parameter E than of parameter 8, and consequently to a decrease of the lattice ratio c/a (c/a = 0.7564 - 0.0381X, e = 0.0008). Contraction of axis E is mom important than that of axis 8. The chemical analysis of the samples showed that all the solids have an atomic ratio (Sr/Cd)/P close to the stoichiometric one 1.67. The infrared spectra of strontium-cadmium fiuorapatites show the absence of OH bands at 3590 cm-l and 540 cm-l. Cadmium - strontium substitution in fluorapatites principally affects the ~1 phosphate ion band, which can be split in mixed apatites (Fieure 2). When the proportion of cadmium introduced into the strontium fluorapatite lattice varies, the intensities of the two bands move in opposite directions. The band at 950 cm-l is only present in the absence of cadmium ions (Fieure2. spectrum A) ; this band disappears and is replaced by another larger one at 946 cm-l when the apatite contains 6 atoms of cadmium per cell (Fiaure 2, spectrum F). Between these two limits, the intensity of the band at 950 cm-l decreases with the quantity of cadmium, while the intensity of that situated at 946 cm-l increasesm2. spectra B, C, D and E). The two band intensities am approximately equal when the quantity of cadmium ions is 3 atoms of cadmium per cell. 4. DISCUSSION The pure apatitic phase, Sr(10-x)Cdx(P04)6Y2 only exists in the range 0 5 x 5 4 for Y = OH and 0 I x I 6 for Y = F. It is possible to substitute cadmium ions, smaller than strontium ions, in sttontium hydtoxy- and fluor-apatites structure, but the reverse is not true.
60
J. L. Lacout et al.
/-----------~-, 910.0
FIG.2-VI
cm-’
910.0
IR bands of PO4 group in fluorapatites with atomic ratio
Cd/(Cd + Sr) : A = 0, B = 0.1, C = 0.2. D = 0.3, E = 0.4, F = 0.6 Concerning the cadmium hydroxyapatite lattice parameters, the values of jb and E. calculated by extrapolation are different from those experimentally determined. It seems that progressive cadmium introduction in lattice induces a greater decrease of B and a smaller decrease of E than expected due to the size of the cation. This can explain the deformation of the lattice, the limit of substitution and the difference with the values of hydroxyapatite parameters. Infrared spectra of strontium-cadmium apatites show a splitting of the vl phosphate ion band. The band, located at about 947 cm-l, is present alone in the absence of Cd2+. When the cadmium ion amount in the lattice increases,two bands appear whose intensities vary in opposite directions. We know that in the apatitic structure, phosphate ions, at six per cell, are isolated from each other and present a Cs site symmetry. The calculation of the vibration numbers according to the site and space groupe P63/m, indicates nine active vibrations in the infrared. In the range we are interested in, from 970 cm-l to 930 cm-I, the apatite allows only one absorption band : symmetric valency band vl. The splitting of the vl band must be correlated with the fact that the six phosphate ions which are normalIy equivalent in the P63/m symmetry are not equivalent when a certain amount of cadmium ions substitutes for strontium ions. Switch et al. [13] have already observed the same phenomenon in the substitution of manganese for calcium in fluorapatites. They have shown, after a detailed study, that the splitting of the vl absorption band of phosphate ions is attributed to the preferential location of manganese in Me(I). 5. CONCLUSION The complete solid solutions strontium/cadmium hydroxy- or fluor-apatite does not exist ; according to the general formula Sr(l@x)Cdx(PO4)6Y2, hydroxyapatite (Y=OH) exists in the range 0 I x I 4, and fluorapatite (Y=F) in the range 0 I x I 6. These limits can be correlated with the different dimensions for strontium and cadmium ion and/or the preferential location of the ions in the two crystallographic cationic sites. A detailed structural study by the Rietveld method can perhaps confirm this hypothesis. 6.i31
PI [lo] [ll] [ 121 [13]
D. MC CONNEL, Apatites, B, New-York, 1973. R. FEITH. T.J.J.H. SLOOFF, I. KAZEM,T.J.H. VANRES, I,Bone M. ANDRES-VERGES, F.J. HIGES, P. FELIX GONZALES-DIAZ, I., IQ 13c y, lL.J.
1978,5&j& 79. 1980,
Powder Diffraction File, Int. Center for Diff. Data Penn. USA, JCPDS file : 14-691. R.D. SHANNON, Acta Crvstallorrr, 1976, A32. Part 5,751. A. NOUNAH. J. SZILAGYI, J.L. LACOUT. Ann.Chim.Fr., 1990, u, 409. G. ENGEL, W.E. KLEE, J.Salid 1972,Z 28. E.R. KRBIDLER, Ph. D. T&& Pennsylvania’University, 1967. P.R. SUITCH, J.L. LACOUT, A. HEWAT, R.A. YOUNG, s, 1985.m.
173.