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Structure and thermal stability of fluorhydroxyapatite and fluorapatite obtained by mechanochemical method
Journal Pre-proof Structure and thermal stability of fluorhydroxyapatite and fluorapatite obtained by mechanochemical method Natalia V. Bulina, Svetlana V. Makarova, Igor Yu Prosanov, Olga B. Vinokurova, Nikolay Z. Lyakhov PII:
S0022-4596(19)30581-X
DOI:
https://doi.org/10.1016/j.jssc.2019.121076
Reference:
YJSSC 121076
To appear in:
Journal of Solid State Chemistry
Received Date: 31 July 2019 Revised Date:
15 October 2019
Accepted Date: 18 November 2019
Please cite this article as: N.V. Bulina, S.V. Makarova, I.Y. Prosanov, O.B. Vinokurova, N.Z. Lyakhov, Structure and thermal stability of fluorhydroxyapatite and fluorapatite obtained by mechanochemical method, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.121076. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Structure and Thermal Stability of Fluorhydroxyapatite and Fluorapatite Obtained by Mechanochemical Method Natalia V. Bulina*, Svetlana V. Makarova, Igor Yu. Prosanov, Olga B. Vinokurova, Nikolay Z. Lyakhov Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze street 18, Novosibirsk 630128, Russia Abstract This work shows that mechanochemical synthesis allows the obtaining of hydroxyapatite in which fluorine ions replace hydroxyl ions either partially or completely after 30 minutes of synthesis. The crystal structure of the materials obtained and their thermal stability when heated in the air were studied. It has been shown that the fluorine ions in the hexagonal channel of hydroxyapatite hinders diffusion of hydroxide ions at high temperatures, which leads to increased thermal stability of the apatite. Keywords: fluorapatite, fluorhydroxyapatite, mechanochemical synthesis, crystal structure 1. Introduction Hydroxyapatite Ca10(PO4)6(OH)2 (HA) is a representative of apatite structural type with the general formula of Ме10(RO4)6X2 with Р63/m space group. The HA crystal structure is described in detail in [1, 2]. The HA unit cell contains 10 calcium cations located in two nonequivalent positions: four cations in the Ca1 site, each surrounded by nine oxygen atoms, and six cations in the Ca2 site, each surrounded by seven oxygen atoms (Fig. 1 (a, c)). In addition to calcium ions, the HA elementary cell has six phosphate and two hydroxyl groups. The latter are located on the c axis in a hexagonal channel formed by calcium and oxygen ions from phosphate tetrahedrons. Hydroxyl groups in the adjacent channels are oriented in opposite directions (Fig. 1 (a, c)).
*
a
b
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d
Corresponding author at: Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze Str. 18,
Novosibirsk, 630128, Russia. E-mail address: [email protected] (N.V. Bulina)
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Fig. 1. HA (a, c) and FA (b, d) structures: projection along the z axis (a, b) and along the y axis (c, d) A wide range of substitutions are possible within the apatite structure, both in the cationic and anionic sublattices [1]; stoichiometric HA is therefore not practically found in the natural world. Usually, biogenic HA, the main mineral component of human bone and dental tissue, contains a carbonate group, calcium, sodium and chlorine ions, etc. [3]. Each type of ion in the apatite’s structure affects the properties of the substance obtained. For example, fluorine ions have a significant effect on the strength characteristics of tooth enamel [3]. This could be explained by the fact that fluorine ions, when replacing the hydroxyl group (Fig. 1 (b, c)), form fluorapatite (FA) Ca10(PO4)6F2, which has the highest hardness and lowest solubility among all calcium phosphates [3]. Despite the very low content of fluorine in enamel (0.01%), it makes this tissue the hardest in the human body. HA and FA are the most demanded calcium orthophosphates in dentistry. They are used as coatings to increase the osteoinductive potential of various dental implants, as additives in dental cements, in materials used for restoration of bone tissue, as well as in dentifrices [4]. In addition, apatites are used as model compounds of tooth enamel [1]. In vitro studies showed that FA can be used as a source of fluorine ions emitted at a controlled rate to ensure the formation of mechanically and functionally strong bone [5]. Partially and completely substituted FA can be obtained by ceramic method [6], precipitation from aqueous solution [7-8], through the sol-gel [9], hydrothermal [10-11], combustion [12], and mechanochemical methods [13-15]. Nikcevic et al showed that FA can be obtained after 9 hours of processing a mixture of Ca(OH)2, Р2О5 and СаF2 during mechanochemical synthesis in a RETCH planetary mill with a vial rotation speed of ~ 230 rpm [13]. In [14], FA was obtained in a mill with a vial rotation speed of 300 rpm after 6 hours of reaction mixture processing. Literature data say that, during mechanochemical synthesis, structural and chemical transformations of solids occur only when the applied stress is above the theoretical ultimate strength of the initial reagents [16]. In this regard, the increased rotation speed of the mill should increase the power released by the mechanical action, which will increase the applied stress, and, accordingly, will make it possible to reduce the time spent on processing. This research studies the possibility of the mechanochemical synthesis of fluorine-substituted hydroxyapatite in the AGO-2 planetary mill, with a vial rotation speed of 1800 rpm. 2. Materials and Methods Mechanochemical synthesis of fluorapatites was carried out in an AGO-2 planetary ball mill with steel vials and balls as grinding bodies. The synthesis conditions were similar to those used to obtain other substituted apatites [17, 18]. The synthesis time was 30 minutes at a vial rotation speed of 1800 rpm. The yield of the apatite synthesized by mechanochemical method in AGO-2 mill was 10 g/h. Calcium hydrogen phosphate, calcium oxide (just annealed at 1000 °C) and calcium fluoride (pure grade) were taken as the initial reagents. The powders were mixed in the stoichiometric ratios required to obtain fluorapatite with the degree of hydroxyl group substitution for fluorine from 0 to 2 mol: 6CaHPO4 + (4-х/2)CaO + х/2CaF2 = Ca10(PO4)6(OH)2-хFх + nН2О
(1)
where х = 0, 0.2, 0.5, 1, 2. To define the crystal structure of the samples, the products of the mechanochemical synthesis were annealed at 1000 ° C in the PVK-1.4-8 high-temperature furnace for 2 hours. 2
