Mechanochemical synthesis and ionic conductivity of lanthanum phosphosilicate oxyapatites

Nadia Gmati; Khaled Boughzala; Ezzedine Ben Salem; Anis Chabene; Nabil Fattah; Khaled Bouzouita

doi:10.1016/j.crci.2013.10.018

Mechanochemical synthesis and ionic conductivity of lanthanum phosphosilicate oxyapatites
[Mécanosynthèse et conductivité ionique d’oxyapatites phosphosilicatées au lanthane]

Nadia Gmati ¹ ; Khaled Boughzala ¹ ; Ezzedine Ben Salem ¹ ; Anis Chabene ¹ ; Nabil Fattah ² ; Khaled Bouzouita ¹

¹ Institut préparatoire aux études d’ingénieur de Monastir, 5019 Monastir, Tunisia
² Compagnie des phosphates de Gafsa, Centre de recherche de Metlaoui, 2130 Metlaoui, Tunisia

Comptes Rendus. Chimie, Volume 17 (2014) no. 9, pp. 920-926.

Résumés

Anglais
Français

Lanthanum phosphosilicate apatites with the chemical formula Sr_10–xLa_x(PO₄)_6–x(SiO₄)_xO, where 0 ≤ x ≤ 6, usually prepared by a solid-state reaction at about 1400 °C, were synthesized via the mechanochemical method at room temperature. The samples were characterized using powder X-ray diffraction, infrared spectroscopy and thermal analysis. The results showed that the prepared products were carbonated apatites and no secondary phase was detected. The realization of the milling under a controlled atmosphere can lead to oxyapatites containing no carbonates. The ionic conductivity of the Sr₆La₄(PO₄)₂(SiO₄)₄O sample was investigated by using impedance spectroscopy. The highest ionic conductivity value of 1.522 × 10⁻⁶ S·cm⁻¹ was found at 800 °C. In the investigated temperature range, the activation energy is of 0.85 eV.

Des apatites phosphosilicatées au lanthane de formule chimique Sr_10–xLa_x(PO₄)_6–x (SiO₄)_xO où 0 ≤ x ≤ 6, généralement préparées par réaction à l’état solide à environ 1400 °C environ, ont été synthétisées par mécanosynthèse à la température ambiante. Les échantillons ont été caractérisés par diffraction des rayons X, spectroscopie infrarouge et analyse thermique. Les résultats ont montré que les produits obtenus étaient des apatites carbonatées et aucune phase secondaire n’a été détectée. La réalisation du broyage sous atmosphère contrôlée devrait conduire à des oxyapatites ne contenant pas de carbonates. La conductivité ionique de l’échantillon Sr₆La₄ (PO₄)₂(SiO₄)₄O a été étudiée par spectroscopie d’impédance. La valeur de la conductivité ionique la plus élevée, 1,522 × 10⁻⁶ S·cm^–1, a été déterminée à 800 °C. Dans le domaine de température étudié, l’énergie d’activation est de 0,85 eV.

Métadonnées

Reçu le : 2013-07-22
Accepté le : 2013-10-28
Publié le : 2014-06-30

DOI : 10.1016/j.crci.2013.10.018

Keywords: Oxyapatite, Rare earths, Mechanochemical method, Ionic conductivity
Mot clés : Oxyapatites, Terres rares, Mécanosynthèse, Conductivité ionique

Affiliations des auteurs :

Nadia Gmati ¹ ; Khaled Boughzala ¹ ; Ezzedine Ben Salem ¹ ; Anis Chabene ¹ ; Nabil Fattah ² ; Khaled Bouzouita ¹

¹ Institut préparatoire aux études d’ingénieur de Monastir, 5019 Monastir, Tunisia
² Compagnie des phosphates de Gafsa, Centre de recherche de Metlaoui, 2130 Metlaoui, Tunisia

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     author = {Nadia Gmati and Khaled Boughzala and Ezzedine Ben Salem and Anis Chabene and Nabil Fattah and Khaled Bouzouita},
     title = {Mechanochemical synthesis and ionic conductivity of lanthanum phosphosilicate oxyapatites},
     journal = {Comptes Rendus. Chimie},
     pages = {920--926},
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Nadia Gmati; Khaled Boughzala; Ezzedine Ben Salem; Anis Chabene; Nabil Fattah; Khaled Bouzouita. Mechanochemical synthesis and ionic conductivity of lanthanum phosphosilicate oxyapatites. Comptes Rendus. Chimie, Volume 17 (2014) no. 9, pp. 920-926. doi : 10.1016/j.crci.2013.10.018. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2013.10.018/