Samples were analyzed by FTIR spectroscopy on a INFRALUM FT801 spectrometer and by X-ray diffraction (XRD) analysis on a D8 Advance powder diffractometer with Cu-Kα radiation, equipped with a one-dimensional Lynx-Eye detector and a Kβ filter. The phase composition of the obtained samples was identified using the ICDD PDF-4 powder database (2011). The structural parameters were specified by Rietveld refinement analysis of powder XRD patterns using Topas 4.2 software. To model the profile of any diffraction reflections, the Lorentzian function was used. The size of the crystallites was calculated from the integral width of the reflections using the Scherrer equation, assuming the presence of spherical particles in the sample. The instrumental function was calculated by the fundamental parameters method [19]. The initial data for HA and FA are taken from [20, 21]. The parameters of the background, sample displacement, lattice parameters, crystallite size and atomic coordinates were refined. Initial refinements for the samples with x = 0.2–1 were conducted considering the position of ions in the hexagonal channel as a non-split one, i.e. with the same coordinates for O4 and F. The final refinements were performed using individual positions for O4 and F, namely position of fluorine was fixed at special site (0, 0, 0.25) based on literature data [21], whereas the value of z for oxygen from the hydroxyl group was under refinement. 3. Results and Discussion 3.1. Mechanochemical synthesis The XRD patterns of the samples obtained after mechanochemical synthesis are shown in Fig.2. All the observed reflections correspond to the apatite phase (card PDF 040-10-6315), so the samples obtained are therefore single-phase. It can be seen that, as the concentration of fluorine increases, the maxima of the reflections shift to large angles which indicates a decrease in the lattice spacing.
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Fig. 2. XRD patterns of samples with different concentrations of fluorine after mechanochemical synthesis for 30 minutes The results of the lattice parameters refinement (Fig. 3) show that the FA unit cell volume mainly decreases due to a decrease in the a parameter, a slight increase in the c parameter does not have a significant effect on the cell volume. Literature data [1, 2, 13, 22] confirm that FA has a significantly smaller a parameter compared to HA. The decrease in the a and, respectively, b parameters is explained by the smaller radius of the substituent ion localized in the hexagonal channel on the c axis. The oxygen of OH groups is located on the c axis between calcium and oxygen ions of the phosphate tetrahedron (Fig. 1 (c)). The smaller radius of the fluorine ion allows it to occupy the more advantageous site – the plane of Ca2 ions (Fig. 1 (d)), that leads to a contraction of the channel in a and b directions. Due to the large distance between the ions located on the c axis, this substitution does not significantly affect the value of c parameter. The size of the crystallites in the synthesized samples does not depend on the degree of substitution and equals to ~ 25 nm (Fig. 3).
a
b
c
d
Fig. 3. Evolution of the unit cell parameters a (a) and c (b), lattice volume (c) and crystallite size (d) depending on the concentration of fluorine used for the samples before and after annealing
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In the FTIR spectrum of HA (Fig. 4, x = 0), the absorption bands of the phosphate tetrahedron and hydroxyl group [1] are observed: the bending vibrations of the O–P–O bond (572 and 602 cm–1), stretching vibration of the P–O bond (962, 1048 and 1090 cm–1), libration (632 cm–1) and stretching vibrations (3574 cm–1) of the OH groups.
Fig. 4. FTIR spectra of samples after mechanochemical synthesis with different concentrations of fluorine At a small degree of substitution (x = 0.2), an increase in the frequency of the libration vibration and a decrease of the intensity and frequency of the OH group stretching vibration are observed (Fig. 4). As the substitution increases, the intensity of these bands decreases. The absence of OH group vibrations at x = 2 shows the absence of hydroxyl groups in the apatite structure that is consistent with the chemical formula for FA and with literature data [13,20,22-23]. In the absorption spectra for intermediate fluorine concentrations (x = 0.5–1), a low intensity band is observed at ~ 730 cm–1, which can be attributed to libration mode of the OH group connected with a fluorine ion by a hydrogen bond [13,20,22–23]. The absorption bands of phosphate ion gradually shift to higher frequencies with an increasing concentration of fluorine (Fig. 4), that is associated with a decrease of the bond lengths in the phosphate tetrahedron and is consistent with a decrease in the unit cell volume, as shown above (Fig. 3). In addition to the absorption bands of the phosphate and hydroxyl groups, in the FTIR spectra of all samples (Fig. 4) there are low-intensity wide absorption bands of the carbonate ion (1400–1500 cm–1) absorbed from air during the synthesis, and sorbed water (3250–3660 cm–1), released in accordance with the synthesis reaction (1). 3.2. Annealed samples Annealing of the samples after mechanochemical synthesis made it possible to significantly increase the crystallinity of the samples (Fig. 3 (d)), while the dynamics of the lattice parameters remained the same (Fig. 3 (a – c)). The Rietveld plots for samples with x = 0 and x = 2 are shown in Fig. 5. Tables 1 and 2 present the results of refinement of the apatite structure for all annealed samples. It is seen that among all the ions located in the apatite unit cell, the oxygen ion of the hydroxyl group located in the O4 site shows the greatest displacement with increasing substitution degree (Table 1). This ion becomes closer to the fluorine ion with increasing fluorine concentration. At the same time the distance between adjacent O4 positions remains constant; it does not depend on the concentration of the substituent 5
(Table 2). The length of the Ca2–F bond gradually decreases, which indicates a decrease in the channel diameter. The distance between the oxygen ion of the OH group (O4) and the nearest oxygen ion of the phosphate tetrahedron (O3) with which there is a hydrogen bond [1, 22] is comparable to the F–O4 distance, that proves the existence of additional hydrogen bond between the OH group and fluorine ion.
a b Fig. 5. Observed, calculated and difference diffraction patterns for the samples with x = 0 (a) and x = 2 (b) after the Rietveld refinement. The peak positions of HA, FA and CaO are shown by vertical markers. Concentration of CaO admixture in the sample with x = 0 is 0.2 wt.% Tab. 1. Refined structural characteristics of the annealed samples
a (Å) с (Å) Ca1 Ca2
z x y P x y O1 x y O2 x y O3 x y z O4(ОН) z Rwp/RB (%) Note:
х=0 9.423582(46) 6.880202(41) 0.00072(28) 0.24718(16) 0.99345(18) 0.39778(17) 0.36777(15) 0.32816(41) 0.48427(47) 0.58592(43) 0.46481(44) 0.34716(33) 0.26135(36) 0.07116(39) 0.19790(82) 5.84/2.31
х = 0.2 9.415057(80) 6.881743(78) 0.00045(45) 0.24680(24) 0.99343(29) 0.39875(32) 0.36934(30) 0.32826(62) 0.48368(62) 0.58934(64) 0.46538(67) 0.34162(42) 0.25559(44) 0.06652(48) 0.1970(16) 9.56/4.09
х = 0.5 9.411680(73) 6.881875(71) 0.00035(44) 0.24620(24) 0.99319(28) 0.39891(31) 0.36954(30) 0.32805(61) 0.48342(62) 0.58942(63) 0.46587(66) 0.34126(42) 0.25528(44) 0.06683(47) 0.1937(20) 9.63/4.08
х = 1.0 9.398407(74) 6.883715(73) 0.00019(46) 0.24506(25) 0.99308(30) 0.39898(33) 0.36970(31) 0.32737(64) 0.48297(64) 0.58977(66) 0.46638(69) 0.34101(44) 0.25514(46) 0.06670(49) 0.1869(30) 9.73/4.19
х = 2.0 9.373517(24) 6.885088(24) 0.00103(29) 0.24230(15) 0.99335(19) 0.39811(20) 0.36845(20) 0.32612(41) 0.48406(41) 0.58927(42) 0.46745(45) 0.33981(29) 0.25498(30) 0.06859(31) – 7.28/3.28
1. Unspecified coordinates are fixed at the values: Ca1 (x) = 1/3, Ca1 (y) = 2/3, O4 (x) = O4 (y) = F (x) = F (y) = 0, the rest - on the value of 1/4. 2. Standard deviations of specified values are given in parentheses.
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It should be noted that the distance between the fluorine ion and the nearest calcium ion, both within this research and according to the literature data [20], is less than the Ca–F distance in calcium fluoride CaF2 [24]. Tab. 2. Some bond lengths in the apatite structure for annealed samples х=0 х = 0.2 х = 0.5 х = 1.0 Å Ca1–O1 ( ) 2.4113(36) 2.4161(51) 2.4172(50) 2.4174(52) Å Ca1–O2 ( ) 2.4562(38) 2.4395(58) 2.4412(57) 2.4405(59) Ca1–O3 (Å) 2.7799(33) 2.8060(41) 2.8077(40) 2.8054(42) Ca2–O1 (Å) 2.7057(35) 2.7032(52) 2.6995(51) 2.6917(54) Ca2–O2 (Å) 2.3555(37) 2.3502(56) 2.3503(55) 2.3523(58) Ca2–O3 (Å) 2.3576(29) 2.3180(36) 2.3196(36) 2.5001(45) Ca2–O4 (Å) 2.3878(18) 2.3832(30) 2.3816(32) 2.3764(43) Ca2–F (Å) – 2.3552(26) 2.3499(26) 2.3364(27) О4–О3 (Å) 3.0776(30) 3.0336(46) 3.0222(52) 3.0036(68) О4–О4 (Å) 3.440(11) 3.441(22) 3.441(28) 3.442(41) F–О4 (Å) – 3.076(11) 3.053(14) 3.008(21)
х = 2.0 2.3994(33) 2.4507(38) 2.8118(28) 2.6879(34) 2.3614(37) 2.4836(29) – 2.3030(17) – – –
The FTIR spectra of samples in Fig. 6 show sorbed water and CO2 removed during the annealing. Libration and stretching vibrations of the OH group became clearer and more intense and, in addition, new absorption bands appeared in these ranges. A detailed analysis of them will be given below.