Version originale du texte intégral

1 Introduction

Apatites are a large family of compounds with the general chemical formula M₁₀(XO₄)₆Y₂, where M represents a divalent cation, XO₄ an anion group and Y a monovalent anion. They crystallize mainly in the hexagonal system with the P6₃/m space group [1] (Fig. 1). The main representative member of the apatite family is the hydroxyapatite HA, Ca₁₀(PO₄)₆(OH)₂, that can be considered as a wonderful compound. Its chemical composition and crystallographic structure similar to that of the mineral phase of bone and teeth make it a material of choice in many medical applications [2–4]. Furthermore, as a result of its ability to accommodate a great number of substitutions: cationic, anionic or both cationic and anionic substitutions, practically a third of the periodic table elements can be incorporated within the apatite lattice [5,6], making the application field of the obtained materials practically unlimited [7–14]. Indeed, this flexibility of the apatite structure to accept a variety of species may confer to the prepared materials very diverse properties, resulting in a variety of applications that are either already planned or are still to be discovered.

Fig. 1
(Color online). Positions of atoms in the strontium oxyapatite unit cell (Sr₁₀(PO₄)₆O).

For example, simultaneous substitutions can occur in the three sites (M, XO₄ and Y). In hydroxyapatite, the substitution of La³⁺, $Si O_{4}^{4 -}$ and O^2–for Ca²⁺, $P O_{4}^{3-}$ and OH^–, respectively leads to new compounds, which have gained considerable interest as oxide ion conductors following the works of Nakayama et al. [15–17]. These compounds are usually prepared by a solid-state reaction at high temperatures. Also, several heat treatments are often required to achieve single-phase materials [18–21]. On the contrary, the mechanochemical synthesis involves only a solid-state reaction at room temperature [22,23]. Therefore, this method appears quite attractive for the preparation of this kind of materials [24,25]. Besides its low energy consumption, simplicity and speed, it is suitable for industrial production. Furthermore, the mechanical treatment leads to an increase in the specific surface area of powders, which should improve the densification of the materials [26].

The aim of this work was to attempt the preparation of the series (Sr_10–xLa_x(PO₄)_6–x(SiO₄)_xO, where 0 ≤ x ≤ 6) via the mechanochemical method. The obtained powders were investigated using XRD, FTIR and TGA/DTA. In addition, we have characterized the Sr₆La₄(PO₄)₂(SiO₄)₄O sample by its ionic conductivity.

2 Experimental procedure

2.1 Preparation of powders

The samples were prepared by the mecanochemical process using a Retsch PM200 planetary micro mill. The synthesis reaction occurs as follows:

4 SrO + (6- x) {/2 Sr}_{2} P_{2} O_{7} + x / 2 {La}_{2} O_{3} + x {SiO}_{2} \to {Sr}_{10 - x} {La}_{x} {({PO}_{4})}_{6 - x} {({SiO}_{4})}_{x} O

(1)

Appropriate amounts of strontium oxide (SrO), lanthanum oxide (La₂O₃), silica (SiO₂) and strontium diphosphate (Sr₂P₂O₇) in order to obtain (Sr + La)/(P + Si) atomic ratios of 1.67 were ground and homogenized in an agate mortar prior to grinding. Then, the mixture was introduced with several stainless steel balls, 10 mm in diameter in a 50 cm³ stainless steel cell. The weight ratio of the ball-to-powder for all samples was 34:1. The rotating disc speed and cell speed were 500 and 1000 rpm, respectively. Following a previous study [24], the grinding duration was fixed at 25 h. In order to avoid excessive temperature inside the mill cell, milling was carried out in 30 minutes intervals with a 5 minutes pause.