Fig. 6. FTIR spectra of samples with different concentrations of fluorine after annealing at 1000 °C Three bands of libration vibrations of the OH group (638, 670, 716 cm–1) and two bands of stretching vibrations (3544 and 3571 cm–1) are detected for the sample with x = 0.2. Based on the literature data [1, 13, 14] and the fact that no more than 10% of hydroxide ions should be replaced by fluorine ions in the sample, it can be concluded that the sample x = 0.2 has two types of hydroxyl groups: 1) OH groups in relation to which the fluorine ion is far away are characterized by stretching vibration of 3571 cm–1 and corresponding libration vibraton of 638 cm–1; 2) OH groups bounded with fluorine by hydrogen bonds are characterized by stretching vibration at a lower frequency (3544 cm– 1 ) and libration vibration at a higher frequency (670, 716 cm–1). It is seen that the absorption bands 7
for the first type of vibrations are strongly shifted relative to the position of these bands in the unsubstituted HA (x = 0). Such high sensitivity in the stretching and bending vibrations of the OH group to fluorine is explained by the strong bond of the OH dipoles in the chain [23]. In the HA structure, hydroxide ions are located at a great distance from one another (see Table 2); therefore there is no hydrogen bond between them but there is a hydrogen bond with the nearest oxygen ion of the PO43– tetrahedron [1, 25]. When replacing hydroxide ions with fluorine ions, the substituent, due to the smaller diameter, does not occupy the position of the OH group, which is located on the c axis between the plane of calcium ions and the plane of oxygen ions of the phosphate tetrahedron (Fig. 1 (c)). It takes the place slightly below (or above, depending on the type of channel) namely the plane of calcium ions (Fig. 1 (d)). As a result, the fluorine ion gets closer to the lower (or higher) located OH group and forms a hydrogen bond with it, as shown above by the X-ray diffraction analysis. An additional hydrogen bond with fluorine increases the frequency of libration vibrations of OH group. At the same time, the hydrogen bond OH⋅⋅⋅F, which is stronger than OH⋅⋅⋅O, reduces the bond strength inside the OH ion, which leads to a decrease in the frequency of stretching vibrations of the OH group in the partially substituted sample. According to the authors of [26], when a fluorine ion is put into the hydroxyl channel, adjustment hydroxyl groups are reoriented so that the proton ends are directed to the F– ion. In this case, a chain …ОН ОН⋅⋅⋅F⋅⋅⋅НО НО… is formed in the hydroxyl channel. In that case, localization of several fluorine ions in different places of the channel leads to the formation of …НО ОН… group with different dipole orientations in the middle of the chain. Libration vibration of these OH groups is observed at a frequency of 670 cm–1 [23]. Similar absorption bands (as for x = 0.2) are detected for the sample x = 0.5 (Fig. 6). In addition, it can be seen for x = 0.5 the decreasing the intensity of the stretching (3570 cm–1) and libration (638 cm–1) vibrations of hydroxide ions located far away from F–, that is consistent with an increase in substitution degree. There are none of these bands in the sample x = 1, since fluorine ions are quite common in chains. Due to the high concentration of fluorine in the sample, new libration vibrations at 740 cm–1 are observed, corresponding to the single hydroxide ion surrounded by fluorine ions: …F ОН⋅⋅⋅F… [23]. The wide band with a wavenumber of 3425 cm–1, observed in samples x = 0.5, 1, is most likely due to the stretching vibrations of the O–H bond of water sorbed on the surface of the particles. The absorption band at 747 cm–1 in sample x = 2 indicates a small concentration of hydroxide ion in FA. 3.3. Thermal stability The use of HA and FA in medicine to create ceramic products and coatings involves the use of high-temperature material processing [27], and therefore it is necessary to know the limits of their thermal stability. Fig. 7 shows that samples with the minimum (x = 0.2) and maximum (x = 2) fluorine concentrations are stable up to 1400 °C, all observed reflections belong to the apatite phase. As was shown earlier in [28], unsubstituted HA obtained during the same synthesis is stable up to 1300 °C during annealing in air. Consequently, using even a small concentration of fluorine leads to an increase in HA thermal stability. Obviously, the fluorine ion in the hexagonal channel and its bond with OH groups hinders the diffusion of hydroxide ions at high temperatures, which leads to an increase in the apatite stability.
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a
b Fig. 7. XRD patterns of samples for x = 0.2 (a) and x = 2 (b) annealed at different temperatures 4. Conclusions As a result of the research, it was found that mechanochemical synthesis can be used to obtain partially and fully substituted fluorapatite. A single-phase nanoscale fluorine-substituted apatite is formed after 30 min of mechanical impact upon the mixture of reagents . In the obtained samples, the lattice parameter depends on the concentration of the fluorine. It is shown that with an increase in substitution, parameter a decreases, c practically does not change, and the volume of the unit cell decreases. Within the structure of partially substituted hydroxyapatite, fluorine is localized in the hexagonal channel in the plane of Ca2 ions and form a hydrogen bond with the nearest hydrogen atom of the OH group. According to an FTIR spectroscopy, an increase in the fluorine ion concentration leads to a decrease in the O–H bond strength, with results in gradual decrease in the frequency of the OH group stretching vibrations and an increase in the frequency of the OH group libration vibrations. 9
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Highlights • • • •
a new approach to the synthesis of F-substituted apatites is proposed dry mechanochemical method allows reducing the synthesis time down to 30 min the product of synthesis is single-phase with an average crystallite size of 25 nm partially and fully substituted F-apatites are stable up to 1400 C.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-4596(19)30581-X
DOI:
https://doi.org/10.1016/j.jssc.2019.121076
Reference:
YJSSC 121076
To appear in:
Journal of Solid State Chemistry
Received Date: 31 July 2019 Revised Date:
15 October 2019
Accepted Date: 18 November 2019
Please cite this article as: N.V. Bulina, S.V. Makarova, I.Y. Prosanov, O.B. Vinokurova, N.Z. Lyakhov, Structure and thermal stability of fluorhydroxyapatite and fluorapatite obtained by mechanochemical method, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.121076. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Structure and Thermal Stability of Fluorhydroxyapatite and Fluorapatite Obtained by Mechanochemical Method Natalia V. Bulina*, Svetlana V. Makarova, Igor Yu. Prosanov, Olga B. Vinokurova, Nikolay Z. Lyakhov Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze street 18, Novosibirsk 630128, Russia Abstract This work shows that mechanochemical synthesis allows the obtaining of hydroxyapatite in which fluorine ions replace hydroxyl ions either partially or completely after 30 minutes of synthesis. The crystal structure of the materials obtained and their thermal stability when heated in the air were studied. It has been shown that the fluorine ions in the hexagonal channel of hydroxyapatite hinders diffusion of hydroxide ions at high temperatures, which leads to increased thermal stability of the apatite. Keywords: fluorapatite, fluorhydroxyapatite, mechanochemical synthesis, crystal structure 1. Introduction Hydroxyapatite Ca10(PO4)6(OH)2 (HA) is a representative of apatite structural type with the general formula of Ме10(RO4)6X2 with Р63/m space group. The HA crystal structure is described in detail in [1, 2]. The HA unit cell contains 10 calcium cations located in two nonequivalent positions: four cations in the Ca1 site, each surrounded by nine oxygen atoms, and six cations in the Ca2 site, each surrounded by seven oxygen atoms (Fig. 1 (a, c)). In addition to calcium ions, the HA elementary cell has six phosphate and two hydroxyl groups. The latter are located on the c axis in a hexagonal channel formed by calcium and oxygen ions from phosphate tetrahedrons. Hydroxyl groups in the adjacent channels are oriented in opposite directions (Fig. 1 (a, c)).