The strontium diphosphate was obtained by heating a mixture of strontium carbonate and di-ammonium hydrogenophosphate at 900 °C for 10 h, while strontium oxide was obtained by calcination of the corresponding carbonate at 1100 °C for 24 h. To avoid deviation from stoichiometry, lanthanum oxide was calcined at 1000 °C for 24 h just before use. Indeed, when La₂O₃ is exposed to air, there is formation of La₂(OH)_6–2x(CO₃)_x, with x ≈ 1 [27,28].

In the following sections, the compositions Sr₁₀(PO₄)₆O, Sr₉La(PO₄)₅(SiO₄)O, Sr₈La₂(PO₄)₄(SiO₄)₂O, Sr₆La₄(PO₄)₂(SiO₄)₄O, Sr₅La₅(PO₄)(SiO₄)₅O and Sr₄La₆(SiO₄)₆O will be labeled Sr₁₀O, Sr₉LaO, Sr₈La₂O, Sr₆La₄O, Sr₅La₅O and Sr₄La₆O, respectively.

2.2 Powder characterization

X-ray diffraction (XRD) patterns of samples were recorded on an X’Pert Pro, PANalytical diffractometer operating with the Cu Kα radiation. The samples were scanned in the 2θ range from 20 to 65° with a step size of 0.01° and a counting time of 1 s per step. The experimental patterns were compared to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS) using the X’Pert HighScore Plus software. The lattice parameters of all samples were determined by the least squared method.

Infrared absorption spectra were recorded in the 400–4000 cm⁻¹ range with a PerkinElmer Spectrum 100 spectrometer, using the KBr pellet technique.

Thermal analysis was conducted in air from room temperature to 1000 °C at a heating rate of 10 °C/min, using a Setaram 92 apparatus.

Impedance measurements were carried out on the Sr₆La₄O sample using a Hewlett-Packard 4192-A impedance analyzer, operating at frequencies ranging from 5 to 13 MHz. After milling for 25 h, the powder was pelletized under a pressure of 13 tons. Then, the pellets were heated at 900 °C for 24 h. This temperature was imposed by our furnace capacity. The obtained pellet has a thickness (e) of 0.0885 cm and a surface (S) of 1.252 cm². The geometric factor (e/S) of the pellet is 0.07 cm⁻¹. The relative density was of about 60%. Silver electrodes were painted on the two faces of the pellets with a silver paste, and then the painted pellets were heated at 300 °C for 1 h.

3 Results and discussion

3.1 X-ray diffraction

Fig. 2 shows the XRD patterns of samples ground for 25 h. As seen, all the patterns exhibit only the reflections of an apatite phase, indexed in the hexagonal system (P6₃/m space group) based on the strontium oxyapatite, Sr₁₀(PO₄)₆O (JCPDS card No. 00-044-0654), and no evidence of any second phase was found. This indicates that all the prepared samples were single phase. However, the presence of other phosphate compounds as amorphous phase or as impurities in small quantities could not be excluded.

Fig. 2
XRD patterns of all the samples milled for 25 h.

3.2 Infrared spectroscopy

The IR absorption spectra of the samples milled for 25 h are displayed in Fig. 3. The assignments of the different absorption bands were performed according to the literature [29–32]. The similarity of the vibration modes of the two tetrahedral groups ( $P O_{4}^{3-}$ and $Si O_{4}^{4 -}$ ) has made the identification of the bands somewhat difficult. The assignments of the observed vibrational frequencies of the prepared samples are summarized in Table 1.

Fig. 3
FTIR spectra of all the samples milled for 25 h.

Samples	PO₄^3–	SiO₄^4–
	ν₁ (cm⁻¹)	ν₃ (cm⁻¹)	ν₄ (cm⁻¹)	ν₁ (cm⁻¹)	ν₃ (cm⁻¹)	ν₄ (cm⁻¹)
Sr₁₀O	946	1042 1073	591 561	–	–	–
Sr₉LaO	942	1041	591 563	871	916	504 543
Sr₈La₂O	942	1043	593 564	872	916	505 544
Sr₆La₄O	943	1042	562 595	874	915	499 542
Sr₅La₅O	942	1046	596 –	877	920	498 544
Sr₄La₆O	–	–	–	–	922	540 496