*
a
b
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d
Corresponding author at: Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze Str. 18,
Novosibirsk, 630128, Russia. E-mail address: [email protected] (N.V. Bulina)
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Fig. 1. HA (a, c) and FA (b, d) structures: projection along the z axis (a, b) and along the y axis (c, d) A wide range of substitutions are possible within the apatite structure, both in the cationic and anionic sublattices [1]; stoichiometric HA is therefore not practically found in the natural world. Usually, biogenic HA, the main mineral component of human bone and dental tissue, contains a carbonate group, calcium, sodium and chlorine ions, etc. [3]. Each type of ion in the apatite’s structure affects the properties of the substance obtained. For example, fluorine ions have a significant effect on the strength characteristics of tooth enamel [3]. This could be explained by the fact that fluorine ions, when replacing the hydroxyl group (Fig. 1 (b, c)), form fluorapatite (FA) Ca10(PO4)6F2, which has the highest hardness and lowest solubility among all calcium phosphates [3]. Despite the very low content of fluorine in enamel (0.01%), it makes this tissue the hardest in the human body. HA and FA are the most demanded calcium orthophosphates in dentistry. They are used as coatings to increase the osteoinductive potential of various dental implants, as additives in dental cements, in materials used for restoration of bone tissue, as well as in dentifrices [4]. In addition, apatites are used as model compounds of tooth enamel [1]. In vitro studies showed that FA can be used as a source of fluorine ions emitted at a controlled rate to ensure the formation of mechanically and functionally strong bone [5]. Partially and completely substituted FA can be obtained by ceramic method [6], precipitation from aqueous solution [7-8], through the sol-gel [9], hydrothermal [10-11], combustion [12], and mechanochemical methods [13-15]. Nikcevic et al showed that FA can be obtained after 9 hours of processing a mixture of Ca(OH)2, Р2О5 and СаF2 during mechanochemical synthesis in a RETCH planetary mill with a vial rotation speed of ~ 230 rpm [13]. In [14], FA was obtained in a mill with a vial rotation speed of 300 rpm after 6 hours of reaction mixture processing. Literature data say that, during mechanochemical synthesis, structural and chemical transformations of solids occur only when the applied stress is above the theoretical ultimate strength of the initial reagents [16]. In this regard, the increased rotation speed of the mill should increase the power released by the mechanical action, which will increase the applied stress, and, accordingly, will make it possible to reduce the time spent on processing. This research studies the possibility of the mechanochemical synthesis of fluorine-substituted hydroxyapatite in the AGO-2 planetary mill, with a vial rotation speed of 1800 rpm. 2. Materials and Methods Mechanochemical synthesis of fluorapatites was carried out in an AGO-2 planetary ball mill with steel vials and balls as grinding bodies. The synthesis conditions were similar to those used to obtain other substituted apatites [17, 18]. The synthesis time was 30 minutes at a vial rotation speed of 1800 rpm. The yield of the apatite synthesized by mechanochemical method in AGO-2 mill was 10 g/h. Calcium hydrogen phosphate, calcium oxide (just annealed at 1000 °C) and calcium fluoride (pure grade) were taken as the initial reagents. The powders were mixed in the stoichiometric ratios required to obtain fluorapatite with the degree of hydroxyl group substitution for fluorine from 0 to 2 mol: 6CaHPO4 + (4-х/2)CaO + х/2CaF2 = Ca10(PO4)6(OH)2-хFх + nН2О
(1)
where х = 0, 0.2, 0.5, 1, 2. To define the crystal structure of the samples, the products of the mechanochemical synthesis were annealed at 1000 ° C in the PVK-1.4-8 high-temperature furnace for 2 hours. 2
Samples were analyzed by FTIR spectroscopy on a INFRALUM FT801 spectrometer and by X-ray diffraction (XRD) analysis on a D8 Advance powder diffractometer with Cu-Kα radiation, equipped with a one-dimensional Lynx-Eye detector and a Kβ filter. The phase composition of the obtained samples was identified using the ICDD PDF-4 powder database (2011). The structural parameters were specified by Rietveld refinement analysis of powder XRD patterns using Topas 4.2 software. To model the profile of any diffraction reflections, the Lorentzian function was used. The size of the crystallites was calculated from the integral width of the reflections using the Scherrer equation, assuming the presence of spherical particles in the sample. The instrumental function was calculated by the fundamental parameters method [19]. The initial data for HA and FA are taken from [20, 21]. The parameters of the background, sample displacement, lattice parameters, crystallite size and atomic coordinates were refined. Initial refinements for the samples with x = 0.2–1 were conducted considering the position of ions in the hexagonal channel as a non-split one, i.e. with the same coordinates for O4 and F. The final refinements were performed using individual positions for O4 and F, namely position of fluorine was fixed at special site (0, 0, 0.25) based on literature data [21], whereas the value of z for oxygen from the hydroxyl group was under refinement. 3. Results and Discussion 3.1. Mechanochemical synthesis The XRD patterns of the samples obtained after mechanochemical synthesis are shown in Fig.2. All the observed reflections correspond to the apatite phase (card PDF 040-10-6315), so the samples obtained are therefore single-phase. It can be seen that, as the concentration of fluorine increases, the maxima of the reflections shift to large angles which indicates a decrease in the lattice spacing.