For the non-substituted sample, the bands at 1073–1042 and 946 cm⁻¹ correspond to P–O stretching vibration modes, while the doublet at 591–561 cm⁻¹ corresponds to the OPO bending mode. The doublet at 1453 and 1406 cm⁻¹, and the band at about 863 cm⁻¹ were assigned to the ν₃ and υ₂ vibration modes of $C O_{3}^{2-}$ groups. These bands indicate that some $P O_{4}^{3 -}$ groups were partly replaced by $C O_{3}^{2 -}$ ones in the apatite structure (B-type substitution). Therefore, an equivalent amount of SiO₂ was expected to remain in its original form in the powder. It was not detected probably due to its low quantity. The weak band at around 3570 cm⁻¹ can be attributed to the stretching mode of the hydroxyl group, indicating that some OH^–groups were inserted into the apatite lattice. Notice the lack of the band at 630 cm⁻¹ corresponding to the bending mode of this latter group. Thus, although the grinding was carried out by a dry process, air moisture may result in the incorporation of hydroxyl groups into the apatite structure. It is well known that phospho-oxyapatite is rarely obtained in a pure state [33]. The bands around 3422 cm⁻¹, as well as the band at 1606 cm⁻¹, are related to the adsorbed water on the solid surface. For the substituted samples with 1 ≤ x ≤ 5, additional bands appeared at 920, 872 and 500–545 cm⁻¹, they were assigned to the $Si O_{4}^{4 -}$ group. The band located at 920 cm⁻¹ is assigned to the asymmetric stretching mode (ν₃) of the SiO bond, while that at 872 cm⁻¹ corresponds to the symmetric stretching mode (ν₁). Finally, the bands between 500 and 545 cm⁻¹ are attributed to the asymmetric deformation mode (ν₄). Furthermore, with the increase of the silica amount in the initial mixture, the intensities of the $Si O_{4}^{4-}$ bands increased, whereas those of the $P O_{4}^{3 -}$ bands decreased. This suggests that the $Si O_{4}^{4-}$ ions substituted for the $P O_{4}^{3 -}$ ones into the apatite lattice. However, as some carbonate ions were incorporated into the apatite structure, an equivalent amount of SiO₂ would not participate in the reaction. The bands associated with the $P O_{4}^{3 -}$ group disappeared completely for the totally silicated sample (x = 6).

3.3 Thermal behavior

TG/DTA curves of the Sr₆La₄O sample milled for 25 h are given in Fig. 4. According to equation (1), the heat treatment must not be accompanied by any weight loss. However, when carbonate groups are incorporated into the apatite structure, in accordance with the charge balance, a certain amount of lanthanum oxide or strontium oxide would not participate in the reaction [24]. It is well known that the lanthanum oxide is very sensitive to moisture and carbon dioxide of air. So, after a prolonged exposure to air, it turns into a mixture of oxide, hydroxide and amorphous hydrated hydroxycarbonate [27,34]. Furthermore, as the FTIR spectroscopic analysis showed that the prepared apatites were carbonated, weight losses should occur during heat treatment. Panteix studying the sintering of La_9.33(SiO₄)₆O₂ observed that although the pellets are sufficiently densified, they degraded when they are left long enough in the air. He attributed this phenomenon to the very high hydrophilicity of the unreacted La₂O₃ and to its sensitivity to air carbon dioxide [27].

Fig. 4
Thermal analysis of the sample Sr₆La₄O milled for 25 h.

The TG profile (Fig. 4) indicated that the weight loss occurred in three temperature ranges. The first weight loss of 0.9% observed in the range from 280 to 350 °C is due to the dehydration of La(OH)₃ and La₂(OH)₄CO₃·nH₂O to LaOOH and La₂O₂CO₃, respectively. The second weight loss of 0.4% achieved at 480 °C was attributed to the departure of water resulting from the transformation of LaOOH into La₂O₃ [27,28,34]. These dehydrations are related to the two first endothermic peaks positioned on the DTA curve at 326 and 473 °C, respectively. The third weight loss of 5.6% occurring between 640 and 900 °C is related to the decarbonation of La₂O₂CO₃ [27,34] and the apatite, The departure of CO₂ is illustrated on the DTA curve by the third and the fourth effects at 737 and 870 °C. It is worth noting that no traces of unreacted oxides or carbonates were detected on the XRD patterns, due probably to their amounts that were below the detection limit of XRD.