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Fig. 2. XRD patterns of samples with different concentrations of fluorine after mechanochemical synthesis for 30 minutes The results of the lattice parameters refinement (Fig. 3) show that the FA unit cell volume mainly decreases due to a decrease in the a parameter, a slight increase in the c parameter does not have a significant effect on the cell volume. Literature data [1, 2, 13, 22] confirm that FA has a significantly smaller a parameter compared to HA. The decrease in the a and, respectively, b parameters is explained by the smaller radius of the substituent ion localized in the hexagonal channel on the c axis. The oxygen of OH groups is located on the c axis between calcium and oxygen ions of the phosphate tetrahedron (Fig. 1 (c)). The smaller radius of the fluorine ion allows it to occupy the more advantageous site – the plane of Ca2 ions (Fig. 1 (d)), that leads to a contraction of the channel in a and b directions. Due to the large distance between the ions located on the c axis, this substitution does not significantly affect the value of c parameter. The size of the crystallites in the synthesized samples does not depend on the degree of substitution and equals to ~ 25 nm (Fig. 3).
a
b
c
d
Fig. 3. Evolution of the unit cell parameters a (a) and c (b), lattice volume (c) and crystallite size (d) depending on the concentration of fluorine used for the samples before and after annealing
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In the FTIR spectrum of HA (Fig. 4, x = 0), the absorption bands of the phosphate tetrahedron and hydroxyl group [1] are observed: the bending vibrations of the O–P–O bond (572 and 602 cm–1), stretching vibration of the P–O bond (962, 1048 and 1090 cm–1), libration (632 cm–1) and stretching vibrations (3574 cm–1) of the OH groups.
Fig. 4. FTIR spectra of samples after mechanochemical synthesis with different concentrations of fluorine At a small degree of substitution (x = 0.2), an increase in the frequency of the libration vibration and a decrease of the intensity and frequency of the OH group stretching vibration are observed (Fig. 4). As the substitution increases, the intensity of these bands decreases. The absence of OH group vibrations at x = 2 shows the absence of hydroxyl groups in the apatite structure that is consistent with the chemical formula for FA and with literature data [13,20,22-23]. In the absorption spectra for intermediate fluorine concentrations (x = 0.5–1), a low intensity band is observed at ~ 730 cm–1, which can be attributed to libration mode of the OH group connected with a fluorine ion by a hydrogen bond [13,20,22–23]. The absorption bands of phosphate ion gradually shift to higher frequencies with an increasing concentration of fluorine (Fig. 4), that is associated with a decrease of the bond lengths in the phosphate tetrahedron and is consistent with a decrease in the unit cell volume, as shown above (Fig. 3). In addition to the absorption bands of the phosphate and hydroxyl groups, in the FTIR spectra of all samples (Fig. 4) there are low-intensity wide absorption bands of the carbonate ion (1400–1500 cm–1) absorbed from air during the synthesis, and sorbed water (3250–3660 cm–1), released in accordance with the synthesis reaction (1). 3.2. Annealed samples Annealing of the samples after mechanochemical synthesis made it possible to significantly increase the crystallinity of the samples (Fig. 3 (d)), while the dynamics of the lattice parameters remained the same (Fig. 3 (a – c)). The Rietveld plots for samples with x = 0 and x = 2 are shown in Fig. 5. Tables 1 and 2 present the results of refinement of the apatite structure for all annealed samples. It is seen that among all the ions located in the apatite unit cell, the oxygen ion of the hydroxyl group located in the O4 site shows the greatest displacement with increasing substitution degree (Table 1). This ion becomes closer to the fluorine ion with increasing fluorine concentration. At the same time the distance between adjacent O4 positions remains constant; it does not depend on the concentration of the substituent 5
(Table 2). The length of the Ca2–F bond gradually decreases, which indicates a decrease in the channel diameter. The distance between the oxygen ion of the OH group (O4) and the nearest oxygen ion of the phosphate tetrahedron (O3) with which there is a hydrogen bond [1, 22] is comparable to the F–O4 distance, that proves the existence of additional hydrogen bond between the OH group and fluorine ion.
a b Fig. 5. Observed, calculated and difference diffraction patterns for the samples with x = 0 (a) and x = 2 (b) after the Rietveld refinement. The peak positions of HA, FA and CaO are shown by vertical markers. Concentration of CaO admixture in the sample with x = 0 is 0.2 wt.% Tab. 1. Refined structural characteristics of the annealed samples
a (Å) с (Å) Ca1 Ca2
z x y P x y O1 x y O2 x y O3 x y z O4(ОН) z Rwp/RB (%) Note:
х=0 9.423582(46) 6.880202(41) 0.00072(28) 0.24718(16) 0.99345(18) 0.39778(17) 0.36777(15) 0.32816(41) 0.48427(47) 0.58592(43) 0.46481(44) 0.34716(33) 0.26135(36) 0.07116(39) 0.19790(82) 5.84/2.31
х = 0.2 9.415057(80) 6.881743(78) 0.00045(45) 0.24680(24) 0.99343(29) 0.39875(32) 0.36934(30) 0.32826(62) 0.48368(62) 0.58934(64) 0.46538(67) 0.34162(42) 0.25559(44) 0.06652(48) 0.1970(16) 9.56/4.09
х = 0.5 9.411680(73) 6.881875(71) 0.00035(44) 0.24620(24) 0.99319(28) 0.39891(31) 0.36954(30) 0.32805(61) 0.48342(62) 0.58942(63) 0.46587(66) 0.34126(42) 0.25528(44) 0.06683(47) 0.1937(20) 9.63/4.08
х = 1.0 9.398407(74) 6.883715(73) 0.00019(46) 0.24506(25) 0.99308(30) 0.39898(33) 0.36970(31) 0.32737(64) 0.48297(64) 0.58977(66) 0.46638(69) 0.34101(44) 0.25514(46) 0.06670(49) 0.1869(30) 9.73/4.19
х = 2.0 9.373517(24) 6.885088(24) 0.00103(29) 0.24230(15) 0.99335(19) 0.39811(20) 0.36845(20) 0.32612(41) 0.48406(41) 0.58927(42) 0.46745(45) 0.33981(29) 0.25498(30) 0.06859(31) – 7.28/3.28
1. Unspecified coordinates are fixed at the values: Ca1 (x) = 1/3, Ca1 (y) = 2/3, O4 (x) = O4 (y) = F (x) = F (y) = 0, the rest - on the value of 1/4. 2. Standard deviations of specified values are given in parentheses.