The XRD pattern of Sr₆La₄O sample milled for 25 h and calcined at 900 °C for 24 h shows that the powder was formed by a pure apatite phase, and no secondary phases were detected (Fig. 5). As observed in Fig. 6, the heat treatment did not cause any significant change in the IR spectrum. It indicates only a decrease in the intensity of bands associated with the carbonate ions.

Fig. 5
XRD patterns of the sample Sr₆La₄O milled and calcined at 900 °C.

Fig. 6
FTIR spectra of the sample Sr₆La₄O milled and calcined at 900 °C.

3.4 Lattice parameters

The lattice parameters are given in Table 2. For the non-substituted sample, the values of a = 9.749(2) Ǻ and c = 7.270(3) Ǻ are lower than those reported in the JCPDS card No. 00-044-0654 (a = 9.7670 Å and c = 7.2890 Å). With the substitution, the a parameter increased, while c decreased. The changes in the lattice parameters confirm once again that the pair (La³⁺, $Si O_{4}^{4 -}$ ) has entered the apatite structure [24]. However, the interpretation of the variation of the parameters with substitution remains difficult because of the difference in size of the substituent ions (Table 3) [35]. Furthermore, after the insertion of some carbonate ions into the apatite lattice, as it was indicated by IR absorption spectroscopy, the interpretation becomes more complicated. However, it is possible to draw some conclusions. In the apatite lattice, the a-axis is rather influenced by the size of the M(2) site, the anionic tetrahedron and the anion of the tunnel, while the c-axis is mainly influenced by the size of the M(1) site. In the present study, the $P O_{4}^{3-}$ group (the average length of P–O is 1.51 Å) is replaced by both a more and a less voluminous groups, $Si O_{4}^{4 -}$ (the average length of Si-O is 1.62 Å) and $C O_{3}^{2-}$ (the average length of C–O is 1.25 Ǻ) [35]. In addition, Sr²⁺ ( ${}^{VII}r_{{Sr}^{2 +}}$ = 1.21 Å; ${}^{IX}r_{{Sr}^{2 +}}$ = 1.31 Å) was substituted by a less voluminous ion La³⁺ ( ${}^{VII}r_{{La}^{3 +}}$ = 1.10 Å; ${}^{IX}r_{{La}^{3 +}}$ = 1.216 Ǻ) [35].

Samples	a (Å)	c (Å)	V(Å³)
Sr₁₀O	9.749 (2)	7.270 (3)	599.06 (3)
Sr₉LaO	9.752 (1)	7.269 (3)	599.35 (1)
Sr₈La₂O	9.756 (3)	7.265 (2)	599.51 (3)
Sr₆La₄O	9.758 (2)	7.263 (2)	599.59 (2)
Sr₅La₅O	9.762 (3)	7.260 (4)	599.83 (3)
Sr₄La₆O	9.767 (4)	7.258 (2)	600.28 (2)

	Sr²⁺	La³⁺	P⁵⁺	Si⁴⁺	C⁴⁺
	CN7/CN9	CN7/CN9	CN4	CN4	CN4
Ionic radii (Å)	1.21/1.31	1.10/1.22	0.17	0.26	0.15

As was shown in ref. [24], following these substitutions, there was creation of vacancies in the cationic sites. In the apatite structure, the cations are distributed between two nonequivalent crystallographic sites. Four cations per unit cell occupying the (4f) sites are arranged along the three-fold axis. The six other cations located in the (6 h) sites are positioned in the summit of two alternated equilateral triangles, centered on a helical six-fold axis of the structure. The alignment in columns, and the shorter distances between the (4f) sites cause strong repulsions between the La³⁺ ions located in these sites, while the higher distances between the (6 h) sites make them more suitable to minimize the repulsions. Therefore, these latter sites should be fully occupied by the cations and the vacancies should be present only at the (4f) sites. This assumption is supported by the Rietveld refinement of La_9.33(SiO₄)₆O₂, which showed that all of the vacancies are present at the (4f) sites [36]. Vacancies were also created in the anionic tunnel following the substitution of O^2–for OH^–. Thus, it appears that the contraction of the c parameter is due to the replacement of Sr²⁺ by a smaller ion, La³⁺, on the one hand, and to the presence of vacancies along this axis, on the other hand, while the increase of the a parameter is due to the contraction following the c-axis. The explanation of the effect of substitution on the a-axis is not as obvious because of the different sizes of the substituted ions. The comparison of the obtained parameter values with those determined by K. Boughzala for the same compounds, prepared at high temperature [37], shows the same trend as a function of x for the two parameters. It is worth noting that these compounds are free of carbonates. Our values are weaker probably because of the incorporation of carbonates into the apatite structure (see § 3.2.).