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It should be noted that the distance between the fluorine ion and the nearest calcium ion, both within this research and according to the literature data [20], is less than the Ca–F distance in calcium fluoride CaF2 [24]. Tab. 2. Some bond lengths in the apatite structure for annealed samples х=0 х = 0.2 х = 0.5 х = 1.0 Å Ca1–O1 ( ) 2.4113(36) 2.4161(51) 2.4172(50) 2.4174(52) Å Ca1–O2 ( ) 2.4562(38) 2.4395(58) 2.4412(57) 2.4405(59) Ca1–O3 (Å) 2.7799(33) 2.8060(41) 2.8077(40) 2.8054(42) Ca2–O1 (Å) 2.7057(35) 2.7032(52) 2.6995(51) 2.6917(54) Ca2–O2 (Å) 2.3555(37) 2.3502(56) 2.3503(55) 2.3523(58) Ca2–O3 (Å) 2.3576(29) 2.3180(36) 2.3196(36) 2.5001(45) Ca2–O4 (Å) 2.3878(18) 2.3832(30) 2.3816(32) 2.3764(43) Ca2–F (Å) – 2.3552(26) 2.3499(26) 2.3364(27) О4–О3 (Å) 3.0776(30) 3.0336(46) 3.0222(52) 3.0036(68) О4–О4 (Å) 3.440(11) 3.441(22) 3.441(28) 3.442(41) F–О4 (Å) – 3.076(11) 3.053(14) 3.008(21)
х = 2.0 2.3994(33) 2.4507(38) 2.8118(28) 2.6879(34) 2.3614(37) 2.4836(29) – 2.3030(17) – – –
The FTIR spectra of samples in Fig. 6 show sorbed water and CO2 removed during the annealing. Libration and stretching vibrations of the OH group became clearer and more intense and, in addition, new absorption bands appeared in these ranges. A detailed analysis of them will be given below.
Fig. 6. FTIR spectra of samples with different concentrations of fluorine after annealing at 1000 °C Three bands of libration vibrations of the OH group (638, 670, 716 cm–1) and two bands of stretching vibrations (3544 and 3571 cm–1) are detected for the sample with x = 0.2. Based on the literature data [1, 13, 14] and the fact that no more than 10% of hydroxide ions should be replaced by fluorine ions in the sample, it can be concluded that the sample x = 0.2 has two types of hydroxyl groups: 1) OH groups in relation to which the fluorine ion is far away are characterized by stretching vibration of 3571 cm–1 and corresponding libration vibraton of 638 cm–1; 2) OH groups bounded with fluorine by hydrogen bonds are characterized by stretching vibration at a lower frequency (3544 cm– 1 ) and libration vibration at a higher frequency (670, 716 cm–1). It is seen that the absorption bands 7
for the first type of vibrations are strongly shifted relative to the position of these bands in the unsubstituted HA (x = 0). Such high sensitivity in the stretching and bending vibrations of the OH group to fluorine is explained by the strong bond of the OH dipoles in the chain [23]. In the HA structure, hydroxide ions are located at a great distance from one another (see Table 2); therefore there is no hydrogen bond between them but there is a hydrogen bond with the nearest oxygen ion of the PO43– tetrahedron [1, 25]. When replacing hydroxide ions with fluorine ions, the substituent, due to the smaller diameter, does not occupy the position of the OH group, which is located on the c axis between the plane of calcium ions and the plane of oxygen ions of the phosphate tetrahedron (Fig. 1 (c)). It takes the place slightly below (or above, depending on the type of channel) namely the plane of calcium ions (Fig. 1 (d)). As a result, the fluorine ion gets closer to the lower (or higher) located OH group and forms a hydrogen bond with it, as shown above by the X-ray diffraction analysis. An additional hydrogen bond with fluorine increases the frequency of libration vibrations of OH group. At the same time, the hydrogen bond OH⋅⋅⋅F, which is stronger than OH⋅⋅⋅O, reduces the bond strength inside the OH ion, which leads to a decrease in the frequency of stretching vibrations of the OH group in the partially substituted sample. According to the authors of [26], when a fluorine ion is put into the hydroxyl channel, adjustment hydroxyl groups are reoriented so that the proton ends are directed to the F– ion. In this case, a chain …ОН ОН⋅⋅⋅F⋅⋅⋅НО НО… is formed in the hydroxyl channel. In that case, localization of several fluorine ions in different places of the channel leads to the formation of …НО ОН… group with different dipole orientations in the middle of the chain. Libration vibration of these OH groups is observed at a frequency of 670 cm–1 [23]. Similar absorption bands (as for x = 0.2) are detected for the sample x = 0.5 (Fig. 6). In addition, it can be seen for x = 0.5 the decreasing the intensity of the stretching (3570 cm–1) and libration (638 cm–1) vibrations of hydroxide ions located far away from F–, that is consistent with an increase in substitution degree. There are none of these bands in the sample x = 1, since fluorine ions are quite common in chains. Due to the high concentration of fluorine in the sample, new libration vibrations at 740 cm–1 are observed, corresponding to the single hydroxide ion surrounded by fluorine ions: …F ОН⋅⋅⋅F… [23]. The wide band with a wavenumber of 3425 cm–1, observed in samples x = 0.5, 1, is most likely due to the stretching vibrations of the O–H bond of water sorbed on the surface of the particles. The absorption band at 747 cm–1 in sample x = 2 indicates a small concentration of hydroxide ion in FA. 3.3. Thermal stability The use of HA and FA in medicine to create ceramic products and coatings involves the use of high-temperature material processing [27], and therefore it is necessary to know the limits of their thermal stability. Fig. 7 shows that samples with the minimum (x = 0.2) and maximum (x = 2) fluorine concentrations are stable up to 1400 °C, all observed reflections belong to the apatite phase. As was shown earlier in [28], unsubstituted HA obtained during the same synthesis is stable up to 1300 °C during annealing in air. Consequently, using even a small concentration of fluorine leads to an increase in HA thermal stability. Obviously, the fluorine ion in the hexagonal channel and its bond with OH groups hinders the diffusion of hydroxide ions at high temperatures, which leads to an increase in the apatite stability.