3.5 Ionic conductivity

Typical complex impedance plots of the Sr₆La₄O sample taken in the 525–800 °C range are illustrated in Fig. 7. As seen, the impedance response of the sample consists of semicircular arcs. The value of resistance at different temperatures has been obtained from the intercept of the semicircular arcs on the real axis, and the electrical conductivity at different temperatures was calculated using the following equation: σ = e/R·s, where R is the resistance determined from impedance plots, and e and s are the thickness and the area of the sample, respectively. The obtained values increased with increasing temperature, varying from 6.76 × 10⁻⁸ to 1.52 × 10⁻⁶ S·cm⁻¹ for 525 and 800 °C, respectively, indicating a thermally activated conduction mechanism. These values correspond to the total conductivity because of the difficulty to separate the grain and bulk boundary contribution from the diagrams.

Fig. 7
The impedance plots of Sr₆La₄O at different temperatures.

The temperature dependence of the ionic conductivity has been analyzed using an Arrhenius equation: σ T = A exp(–E_a/kT), where σ, A, E_a, k and T are the conductivity, pre-exponential factor, activation energy, Boltzmann constant and absolute temperature, respectively. Fig. 8 shows the Arrhenius plot of the sample. The activation energy E_a, determined from the slope of the previous plot, is 0.85 eV.

Fig. 8
Plot of log σT (S·K·cm^–1) versus 1000/T (K^–1) for Sr₆La₄O.

It is well known that for the rare earth silicate oxypatites, the highest conductivity is observed for the samples containing cationic vacancies and/or an oxygen excess [38,39]. Compared to the lanthanum silicate oxyapatite (La_9.33(SiO₄)₆O₂) [40], the studied material has a lower conductivity. This can be explained, on the one hand, by the high porosity of the material (relative density = 60%) and on the other hand, by its structure features. As was revealed by IR spectroscopy analysis, all carbonates have not been completely eliminated after heat treatment at 900 °C. Therefore, with respect to the oxyapatite-type La_9.33SiO₂₆, the carbonate ions, and those of phosphate contracted the a-axis, in accordance with their respective sizes. It results a contraction of the tunnel containing oxide ions, limiting therefore the movement of these latter ions. Indeed, it was shown that a high conductivity is characterized by larger channels, in addition to the cationic vacancies [41]. Panteix et al. studying the influence of anionic vacancies on the ionic conductivity of the lanthanum silicate oxyapatite observed that the conductivity is enhanced and the activation energy is decreased for the low levels of oxygen vacancies, while for the high levels, the conductivity is lower and the activation energy is higher [42]. Therefore, the anionic vacancies due to the substitution of O^2–by OH^–contributed to lowering the conductivity for our sample. In contrast to the fully stoichiometric sample, Sr₆La₄(PO₄)₄(SiO₄)₂O₂ [40], the compound investigated in the present work had a higher conductivity, confirming once again that this latter contained cationic vacancies, obviously in addition to the anionic vacancies.

4 Conclusion

Oxyapatites with the chemical formula Sr_10–xLa_x(PO₄)_6–x(SiO₄)_xO (where 0 ≤ x ≤ 6) have been prepared from SrO, Sr₂P₂O₇, La₂O₃ and SiO₂ using the mechanochemical process. The obtained powders have been characterized by X-ray diffraction, infrared spectroscopy, and thermogravimetric and differential thermal analysis. The results indicated that the powders were single phase. However, they were carbonated, due to the air carbon dioxide. The electrical properties of the Sr₆La₄(PO₄)₂(SiO₄)₄O sample was investigated by using complex impedance analysis. The highest ionic conductivity of 1.52 × 10⁻⁶ S·cm⁻¹ was obtained at 800 °C and the ionic jump activation energy is of 0.85 eV.

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