8
a
b Fig. 7. XRD patterns of samples for x = 0.2 (a) and x = 2 (b) annealed at different temperatures 4. Conclusions As a result of the research, it was found that mechanochemical synthesis can be used to obtain partially and fully substituted fluorapatite. A single-phase nanoscale fluorine-substituted apatite is formed after 30 min of mechanical impact upon the mixture of reagents . In the obtained samples, the lattice parameter depends on the concentration of the fluorine. It is shown that with an increase in substitution, parameter a decreases, c practically does not change, and the volume of the unit cell decreases. Within the structure of partially substituted hydroxyapatite, fluorine is localized in the hexagonal channel in the plane of Ca2 ions and form a hydrogen bond with the nearest hydrogen atom of the OH group. According to an FTIR spectroscopy, an increase in the fluorine ion concentration leads to a decrease in the O–H bond strength, with results in gradual decrease in the frequency of the OH group stretching vibrations and an increase in the frequency of the OH group libration vibrations. 9
Studies of the thermal stability of the obtained samples showed that even a low concentration of fluorine increases the apatite thermal stability. Acknowledgements This work was supported by the RFBR according to the research project № 18-29-11064. References 1. J.C. Elliott, Structure and Chemistry of the Apatites and Other Calcium Orthophosphates. Studies in Inorganic Chemistry, Elsevier, vol. 18 (1994) 388. 2. J.M. Hughes, J. Rakovan, The crystal structure of apatite, Ca5 (PO4) 3 (F, OH, Cl)2, Reviews in Mineralogy and Geochemistry, 48(1) (2002) 1–12. https://doi.org/10.2138/rmg.2002.48.1. 3. O. Kaygili, S. Keser, M. Kom, N. Bulut, S.V. Dorozhkin, The effect of simulating body fluid on the structural properties of hydroxyapatite synthesized in the presence of citric acid, Prog. Biomater., 5 (2016) 173-182. https://doi.org/10.1007/s40204-016-0055-5. 4. S.V. Dorozhkin, Calcium orthophosphates in dentistry, J. Mater. Sci.: Mater. Med., 24(6) (2013) 1335–1363. https://doi.org/ 10.1007/s10856-013-4898-1. 5. L. Gineste, M. Gineste, X. Ranz, A. Ellefterion, A. Guilhem, N. Rouquet, P. Frayssinet, Degradation of hydroxylapatite, fluorapatite, and fluorhydroxyapatite coatings of dental implants in dogs, J. of Biomedical Materials Research: An Official Journal of The Society for Biomaterials, The Japanese Society for Biomaterials, and The Australian Society for Biomaterials, 48(3) (1999) 224-234. https://doi.org/10.1002/(SICI)10974636(1999)48:3<224::AID-JBM5>3.0.CO;2-A. 6. C.C. Wu, S.T.Huang, T.W.Tseng, Q.-L. Rao, H.-C. Lin, FT-IR and XRD investigations on sintered fluoridated hydroxyapatite composites, J.Mol.Struct., 979(1-3) (2010) 72-76. https://doi.org/10.1016/j.molstruc.2010.06.003. 7. S.M. Barinov, L.I. Shvorneva, D. Ferro, I.V. Fadeeva, S.V. Tumanov, Solid solution formation at the sintering of hydroxyapatite–fluorapatite ceramics, Science and Technology of Advanced Materials, 5(5-6) (2004) 537. https://doi.org/10.1016/j.stam.2004.02.012. 8. S.C. Lima, T. Baikie, S.S. Pramana, R. Smith, T.J. White, Apatite metaprism twist angle (j) as a tool for crystallochemical diagnosis, J. Solid State Chem., 184 (2011) 2978–2986. https://doi:10.1016/j.jssc.2011.08.031. 9. C.J. Tredwin, A.M.Young, G.Georgiou, S.-H. Shin, H.-W. Kim, J.C. Knowles, Hydroxyapatite, fluor-hydroxyapatite and fluorapatite produced via the sol–gel method. Optimisation, characterisation and rheology, Dent. Mater., 29(2) (2013) 166-173. https://doi.org/10.1016/j.dental.2012.11.008. 10. J.C. RendoHn-Angeles, K. Yanagisawa, N. Ishizawa, S. Oishi, J. Solid State Chem., 151 (2000) 65–72. https://doi.org/10.1006/jssc.1999.8623 11. M. Chen, D. Jiang, D. Li, J. Zhu, G. Li, J. Xie. Controllable synthesis of fluorapatite nanocrystals with various morphologies: Effects of pH value and chelating reagent, J. Alloys & Compounds, 485(1-2) (2009) 396-401. https://doi.org/10.1016/j.jallcom.2009.05.121. 12. J. Zhao, X. Dong, M. Bian, J. Zhao, Y. Zhang, Y. Sun, J. Chen, X. Wang, Solution combustion method for synthesis of nanostructured hydroxyapatite, fluorapatite and chlorapatite, Appl. Surface Sci., 314 (2014) 1026-1033. https://doi.org/10.1016/j.apsusc.2014.06.075. 13. I. Nikcevic, V. Jokanovic, M. Mitric, Z. Nedic, D. Makovec, D. Uskokovic. Mechanochemical synthesis of nanostructured fluorapatite/fluorhydroxyapatite and carbonated fluorapatite/fluorhydroxyapatite, J. Solid State Chem., 177(7) (2004) 2565-2574. https://doi.org/10.1016/j.jssc.2004.03.024. 14. M.H. Fathi, E. M. Zahrani. Mechanical alloying synthesis and bioactivity evaluation of nanocrystalline fluoridated hydroxyapatite, J. Cryst. Growth, 311(5) (2009) 1392-1403. https://doi.org/10.1016/j.jcrysgro.2008.11.100. 10
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Highlights • • • •
a new approach to the synthesis of F-substituted apatites is proposed dry mechanochemical method allows reducing the synthesis time down to 30 min the product of synthesis is single-phase with an average crystallite size of 25 nm partially and fully substituted F-apatites are stable up to 1400 C.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: